Chapter 4: NumPy

NumPy is a popular library in the Python ecosystem and a critical component of the SciPy stack. So much so that NumPy is even included in Apple’s default installation of Python and in other Python-powered applications such as Blender. While it may be tempting to work with NumPy’s objects as lists or to circumnavigate the NumPy library altogether, the time it takes to learn NumPy’s powerful features is well worth it! It will often allow you to solve problems with less effort and time and with shorter and faster-executing code. This is due to:

• NumPy automatically propagating operations to all values in an array instead of requiring for loops

• A massive collection of functions for working with numerical data

• Many of NumPy’s functions are Python-wrapped C code making them run faster

The NumPy package can be imported by import numpy, but the scientific Python community has developed an unofficial, but strong, convention of importing NumPy using the np alias. It is a matter of personal preference whether to use the alias or not, but it is strongly encouraged for consistency with the rest of the community. Instead of numpy.function(), the function is then called by the shorter np.function(). All of the NumPy code in this and subsequent chapters assumes the following import.

import numpy as np

4.1 NumPy Arrays

One of the main contributions of NumPy is the ndarray (i.e., “n-dimensional array”), NumPy array, or just array for short. This is an object similar to a list or nested list of lists except that mathematical operations and NumPy functions automatically propagate to each element instead of requiring a for loop to iterate over it. Because of their power and convenience, arrays are the default object type for any operation performed with NumPy and many scientific libraries that are built on NumPy (e.g., SciPy, pandas, scikit-learn, etc…).

4.1.1 Basic Arrays

The NumPy array looks like a Python list wrapped in array(). It is an iterable object, so you could iterate over it using a for loop if you really want to. However, because NumPy automatically propagates operations through the array, for loops are typically unnecessary. For example, let us say you want to multiply a list of numbers by 2. Doing this with a list would likely look like the following.

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
for value in nums:
    print(2 * value)

0
2
4
6
8
10
12
14
16
18

In contrast, performing this same operation using a NumPy array only requires multiplying the array by 2.

arr = np.array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
print(2 * arr)

[ 0  2  4  6  8 10 12 14 16 18]

4.1.2 Type Conversion to Arrays

There are three common ways to generate a NumPy array that we will cover in the beginning of this chapter. The first is simply to convert a list or tuple to an array using the np.array() function.

a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]  # list
arr = np.array(a)
arr

array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

The fact that the object is an ndarray is denoted by the array().

4.1.3 Array from Sequence

We can also create an array using NumPy sequence-generating functions. There are two common functions in NumPy for this task: np.arange() and np.linspace(). The np.arange() function behaves similarly to the native Python range() function with the key difference that it outputs an array. Another minor difference is that while range() generates a range object, np.arange() generates a sequences of values immediately. The arguments for np.arange() are similar to that of Python’s range() function where start is inclusive and stop is exclusive, but unlike range(), the step size for np.arange() does not need to be an integer value.

np.arange(start, stop, step)

The np.linspace() function is related to np.arange() except that instead of defining the sequence based on step size, it generates a sequence based on how many evenly distributed points to generate in the given span of numbers. Additionally, np.arange() excludes the stop values while np.linspace() includes it. The difference between these two functions is somewhat subtle, and the use of one over the other often comes down to user preference or convenience.

np.linspace(start, stop, number of points)

arr = np.arange(0, 10, 0.5)
arr

array([0. , 0.5, 1. , 1.5, 2. , 2.5, 3. , 3.5, 4. , 4.5, 5. , 5.5, 6. ,
       6.5, 7. , 7.5, 8. , 8.5, 9. , 9.5])

arr = np.linspace(0,10, 20)
arr

array([ 0.        ,  0.52631579,  1.05263158,  1.57894737,  2.10526316,
        2.63157895,  3.15789474,  3.68421053,  4.21052632,  4.73684211,
        5.26315789,  5.78947368,  6.31578947,  6.84210526,  7.36842105,
        7.89473684,  8.42105263,  8.94736842,  9.47368421, 10.        ])

Two other useful functions for generating arrays are np.zeros() and np.ones() which generate arrays populated with exclusively zeros and ones, respectively. The functions accept the shape argument as a tuple of the array dimensions in the form (rows, columns).

np.zeros((2,4))

array([[0., 0., 0., 0.],
       [0., 0., 0., 0.]])

np.ones(10)

array([1., 1., 1., 1., 1., 1., 1., 1., 1., 1.])

You should commit to remembering np.arange() and np.linspace(), as these are used often. The np.zeros() and np.ones() functions are not as common but are useful in particular applications. They can also be used to generate arrays filled with other values. For example, to generate an array of threes, an array of zeros can be generated and then incremented by 3.

arr = np.zeros((2,4))
arr += 3
print(arr)

[[3. 3. 3. 3.]
 [3. 3. 3. 3.]]

4.1.4 Arrays from Functions

A third approach is to generate an array from a function using np.fromfunction() which generates an array of values using the array indices as inputs. This function requires a function as an argument.

np.fromfunction(function, shape)

Let us make an array of the dimensions (3,3) where each element is the product of the row and column indices.

def prod(x, y):
    return x * y

np.fromfunction(prod, (3,3))

array([[0., 0., 0.],
       [0., 1., 2.],
       [0., 2., 4.]])

4.2 Reshaping & Merging Arrays

Modifying the dimensions of one or more arrays is a common task in NumPy. This may involve changing the number of columns and rows or merging multiple arrays into a larger array. The size and shape or an array are the number of elements and dimensions, respectively. These can be determined using the size and shape NumPy methods.

counting = np.array([[1, 2, 3], [4, 5, 6]])

counting.size

6

counting.shape

(2, 3)

The NumPy convension is to provide the dimensions of a two-dimensional array as (row, columns).

4.2.1 Reshaping Arrays

The dimensions of arrays can be modified using the np.reshape() method. This method maintains the number of elements and order of elements in the array but repacks them into a different number of row and columns. Because the number of elements is maintained, the new size array needs to contain the same number of elements as the original.

np.reshape(array, dimensions)

In this function, array is the NumPy array being reshaped and dimensions is a tuple containing the desired number of rows and columns in that order. The original array must fit exactly into the new dimensions or else NumPy will refuse to change it. This method does not change the original array in place but rather returns a modified copy. This is a good time to note that because this and other NumPy functions are methods for NumPy arrays, they can also be called by listing the array up front like list and string methods presented in chapter 1. For example, the reshape() function can be called with array.reshape(dimensions).

array_1D = np.linspace(0, 9.5, 20)
array_1D

array([0. , 0.5, 1. , 1.5, 2. , 2.5, 3. , 3.5, 4. , 4.5, 5. , 5.5, 6. ,
       6.5, 7. , 7.5, 8. , 8.5, 9. , 9.5])

The following code reshapes the array into a 4 \(\times\) 5 array.

array_2D = np.reshape(array_1D, (4, 5))
array_2D

array([[0. , 0.5, 1. , 1.5, 2. ],
       [2.5, 3. , 3.5, 4. , 4.5],
       [5. , 5.5, 6. , 6.5, 7. ],
       [7.5, 8. , 8.5, 9. , 9.5]])

If you need to reshape an array with only one new dimension known, place a -1 in the other. This signals to NumPy that it should choose the second dimension to make the data fit.

4.2.2 Flatten Arrays

Flattening an array takes a higher-dimensional array and squishes it into a one-dimensional array. To flatten out an array, the np.flatten() method is often the most convenient way.

array_2D.flatten()

array([0. , 0.5, 1. , 1.5, 2. , 2.5, 3. , 3.5, 4. , 4.5, 5. , 5.5, 6. ,
       6.5, 7. , 7.5, 8. , 8.5, 9. , 9.5])

The format of the output makes it look like it is still a 2D array, but notice that there is a comma instead of a square bracket at the end of the first row. The dimensions of this array are 1 \(\times\) 20.

4.2.3 Transpose Arrays

Transposing an array rotates the array around the diagonal (Figure 1).

Figure 1 The np.transpose() or array.T method transposes the NumPy array effectively flipping the rows and columns.

The np.transpose() method flips the rows and columns. NumPy also provides an alias/shortcut of array.T to accomplish the same outcome. The latter is far more common, so it is the method used here.

array_2D

array([[0. , 0.5, 1. , 1.5, 2. ],
       [2.5, 3. , 3.5, 4. , 4.5],
       [5. , 5.5, 6. , 6.5, 7. ],
       [7.5, 8. , 8.5, 9. , 9.5]])

array_2D.T

array([[0. , 2.5, 5. , 7.5],
       [0.5, 3. , 5.5, 8. ],
       [1. , 3.5, 6. , 8.5],
       [1.5, 4. , 6.5, 9. ],
       [2. , 4.5, 7. , 9.5]])

4.2.4 Merge Arrays

Merging arrays can be done in multiple ways. NumPy provides convenient methods for merging arrays using np.vstack, np.hstack, and np.dstack which merge arrays along the vertically, horizontally, and depth-wise axes, respectively (Figure 2).

Figure 2 NumPy arrays can be stacked vertically (top left), as columns (top center), depth-wise (top right), or horizontally (bottom) using the np.vstack(), np.column_stack(), np.dstack(), and np.hstack() functions, respectively.

a = np.arange(0, 5)

b = np.arange(5, 10)

np.vstack((a, b))

array([[0, 1, 2, 3, 4],
       [5, 6, 7, 8, 9]])

np.hstack((a, b))

array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

np.dstack((a, b))

array([[[0, 5],
        [1, 6],
        [2, 7],
        [3, 8],
        [4, 9]]])

A related function is the np.column_stack() function that stacks the corresponding elements in column lists in a column arrangement.

np.column_stack((a,b))

array([[0, 5],
       [1, 6],
       [2, 7],
       [3, 8],
       [4, 9]])

The outcome of the np.column_stack() function can also be accomplished by transposing the output of the np.vstack() function.

np.vstack((a,b)).T

array([[0, 5],
       [1, 6],
       [2, 7],
       [3, 8],
       [4, 9]])

4.3 Indexing Arrays

Similar to lists, it is often useful to be able to index and slice ndarrays. Because arrays are often higher dimensional, there are some differences in indexing that provide extra convenience.

4.3.1 One-Dimensional Arrays

Indexing one-dimensional arrays is done in an identical fashion to lists. Simply include the index value(s) or range in square brackets behind the array name.

array_1D

array([0. , 0.5, 1. , 1.5, 2. , 2.5, 3. , 3.5, 4. , 4.5, 5. , 5.5, 6. ,
       6.5, 7. , 7.5, 8. , 8.5, 9. , 9.5])

array_1D[5]

2.5

4.3.2 Two-Dimensional Arrays

Two-dimensional arrays can be also indexed in a similar fashion to nested lists, but because arrays are often multidimensional, there is also a shortcut below to make work with arrays more convenient. To access the entire second row of an array, provide the row index in square brackets behind the array name just like indexing in lists.

array_2D

array([[0. , 0.5, 1. , 1.5, 2. ],
       [2.5, 3. , 3.5, 4. , 4.5],
       [5. , 5.5, 6. , 6.5, 7. ],
       [7.5, 8. , 8.5, 9. , 9.5]])

array_2D[1]

array([2.5, 3. , 3.5, 4. , 4.5])

To access the first element in the second row, it is perfectly valid to use two adjacent square brackets just as one would use in a nested list of lists. However, to make work more convenient, these square brackets are often combined with the row and column indices separated by commas.

array_name[rows, columns]

array_2D[1][0]

2.5

array_2D[1, 0]

2.5

Ranges of values can also be accessed in arrays by using slicing. The following array input generates a slice of the second row of the array.

array_2D[1, 1:]

array([3. , 3.5, 4. , 4.5])

As seen above, if you want to access an entire row, it is not necessary to indicate the columns. It is implicitly understood that all columns are requested. However, if you want to access the first column, something needs to be placed before the column. The easiest solutions is the use a colon to explicitly indicate all rows.

array_2D[0] # implicitly understood all columns

array([0. , 0.5, 1. , 1.5, 2. ])

array_2D[0, :] # explicit indicating all columns

array([0. , 0.5, 1. , 1.5, 2. ])

array_2D[:, 0]     # all rows

array([0. , 2.5, 5. , 7.5])

Tip

As you index higher dimensional arrays, you may see code that looks like arr[...,0] where arr is the array name. The three dots mean to include everything, so arr[...,0] has the same effect as arr[:,:,0] for a three-dimensional array, for example.

4.3.3 Advanced Indexing

In the event you have a multidimensional array, you can access elements in the array using multiple collections of values (i.e., ndarrays, lists, or tuples) where each collection indicates the location along a different dimension. This is an instance of fancy indexing. For example, if we want to select the following bolded, orange elements from array_2D, we can create two lists - the first list contains the row indicies for each element and the second list likewise contains the column indicies.

\[\begin{split} \left[ \begin{array}{cCc} 0. & 0.5 & 1. & \color{BurntOrange}\textbf{1.5} & 2. \\ 2.5 & 3. & 3.5 & 4. & 4.5 \\ \color{BurntOrange}\textbf{5.} & \color{BurntOrange}\textbf{5.5} & 6. & 6.5 & 7. \\ 7.5 & 8. & 8.5 & 9. & 9.5 \end{array} \right] \end{split}\]

row = [2,2,0]
col = [0,1,3]

array_2D[row, col]

array([5. , 5.5, 1.5])

Another feature of indexing ndarrays is that the returned array will have the same dimensions as the array containing the indicies. In the following example, we have two index arrays where i_flat is a 1 \(\times\) 4 array while i_square is a 2 \(\times\) 2 array resulting in 1 \(\times\) 4 and 2 \(\times\) 2 arrays, respectively

threes = np.arange(3, 30, 3)

i_flat = np.array([0, 3,1, 5])
i_square = np.array([[0, 3],
                     [1, 5]])

threes[i_flat]

array([ 3, 12,  6, 18])

threes[i_square]

array([[ 3, 12],
       [ 6, 18]])

The latter result can also be accomplished by indexing using a flat (i.e., one-dimensional) array followed by reshaping it to the desired dimensions as demonstred below.

i = np.array([0, 3,1, 5])

threes[i].reshape((2,2))

array([[ 3, 12],
       [ 6, 18]])

4.3.4 Masking

Elements in a NumPy array can also be selected using a boolean array through a process known as masking. The masking array is a boolean array filled with either 1 and 0 or True and False and has the same dimensions as the origional array. Any element in the origional array that has a 1 or True in the corresponding position of the masking array is returned.

For example,

orig_array = np.array([[5, 7, 1],
                       [3, 4, 2],
                       [0, 9, 8]])

mask = np.array([[0, 1, 0],
                 [1, 1, 1],
                 [1, 0, 1]], dtype=bool)

orig_array[mask]

array([7, 3, 4, 2, 0, 8])

It’s important to note that if you use 1 and 0 in the masking array, it is required that you include dtype=bool or else NumPy will treat the 1 and 0 as indicies instead of booleans and attempt indexing.

mask = np.array([[0, 1, 0],
                 [1, 1, 1],
                 [1, 0, 1]])

orig_array[mask]

array([[[5, 7, 1],
        [3, 4, 2],
        [5, 7, 1]],

       [[3, 4, 2],
        [3, 4, 2],
        [3, 4, 2]],

       [[3, 4, 2],
        [5, 7, 1],
        [3, 4, 2]]])

The true power of masking is when the masking array is generated through boolean logic such as >, <=, or ==. This enables the user to select elements of an array through conditions as demonstrated below where we select all elements of the orig_array that are greater than 5.

Note

If a masking array is generated by a boolean condition, the resulting masking array will automatically be a boolean array suitable for masking.

cond = orig_array > 5
cond

array([[False,  True, False],
       [False, False, False],
       [False,  True,  True]])

orig_array[cond]

array([7, 9, 8])

We can also include the condition directly in the square brackets to save a step as shown below.

orig_array[orig_array > 5]

array([7, 9, 8])

4.4 Vectorization & Broadcasting

One of the major advantages of NumPy arrays over lists is that operations automatically vectorize across the arrays. That is, mathematical operations propagate through the array(s) instead of requiring for loops. This both speeds up the calculations and makes the code easier to read and write.

4.4.1 NumPy Functions

Let us take the square root of squares using NumPy’s np.sqrt() function. The square root is taken of each elements automatically.

squares = np.array([1, 4, 9, 16, 25])

np.sqrt(squares)

array([1., 2., 3., 4., 5.])

Performing this operation requires NumPy’s sqrt() function. If this is attempted with the math module’s sqrt() function, an error is returned because this function cannot take the square root of a multi-element object.

import math
math.sqrt(squares)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[52], line 2
      1 import math
----> 2 math.sqrt(squares)

TypeError: only length-1 arrays can be converted to Python scalars

4.4.2 Scalars & Arrays

When performing mathematical operations between a scalar and an array, the same operation is performed across each elements of the array returning an array of the same dimension as the starting array. Below, an array is multiplied by the scaler 3 which results in every element in the array being multiplied by this value.

\[\begin{split} 3 \times \left[ \begin{array}{cc} 5 & 6 \\ 7 & 8 \end{array} \right] = \left[ \begin{array}{cc} 15 & 18 \\ 21 & 24 \end{array} \right] \end{split}\]

3 * np.array([[5, 6], [7, 8]])

array([[15, 18],
       [21, 24]])

The same outcome arises when performing a similar operation between a 1 \(\times\)1 array and a larger array.

\[ \left[ \begin{array}{c} 2 \end{array} \right] + \left[ \begin{array}{cc} 10 & 20 \end{array} \right] = \left[ \begin{array}{cc} 12 & 22 \end{array} \right] \]

np.array([2]) + np.array([10, 20])

array([12, 22])

4.4.3 Arrays of the Same Dimensions

If a mathematical operation is performed between two arrays of the same dimensions, then the mathematical operation is performed between corresponding elements in the two arrays. For example, if a pair of 2 \(\times\) 2 arrays are added to one another, then the corresponding elements are added to one another. This means that the top left elements are added together and so on.

\[\begin{split} \left[ \begin{array}{cc} 1 & 2 \\ 3 & 4 \end{array} \right] + \left[ \begin{array}{cc} 5 & 6 \\ 7 & 8 \end{array} \right] = \left[ \begin{array}{cc} 6 & 8 \\ 10 & 12 \end{array} \right] \end{split}\]

a = np.array([[1,2], [3,4]]) 
b = np.array([[5,6], [7,8]]) 
a + b

array([[ 6,  8],
       [10, 12]])

4.4.4 Arrays of Different Dimensions

Broadcasting is another form of vectorization that is a series of rules for dealing with mathematical operations between two arrays of different dimensions. In broadcasting, the one of the dimensions of the two arrays must be either identical or one-dimensional, otherwise nothing happens except an error message. To deal with the different dimensions, NumPy clones the array with fewer dimensions out so that it has the same dimensions as the other array. It should be noted that NumPy does not really clone out the array in the background; it behavior acts as if it does. It is a convenient way of thinking about the behavior and results. For example, below is the addition between a 2\(\times\)2 and a 1\(\times\)2 array.

\[\begin{split} \left[ \begin{array}{cc} 1 & 2 \\ 3 & 4 \end{array} \right] + \left[ \begin{array}{cc} 2 & 2 \end{array} \right]= ? \end{split}\]

To make the two arrays the same size, the smaller array is cloned along the smaller dimension until the two arrays are the same size as shown below. We are then left with simple corresponding element by corresponding element mathematical operations described in section 4.4.3.

\[\begin{split} \left[ \begin{array}{cc} 1 & 2 \\ 3 & 4 \end{array} \right] + \left[ \begin{array}{cc} 2 & 2 \\ 2 & 2 \end{array} \right]= \left[ \begin{array}{cc} 3 & 4 \\ 5 & 6 \end{array} \right] \end{split}\]

a = np.array([[1,2], [3,4]])
b = np.array([2,2])
a + b

array([[3, 4],
       [5, 6]])

What happens if a mathematical operation is performed between an array of higher dimensions with a scalar or a 1\(\times\)1 array as shown below? You already probably know the answer from section 4.4.2, but here is how to rationalize the behavior. In this case, no dimensions are the same, but being that one of the arrays has dimensions of one where the two arrays differ, the arrays still broadcast.

\[\begin{split} \left[ \begin{array}{cc} 1 & 2 \\ 3 & 4 \end{array} \right] \times \left[ \begin{array}{c} 2 \end{array} \right]= ? \end{split}\]

Again, the smaller array is cloned until the two arrays are the same size.

\[\begin{split} \left[ \begin{array}{cc} 1 & 2 \\ 3 & 4 \end{array} \right] + \left[ \begin{array}{cc} 2 & 2 \\ 2 & 2 \end{array} \right]= \left[ \begin{array}{cc} 2 & 4 \\ 6 & 8 \end{array} \right] \end{split}\]

a = np.array([[1,2], [3,4]])
b = np.array([2]) 
a * b

array([[2, 4],
       [6, 8]])

Finally, if we attempt to perform a mathematical operation between two arrays with different dimensions and none of the arrays have a dimension of one where the two arrays are different, an error is raised, and no operation is performed.

\[\begin{split} \left[ \begin{array}{cc} 1 & 2 \\ 3 & 4 \end{array} \right] + \left[ \begin{array}{cCc} 1 & 1 & 1 \\ 2 & 2 & 2 \\ 3 & 3 & 3 \end{array} \right]= ? \end{split}\]

a = np.array([[1,2], [3,4]])
b = np.array([[1,1,1], [2,2,2], [3,3,3]]) 
a + b

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[58], line 3
      1 a = np.array([[1,2], [3,4]])
      2 b = np.array([[1,1,1], [2,2,2], [3,3,3]]) 
----> 3 a + b

ValueError: operands could not be broadcast together with shapes (2,2) (3,3) 

4.4.5 Vectorizing Python Functions

Standard Python functions are often designed to perform a calculation a single time and output Python objects and not NumPy arrays. As an example, the following function calculates the rate of a first-order reaction given the rate constant (k) and concentration of reactant (conc).

def rate(k, conc):
    return k * conc

rate(1.2, 0.80)

0.96

What happens if we attempt the above calculation using a list of concentration values?

concs = [0.1, 0.5, 1.0, 1.5, 2.0]

rate(1.2, concs)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[62], line 1
----> 1 rate(1.2, concs)

Cell In[59], line 2, in rate(k, conc)
      1 def rate(k, conc):
----> 2     return k * conc

TypeError: can't multiply sequence by non-int of type 'float'

We get an error because Python cannot multiply a list by a value the way NumPy can. However, the above function can be converted to a NumPy function using np.vectorize() which will allows the function to perform the calculation on a series of values and returns a NumPy array.

vrate = np.vectorize(rate)

vrate(1.2, concs)

array([0.12, 0.6 , 1.2 , 1.8 , 2.4 ])

4.5 Array Methods

Technically, NumPy array methods have already been employed in this chapter. The functions above are NumPy methods specifically for working with NumPy arrays. If an array is fed to many non-NumPy functions, an error will result because they cannot handle multi-element objects or arrays specifically. Interestingly, if a float or integer is fed into a NumPy method, it will still work. As an example, the integer 4 can be fed into the np.sqrt() function as well as an array of values.

np.sqrt(4)

2.0

np.sqrt(np.array([1, 4, 9]))

array([1., 2., 3.])

NumPy contains an extensive listing of methods for working with arrays… so much so that it would be impractical to list them all here. However, below are tables of some common and useful methods. It is worth browsing and being aware of them; most are worth committing to memory. If you ever find yourself needing to manipulate an array in some complex fashion, it is worth doing a quick internet search and including “NumPy” in your search. You will likely either find additional NumPy methods that will help or advice on how others solved a similar problem.

Table 1 Common Methods for Generating Arrays

Method	Description
np.array()	Generates an array from another object
np.arange()	Creates an array from [start, stop) with a given step size
np.linspace()	Creates an array from [start, stop] with given number of steps
np.empty()	Creates an “empty” array (actually filled with garbage)
np.zeros()	Generates an array of a given dimensions filled with zeros
np.ones()	Generates an array of a given dimensions filled with ones
np.fromfunction()	Generates an array using a Python function
np.genfromtxt()	Loads text file data into an array
np.loadtxt()	Load text file data into an array (cannot handle missing data)

Table 2 Array Attribute Methods

Method	Description
np.shape(array)	Returns the dimensions of an array
np.ndim(array)	Returns the number of dimensions (e.g., a 2D array is 2)
np.size(array)	Returns the number of elements in an array

Table 3 Array Modification Methods

Method	Description
np.flatten()	Flattens an array in place
np.ravel()	Returns a flattened view of the array without changing the array
np.reshape()	Reshapes an array in place
np.resize()	Returns a resized view of an array without modifying the original
np.transpose()	Returns a view of transposed array
np.vstack()	Vertically stacks an arrays into a new array
np.hstack()	Horizontally stacks an arrays into a new array
np.dstack()	Depth-wise stacks an arrays into a new array
np.vsplit()	Splits an array vertically
np.hsplit()	Splits an array horizontally
np.dsplit()	Splits an array depth-wise
np.meshgrid()	Creates a meshgrid (see chapter 3 for an example)
np.sort()	Sorts elements in array; defaults along last axis
np.argsort()	Returns index values of sorted array
np.fill(x)	Sets all values in an array to x
np.roll()	Rolls the array along the given axis; elements that fall off one end of the array appear at the other
np.floor()	Returns the floor (i.e., rounds down) of all elements in an array
np.round(x, decimals=y)	Rounds every number in an array x to y decimal places

Table 4 Array Measurement and Statistics Methods

Method	Description
np.min()	Returns the minimum value in the array
np.max()	Returns the maximum value in the array
np.argmin()	Returns argument (i.e., index) of min
np.argmax()	Returns argument (i.e., index) of max
np.argrelmax()	Returns argument (i.e., index) of the local max
np.fmin()	Returns the element-by-element min between two arrays of the same size
np.fmax()	Returns the element-by-element max between two arrays of the same size
np.percentile()	Returns the specified percentile
np.mean()	Returns the mean (commonly known as the average)
np.median()	Returns the median
np.std()	Returns the standard deviation; be sure to include ddof=1
np.histogram()	Returns counts and bins for a histogram
np.cumprod()	Returns the cumulative product
np.cumsum()	Returns the cumulative sum
np.sum()	Returns the sum of all elements
np.prod()	Returns the product of all elements
np.ptp()	Returns the peak-to-peak separation of max and min values
np.unique()	Returns an array of unique values in an array

Note

The standard deviation equation includes a degrees of freedom. The default value for NumPy is zero, but the default value for Excel, and some other software, is one. If you want your standard devationations to match Excel, include the ddof=1 argument to the np.std() standard deviation function.

4.6 Missing Data

Real data sets frequently contain gaps or missing values, so it is important to be able to deal with missing data. When importing data into NumPy, there are two commonly employed functions, np.genfromtxt() and np.loadtxt(). Though these are largly analogous functions in terms of capabilities, there is a key difference in that np.genfromtxt() can handle missing data while np.loadtxt() cannot. This means if your data set may contain gaps, you should use np.genfromtxt().

In the event the data file contains a gap, the np.genfromtxt() function will place a nan in that loacation by default. The nan stands for “not a number” and is simply a placeholder. For example, the file dHf_ROH.csv contains the number of carbons in linear alcohols and the gas-phase heat of formation in kJ/mol of each alcohol. The value for 1-undecanol (eleven carbons) is missing, so np.genfromtxt() places a nan in its place.

np.genfromtxt('data/dHf_ROH.csv', delimiter=',')

array([[   1., -205.],
       [   2., -234.],
       [   3., -256.],
       [   4., -277.],
       [   5., -298.],
       [   6., -316.],
       [   7., -340.],
       [   8., -356.],
       [   9., -377.],
       [  10., -395.],
       [  11.,   nan],
       [  12., -437.]])

Some data files use placeholder values instead of no value at all. These placeholders are often -1, 0, 999, or some physically meaningless or improbable value. If you have an alternative values you want in the missing data location, you can specify this using the filling_values= argument. As an example below, the missing value is replaced with a 999.

np.genfromtxt('data/dHf_ROH.csv', delimiter=',', filling_values=999)

array([[   1., -205.],
       [   2., -234.],
       [   3., -256.],
       [   4., -277.],
       [   5., -298.],
       [   6., -316.],
       [   7., -340.],
       [   8., -356.],
       [   9., -377.],
       [  10., -395.],
       [  11.,  999.],
       [  12., -437.]])

In the event you have data with missing values, the nan placeholders can pose an issue when running statistics on the data. Below, we use the np.mean() method to try to calculate the mean enthalpy of formation but get a nan instead because the np.mean() function cannot handle the placeholder.

dHf = np.genfromtxt('data/dHf_ROH.csv', delimiter=',')

np.mean(dHf[:,1])

nan

Alternatively, NumPy has a number of versions of functions, listed below, that are specifically designed to handle data with missing values.

Function	Description
np.nanstd()	Standard deveation
np.nanmean()	Mean
np.nanvar()	Variance
np.nanmedian()	Median
np.nanpercentile()	Qth percentile
np.nanquantile()	Qth quantile

np.nanmean(dHf[:,1])

-317.3636363636364

np.nanquantile(dHf[:,1], 0.6)

-298.0

4.7 Random Number Generation

Stochastic simulations, addressed in chapter 9, are a common tool in the sciences and rely on a series of random numbers; it is worth addressing their generation using NumPy. Depending upon the requirements of the simulation, random numbers may be a series of floats or integers, and they may be generated from various ranges of values. The numbers may also be generated as a uniform distribution where all values are equally likely or biased a distribution where some values are more probable than others. Below are random number functions from the NumPy random module useful in generating random number distributions to suit the needs of your simulations.

Note

Software generated random numbers are really pseudorandom numbers. However, they are close enough to random for most chemical simulations and will be referred to as “random numbers” herein.

4.7.1 Uniform Distribution

The simplest distribution is the uniform distribution of random numbers where every number in the range has an equal probability of occurring. The distribution may not always appear as even with small sample sizes due to the random nature of the number generation, but as a larger population of samples is generated, the relative distribution will appear more even. The histograms below (Figure 3) are of a hundred (left) and a hundred thousand (right) randomly generated floats from the [0,1) range in an even distribution. While the plot on the right appears more even, this is mostly an effect of the different scales.

Figure 3 Histograms of a hundred (left) and a hundred thousand (right) randomly generated floats from the [0,1) range in an even distribution using the random() method.

Starting in verion 1.18, NumPy’s prefered method for producing random values is through a Generator called using teh rng = np.random.default_rng() function. Once a generator has been crated, it can be used to generate the neccesary random values. NumPy has multiple methods available for generating evenly-distributed random numbers including the following two functions where n is the number of random values to be generated. The rng.random(n) function generates n random floats from the range [0,1). The rng.integers(low, high=, size=n) function generates random integers in the range [low, high) and can generate multiple values using the size argument.

rng = np.random.default_rng()

rng.random(n)

rng.integers(low, high=x, size=n)

Warning

Prior to version 1.18 of NumPy, random numbers were generated using function calls that look like np.random.randint(). While these should still work, they are considered legacy, so it is uncertain how long they will continue to be supported.

rng = np.random.default_rng()

4.7.2 Binomial Distribution

A binomial distribution results when values are generated from two possible outcomes. This is useful for applications such as deciding if a simulated molecule reacts or whether a polymer chain terminates or propagates. The two outcomes are represented by a 0 or 1 with the probability, p, of a 1 being generated. Binomial distributions are generated by the NumPy random module using the rng.binomial() function call.

rng = np.rangom.default_rng()

rng.binomial(t, p, size=n)

The t argument is the number of trials while the size= argument is the number of generated values. For example, if t = 2, two binomial values are generated and the sum is returned, which may be 0, 1, or 2. Basic probability predicts that these sums will occur in a 1:2:1 ratio, respectively. If t is increased to 10, a shape more closely representing a bell curve is obtained. A Bernoulli distribution is the specific instance of a binomial distribution where t = 1. The histograms below (Figure 4) are of a hundred randomly generated numbers in a binomial distribution with p = 0.5 and where t = 1 (top), t = 2 (bottom left), and t = 10 (bottom right).

Figure 4 Histograms of a hundred randomly generated numbers in a binomial distribution with p = 0.5 and t = 1 (top), t = 2 (bottom left), and t = 10 (bottom right).

4.7.3 Poisson Distribution

A Poisson distribution is a probability distribution of how likely it is for independent events to occur in a given interval (time or space) with a known average frequency (\(\lambda\)). Each sample in a poisson distribution is a count of how many events have occurred in the time interval, so they are always integers. NumPy can generate integers in a Poisson distribution using the rng.poisson() function, which accepts two arguments.

rng.poisson(lam=1.0, size=n)

The first argument, \(\lambda\) (lam), is the statistical mean for the values generated, and the second argument, size, is the requested number of values. For example, a Geiger counter can be simulated detecting background radiation in a location that is known to have an average of 3.6 radiation counts per second with the following function call.

rng.poisson(lam=3.6, size=30)

array([2, 2, 4, 2, 3, 2, 4, 3, 2, 6, 3, 3, 4, 3, 4, 4, 3, 0, 5, 5, 4, 4,
       8, 2, 2, 3, 7, 4, 4, 5])

The returned array of values are the total radiation detections for each second for thirty seconds, and the mean value is 3.8 counts. While not precisely the target of 3.6 counts, it is close and larger sample sizes are statistically more likely to generate results closer to the target value. A histogram of these values is shown in below (Figure 5, left). When this simulation was repeated with thirty thousand samples (Figure 5, right), a mean of 3.61 counts is obtained. In addition, the larger number of values results in a classic Poisson distribution curve which appears something like a bell curve with more tapering on the high end.

Figure 5 Histograms of thirty (left) and thirty thousand (right) randomly generated integers in a Poisson distribution with a target mean (\(\lambda\)) of 3.6 (dashed red line).

Alternative distributions of random number can be generated by manipulating the output of the above functions. For example, random numbers in a [-1, 1) distribution, which is useful in a 2D diffusion simulation, can be generated by subtracting 0.5 from values in the range [0, 1) and multiplying by two.

rand_float = 2 * (rng.random() - 0.5)

4.7.4 Other Functions

The random module in NumPy also includes a large variety of other random number and sequence generators. This includes rng.normal() which generates values centered around zero in a normal distribution. The rng.choice() function selects a random value from a provided array of values, while the rng.shuffle() function randomizes the order of values for a given array. Other random distribution functions can be found on the SciPy website (see Further Reading). A summary of common NumPy random functions are in Table 5.

Table 5 Summary of Common NumPy np.random Functions

Function	Description
rng.random()	Generates random floats in the range [0,1) in an even distribution
rng.integers()	Generates random integers from a given range in an even distributionb
rng.normal()	Generates random floats in a normal distribution centered around zero
rng.binomial()	Generates random integers in a binomial distribution; takes a probability ,p, and size artuments
rng.poisson()	Generates random floats in a Poisson distribution; takes a target mean argument (lam)
rng.choice()	Selects random values taken from a 1-D array or range
rng.shuffle()	Randomizes the order of an array

rng.random(1)

array([0.29686811])

rng.integers(0, high=100)

14

rng.normal(loc=0.0, scale=1.0, size=3)

array([ 0.4697172 , -1.31672361, -1.28933494])

rng.binomial(2, p=0.5, size=3)

array([2, 1, 2])

rng.poisson(lam=2.0, size=5)

array([1, 1, 3, 2, 1])

rng.choice(20, size=3)

array([ 0, 14, 16])

arr = np.array([0, 1, 2, 3, 4])
rng.shuffle(arr)
arr

array([1, 3, 2, 0, 4])

Further Reading

The NumPy documentation is well written and a good resource. Because NumPy is the foundation of the SciPy ecosystem, if you find a Python book on scientific computing, odds are that it will discuss or use NumPy at some level.

1. NumPy Website. http://www.numpy.org/ (free resource)

2. NumPy User Guide. https://numpy.org/doc/stable/user/index.html#user (free resource)

Exercises

Complete the following exercises in a Jupyter notebook using NumPy and NumPy arrays. Avoid using for loops whenever possible. Any data file(s) refered to in the problems can be found in the data folder in the same directory as this chapter’s Jupyter notebook. Alternatively, you can download a zip file of the data for this chapter from here by selecting the appropriate chapter file and then clicking the Download button.

1. Generate an array containing the atomic numbers for the first 26 elements.

2. The following equation defines the relationship between energy (J) of a photo and its wavelength (m) where h is Plank’s constant (6.626 \(\times \, 10^{-34} J\cdot s\)) and c is the speed of light in a vacuum (2.998 \(\times \, 10^8 m/s\)).

   \[ E = \frac{hc}{\lambda} \]

   a. Generate an array containing the wavelengths of visible light (4.00 \(\times\) 10\(^{-7}\) m \(\rightarrow\) 8.00 \(\times\) 10\(^{-7}\) m) in 5 \(\times\) 10\(^{-8}\) m increments.

   b. Generate a second array containing the energy of each wavelength of light from part a.

3. Generate an array containing 101.325 a hundred times.

4. The following array contains temperatures in Fahrenheit. Convert these values to \(^\circ\)C without using a for loop.

   F = array([0, 32, 100, 212, 451])
   

5. Generate two arrays containing the following sine functionsfrom x = 0 \(\rightarrow 10 \pi\)

   \[ y = sin(x) \]
   \[ y = sin(1.1x + 0.5) \]

   a. Plot these two sine waves on the same plot.

   b. Add the two sine functions together and plot the result.

   c. Explain why the signal in part b is smaller in one area and larger in another. Hint: look at your plot for part a to see how the two origonal sine waves related to each other.

6. The numerical relationship between \(\Delta G^o\) and K (equilibrium constant) is shown below. Plot \(\Delta G^o\) versus K at standard temperature and pressure for K values of 0.001 \(\rightarrow\) 1000. Use NumPy arrays and do not use any for loops.

   \[ \Delta G = -RTln(k) \]

7. The numerical relationship between k (rate constant) and E\(_a\) is shown below. Plot k versus Ea at standard temperature and pressure for activation energies of 1 \(\rightarrow\) 20 kJ/mol. Use NumPy arrays, do not use any for loops, and use A = 1. Watch your energy units carefully.

   \[ k = Ae^{-E_a / RT} \]

8. Generate an array containing integers 0 \(\rightarrow\) 14 (inclusive).

   a. Reshape the array to be a 3 \(\times\) 5 array.

   b. Transpose the array, so now it should be a 5 \(\times\) 3 array.

   c. Make the array one-dimensional again without using the reshape() method.

9. Generating an Combining arrays – Bohr hydrogen atom.

   a. Create an array containing the principle quantum numbers (n) for the first eight orbits of a hydrogen atom (e.i., 1 \(\rightarrow\) 8).

   b. Generate a second array containing the energy (J) of each orbit in part A for a Bohr model of a hydrogen atom using the equation below.

   \[ E = -2.18 \times 10^{-18}J \frac{1}{n^2} \]

   c. Combine the two arrays from parts A and B into a new 8 \(\times\) 2 array with the first column containing the principle quantum numbers and the second containing the energies.

10. Generate a one-dimensional array with the following code and index the 5th element of the array.

    arr = np.random.randint(0, high=10, size=10)
    

11. Generate a two-dimensional array with the following code.

    arr2 = np.random.randint(0, high=10, size=15).reshape(5, 3)
    

    a. Index the second element of the third column.

    b. Slice the array to get the entire third row.

    c. Slice the array to access the entire first column.

    d. Slice the array to get the last two elements of the first row.

12. Predict the outcome of the following operation between two NumPy arrays. Test your your prediction.

    \[\begin{split} \left[ \begin{array}{cc} 1 & 1 \\ 2 & 2 \end{array} \right] + \left[1 \right] = \,\, ?\end{split}\]

13. Predict the outcome of the following operation between two NumPy arrays. Test your your prediction.

    \[\begin{split} \left[ \begin{array}{ccc} 1 & 8 & 9 \\ 8 & 1 & 9 \\ 1 & 8 & 1 \end{array} \right] + \left[ \begin{array}{cc} 1 & 1 \\ 1 & 1 \end{array} \right] = \,\, ? \end{split}\]

14. Predict the outcome of the following operation between two NumPy arrays. Test your your prediction.

    \[\begin{split} \left[ \begin{array}{cc} 1 & 8 \\ 3 & 2 \end{array} \right] + \left[ \begin{array}{cc} 1 & 1 \\ 1 & 1 \end{array} \right] = \,\, ?\end{split}\]

15. For the following randomly-generated array:

    arr = np.random.rand(20)
    

    a. Find the index of the largest values in the following array.

    b. Calculate the mean value of the array.

    c. Calculate the cumulative sum of the array.

    d. Sort the array.

16. Generate a random array of values from -1 \(\rightarrow\) 1 (exclusive) and calculate its median value. Hint: start with an array of values 0 \(\rightarrow\) 1 (exclusive) and manipulate it.

17. Generate a random array of integers from 0 \(\rightarrow\) 35 (inclusive) and then sort it.

18. Hydrogen nuclei can have a spin of +1/2 and -1/2 and occur in approximately a 1:1 ratio. Simulate the number of +1/2 hydrogen nuclei in a molecule of six hydrogen atoms and plot the distribution. Hint: being that there are two possible outcomes, this can be simulated using a binomial distribution. See section 4.7.2.

19. Using NumPy’s random module, generate a random DNA sequence (i.e., series of ‘A’, ‘T’, ‘C’, and ‘G’ bases) 40 bases long stored in an array.