NumPy

Questions

  • Why using NumPy instead of pure python?

  • How to use basic NumPy?

Objectives

  • Be able to use basic NumPy functionality

  • Understand enough of NumPy to seach for answers to the rest of your questions ;)

So, we already know about python lists, and that we can put all kinds of things in there. But in scientific usage, lists are often not enough. They are slow and not very flexible.

What is an array?

For example, consider [1, 2.5, 'asdf', False, [1.5, True]] - this is a Python list but it has different types for every element. When you do math on this, every element has to be handled separately.

NumPy is the most used library for scientific computing. Even if you are not using it directly, chances are high that some library uses it in the background. NumPy provides the high-performance multidimensional array object and tools to use it.

An array is a ‘grid’ of values, with all the same types. It is indexed by tuples of non negative indices and provides the framework for multiple dimensions. An array has:

  • dtype - data type. Arrays always contain one type

  • shape - shape of the data, for example 3×2 or 3×2×500 or even 500 (one dimensional) or [] (zero dimensional).

  • data - raw data storage in memory. This can be passed to C or Fortran code for efficient calculations.

To test the performance of pure Python vs NumPy we can write in our jupyter notebook:

Create one list and one ‘empty’ list, to store the result in

a = list(range(10000))
b = [ 0 ] * 10000

In a new cell starting with %%timeit, loop through the list a and fill the second list b with a squared

%%timeit
for i in range(len(a)):
  b[i] = a[i]**2

That looks and feels quite fast. But let’s take a look at how NumPy performs for the same task.

So for the NumPy example, create one array and one ‘empty’ array to store the result in

import numpy as np
a = np.arange(10000)
b = np.zeros(10000)

In a new cell starting with %%timeit, fill b with a squared

%%timeit
b = a ** 2

We see that compared to working with numpy arrays, working with traditional python lists is actually slow.

Creating arrays

There are different ways of creating arrays:

a = np.array([1,2,3])               # 1-dimensional array (rank 1)
b = np.array([[1,2,3],[4,5,6]])     # 2-dimensional array (rank 2)

b.shape                             # the shape (rows,columns)
b.size                              # number of elements

In addition to above ways of creating arrays, there are many other ways of creating arrays depending on content:

np.zeros((2, 3))           # 2x3 array with all elements 0
np.ones((1,2))             # 1x2 array with all elements 1
np.full((2,2),7)           # 2x2 array with all elements 7
np.eye(2)                  # 2x2 identity matrix

np.arange(10)              # Evenly spaced values in an interval
np.linspace(0,9,10)        # same as above, see exercise

c = np.ones((3,3))
d = np.ones((3, 2), bool)  # 3x2 boolean array

Arrays can also be stored and read from a (.npy) file:

np.save('x.npy', a)           # save the array a to a .npy file
x = np.load('x.npy')          # load an array from a .npy file and store it in variable x

In many occasions (especially when something goes different than expected) it is useful to check and control the datatype of the array:

d.dtype                    # datatype of the array
d.astype('int')            # change datatype from boolean to integer

In the last example, .astype('int'), it will make a copy of the array, and re-allocate data - unless the dtype is exactly the same as before. Understanding and minimizing copies is one of the most important things to do for speed.

Exercises: Numpy-1

  • Datatypes Try out np.arange(10) and np.linspace(0,9,10), what is the difference? Can you adjust one to do the same as the other?

  • Datatypes Create a 3x2 array of random float numbers (check np.random) between 0 and 1. Now change the arrays datatype to int (array.astype). How does the array look like?

  • Reshape Create a 3x2 array of random integer numbers between 0 and 10. Reshape the array in any way possible. What is not possible?

  • NumPyI/O Save above array to .npy file (np.save) and read it in again.

Array maths

Clearly, you can do math on arrays. Math in NumPy, is very fast because it is implemented in C or Fortran - just like most other high-level languages such as R, Matlab, etc do.

By default, in NumPy all math is element-by-element. This is unlike Matlab, where most things are element-by-element, but * becomes array multiplication. NumPy values consistency and does not treat 2-dimensional arrays specially:

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

c = a + b
d = np.add(a,b)

Also: - (np.subtract()), * (np.multiply()), / (np.divide()), np.sqrt(), np.sum(), np.mean(), …

Exercises: Numpy-2

  • Matrix multiplication What is the difference between np.multiply and np.dot ? Try it.

  • Axis What is the difference between np.sum(axis=1) vs np.sum(axis=0) on a two-dimensional array? What if you leave out the axis parameter?

Indexing and Slicing

NumPy has many ways to extract values out of arrays:

  • You can select a single element

  • You can select rows or columns

  • You can select ranges where a condition is true.

Clever and efficient use of these operations is a key to NumPy’s speed: you should try to cleverly use these selectors (written in C) to extract data to be used with other NumPy functions written in C or Fortran. This will give you the benefits of Python with most of the speed of C.

a = np.arange(16).reshape(4, 4)  # 4x4 matrix from 0 to 15
a[0]                             # first row
a[:,0]                           # first column
a[1:3,1:3]                       # middle 2x2 array

a[(0, 1), (1, 1)]                # second element of first and second row as array

Boolean indexing on above created array:

idx = (a > 0)      # creates boolean matrix of same size as a
a[idx]             # array with matching values of above criterion

a[a > 0]           # same as above in one line

Exercise: Numpy-3

a = np.eye(4)
b = a[:,0]
b[0] = 5
  • View vs copy Try out above code. How does a look like before b has changed and after? How could it be avoided?

Types of operations

There are different types of standard operations in NumPy:

ufuncs, “universal functions”: These are element-by-element functions with standardized arguments:

  • One, two, or three input arguments

  • For example, a + b is similar to np.add(a, b) but the ufunc has more control.

  • out= output argument, store output in this array (rather than make a new array) - saves copying data!

  • See the full reference

  • They also do broadcasting. Can you add a 1-dimensional array of shape (3) to an 2-dimensional array of shape (3, 2)? With broadcasting you can!

    a = np.array([[1, 2, 3],
                  [4, 5, 6]])
    b = np.array([10, 10, 10])
    a + b                       # array([[11, 12, 13],
                                #        [14, 15, 16]])
    

    Broadcasting is smart and consistent about what it does, which I’m not clever enough to explain quickly here: the manual page on broadcasting. The basic idea is that it expands dimensions of the smaller array so that they are compatible in shape.

Array methods do something to one array:

  • Some of these are the same as ufuncs:

    x = np.arange(12)
    x.shape = (3, 4)
    x                    #  array([[ 0,  1,  2,  3],
                         #         [ 4,  5,  6,  7],
                         #         [ 8,  9, 10, 11]])
    x.max()              #  11
    x.max(axis=0)        #  array([ 8,  9, 10, 11])
    x.max(axis=1)        #  array([ 3,  7, 11])
    

Other functions: there are countless other functions covering linear algebra, scientific functions, etc.

Exercises: Numpy-4

  • In-place addition: Create an array, add it to itself using a ufunc.

  • In-place addition (advanced): Create an array of dtype='float', and an array of dtype='int'. Try to use the int array is the output argument of the first two arrays.

Linear algebra and other advanced math

In general, you use arrays (n-dimensions), not matrixes (specialized 2-dimensional) in NumPy.

Internally, NumPy doesn’t invent its own math routines: it relies on BLAS and LAPACK to do this kind of math - the same as many other languages.

  • Linear algebra in numpy

  • Many, many other array functions

  • Scipy has even more functions

  • Many other libraries use NumPy arrays as the standard data structure: they take data in this format, and return it similarly. Thus, all the other packages you may want to use are compatible

  • If you need to write your own fast code in C, NumPy arrays can be used to pass data. This is known as extending Python.

Additional Exercises Numpy-6

These are advanced and optional, and will not be done in most courses.

  1. Reverse a vector. Given a vector, reverse it such that the last element becomes the first, e.g. [1, 2, 3] => [3, 2, 1]

  2. Create a 2D array with zeros on the borders and 1 inside.

  3. Create a random array with elements [0, 1), then add 10 to all elements in the range [0.2, 0.7).

  4. What is np.round(0.5)? What is np.round(1.5)? Why?

  5. In addition to np.round, explore np.ceil, np.floor, np.trunc. In particular, take note of how they behave with negative numbers.

  6. Recall the identity \(\sin^2(x) + \cos^2(x) = 1\). Create a random 4x4 array with values in the range [0, 10). Now test the equality with np.equal. What result do you get with np.allclose instead of np.equal?

  7. Create a 1D array with 10 random elements. Sort it.

  8. What’s the difference between np_array.sort() and np.sort(np_array)?

  9. For the random array in question 8, instead of sorting it, perform an indirect sort. That is, return the list of indices which would index the array in sorted order.

  10. Create a 4x4 array of zeros, and another 4x4 array of ones. Next combine them into a single 8x4 array with the content of the zeros array on top and the ones on the bottom. Finally, do the same, but create a 4x8 array with the zeros on the left and the ones on the rigth.

  11. NumPy functionality Create two 2D arrays and do matrix multiplication first manually (for loop), then using the np.dot function. Use %%timeit to compare execution times. What is happening?

See also

Keypoints

  • NumPy is a powerful library every scientist using python should know about, since many other libraries also use it internally.

  • Be aware of some NumPy specific peculiarities