13 KiB
100 numpy exercises
This is a collection of exercises that have been collected in the numpy mailing list, on stack overflow and in the numpy documentation. The goal of this collection is to offer a quick reference for both old and new users but also to provide a set of exercises for those who teach.
If you find an error or think you've a better way to solve some of them, feel free to open an issue at https://github.com/rougier/numpy-100. File automatically generated. See the documentation to update questions/answers/hints programmatically.
1. Import the numpy package under the name np (★☆☆)
hint: import … as
2. Print the numpy version and the configuration (★☆☆)
hint: np.__version__, np.show_config)
3. Create a null vector of size 10 (★☆☆)
hint: np.zeros
4. How to find the memory size of any array (★☆☆)
hint: size, itemsize
5. How to get the documentation of the numpy add function from the command line? (★☆☆)
hint: np.info
6. Create a null vector of size 10 but the fifth value which is 1 (★☆☆)
hint: array[4]
7. Create a vector with values ranging from 10 to 49 (★☆☆)
hint: arange
8. Reverse a vector (first element becomes last) (★☆☆)
hint: array[::-1]
9. Create a 3x3 matrix with values ranging from 0 to 8 (★☆☆)
hint: reshape
10. Find indices of non-zero elements from [1,2,0,0,4,0] (★☆☆)
hint: np.nonzero
11. Create a 3x3 identity matrix (★☆☆)
hint: np.eye
12. Create a 3x3x3 array with random values (★☆☆)
hint: np.random.random
13. Create a 10x10 array with random values and find the minimum and maximum values (★☆☆)
hint: min, max
14. Create a random vector of size 30 and find the mean value (★☆☆)
hint: mean
15. Create a 2d array with 1 on the border and 0 inside (★☆☆)
hint: array[1:-1, 1:-1]
16. How to add a border (filled with 0's) around an existing array? (★☆☆)
hint: np.pad
17. What is the result of the following expression? (★☆☆)
0 * np.nan
np.nan == np.nan
np.inf > np.nan
np.nan - np.nan
np.nan in set([np.nan])
0.3 == 3 * 0.1
hint: NaN = not a number, inf = infinity
18. Create a 5x5 matrix with values 1,2,3,4 just below the diagonal (★☆☆)
hint: np.diag
19. Create a 8x8 matrix and fill it with a checkerboard pattern (★☆☆)
hint: array[::2]
20. Consider a (6,7,8) shape array, what is the index (x,y,z) of the 100th element? (★☆☆)
hint: np.unravel_index
21. Create a checkerboard 8x8 matrix using the tile function (★☆☆)
hint: np.tile
22. Normalize a 5x5 random matrix (★☆☆)
hint: (x -mean)/std
23. Create a custom dtype that describes a color as four unsigned bytes (RGBA) (★☆☆)
hint: np.dtype
24. Multiply a 5x3 matrix by a 3x2 matrix (real matrix product) (★☆☆)
hint:
25. Given a 1D array, negate all elements which are between 3 and 8, in place. (★☆☆)
hint: >, <
26. What is the output of the following script? (★☆☆)
# Author: Jake VanderPlas
print(sum(range(5),-1))
from numpy import *
print(sum(range(5),-1))
hint: np.sum
27. Consider an integer vector Z, which of these expressions are legal? (★☆☆)
Z**Z
2 << Z >> 2
Z <- Z
1j*Z
Z/1/1
Z<Z>Z
No hints provided...
28. What are the result of the following expressions? (★☆☆)
np.array(0) / np.array(0)
np.array(0) // np.array(0)
np.array([np.nan]).astype(int).astype(float)
No hints provided...
29. How to round away from zero a float array ? (★☆☆)
hint: np.uniform, np.copysign, np.ceil, np.abs, np.where
30. How to find common values between two arrays? (★☆☆)
hint: np.intersect1d
31. How to ignore all numpy warnings (not recommended)? (★☆☆)
hint: np.seterr, np.errstate
32. Is the following expressions true? (★☆☆)
np.sqrt(-1) == np.emath.sqrt(-1)
hint: imaginary number
33. How to get the dates of yesterday, today and tomorrow? (★☆☆)
hint: np.datetime64, np.timedelta64
34. How to get all the dates corresponding to the month of July 2016? (★★☆)
hint: np.arange(dtype=datetime64['D'])
35. How to compute ((A+B)*(-A/2)) in place (without copy)? (★★☆)
hint: np.add(out=), np.negative(out=), np.multiply(out=), np.divide(out=)
36. Extract the integer part of a random array of positive numbers using 4 different methods (★★☆)
hint: %, np.floor, astype, np.trunc
37. Create a 5x5 matrix with row values ranging from 0 to 4 (★★☆)
hint: np.arange
38. Consider a generator function that generates 10 integers and use it to build an array (★☆☆)
hint: np.fromiter
39. Create a vector of size 10 with values ranging from 0 to 1, both excluded (★★☆)
hint: np.linspace
40. Create a random vector of size 10 and sort it (★★☆)
hint: sort
41. How to sum a small array faster than np.sum? (★★☆)
hint: np.add.reduce
42. Consider two random array A and B, check if they are equal (★★☆)
hint: np.allclose, np.array_equal
43. Make an array immutable (read-only) (★★☆)
hint: flags.writeable
44. Consider a random 10x2 matrix representing cartesian coordinates, convert them to polar coordinates (★★☆)
hint: np.sqrt, np.arctan2
45. Create random vector of size 10 and replace the maximum value by 0 (★★☆)
hint: argmax
46. Create a structured array with x and y coordinates covering the [0,1]x[0,1] area (★★☆)
hint: np.meshgrid
47. Given two arrays, X and Y, construct the Cauchy matrix C (Cij =1/(xi - yj)) (★★☆)
hint: np.subtract.outer
48. Print the minimum and maximum representable value for each numpy scalar type (★★☆)
hint: np.iinfo, np.finfo, eps
49. How to print all the values of an array? (★★☆)
hint: np.set_printoptions
50. How to find the closest value (to a given scalar) in a vector? (★★☆)
hint: argmin
51. Create a structured array representing a position (x,y) and a color (r,g,b) (★★☆)
hint: dtype
52. Consider a random vector with shape (100,2) representing coordinates, find point by point distances (★★☆)
hint: np.atleast_2d, T, np.sqrt
53. How to convert a float (32 bits) array into an integer (32 bits) in place?
hint: view and [:] =
54. How to read the following file? (★★☆)
1, 2, 3, 4, 5
6, , , 7, 8
, , 9,10,11
hint: np.genfromtxt
55. What is the equivalent of enumerate for numpy arrays? (★★☆)
hint: np.ndenumerate, np.ndindex
56. Generate a generic 2D Gaussian-like array (★★☆)
hint: np.meshgrid, np.exp
57. How to randomly place p elements in a 2D array? (★★☆)
hint: np.put, np.random.choice
58. Subtract the mean of each row of a matrix (★★☆)
hint: mean(axis=,keepdims=)
59. How to sort an array by the nth column? (★★☆)
hint: argsort
60. How to tell if a given 2D array has null columns? (★★☆)
hint: any, ~
61. Find the nearest value from a given value in an array (★★☆)
hint: np.abs, argmin, flat
62. Considering two arrays with shape (1,3) and (3,1), how to compute their sum using an iterator? (★★☆)
hint: np.nditer
63. Create an array class that has a name attribute (★★☆)
hint: class method
64. Consider a given vector, how to add 1 to each element indexed by a second vector (be careful with repeated indices)? (★★★)
hint: np.bincount | np.add.at
65. How to accumulate elements of a vector (X) to an array (F) based on an index list (I)? (★★★)
hint: np.bincount
66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★☆)
hint: np.unique
67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)
hint: sum(axis=(-2,-1))
68. Considering a one-dimensional vector D, how to compute means of subsets of D using a vector S of same size describing subset indices? (★★★)
hint: np.bincount
69. How to get the diagonal of a dot product? (★★★)
hint: np.diag
70. Consider the vector [1, 2, 3, 4, 5], how to build a new vector with 3 consecutive zeros interleaved between each value? (★★★)
hint: array[::4]
71. Consider an array of dimension (5,5,3), how to mulitply it by an array with dimensions (5,5)? (★★★)
hint: array[:, :, None]
72. How to swap two rows of an array? (★★★)
hint: array[[]] = array[[]]
73. Consider a set of 10 triplets describing 10 triangles (with shared vertices), find the set of unique line segments composing all the triangles (★★★)
hint: repeat, np.roll, np.sort, view, np.unique
74. Given a sorted array C that corresponds to a bincount, how to produce an array A such that np.bincount(A) == C? (★★★)
hint: np.repeat
75. How to compute averages using a sliding window over an array? (★★★)
hint: np.cumsum, from numpy.lib.stride_tricks import sliding_window_view (np>=1.20.0)
76. Consider a one-dimensional array Z, build a two-dimensional array whose first row is (Z[0],Z[1],Z[2]) and each subsequent row is shifted by 1 (last row should be (Z[-3],Z[-2],Z[-1]) (★★★)
hint: from numpy.lib import stride_tricks, from numpy.lib.stride_tricks import sliding_window_view (np>=1.20.0)
77. How to negate a boolean, or to change the sign of a float inplace? (★★★)
hint: np.logical_not, np.negative
78. Consider 2 sets of points P0,P1 describing lines (2d) and a point p, how to compute distance from p to each line i (P0[i],P1[i])? (★★★)
No hints provided...
79. Consider 2 sets of points P0,P1 describing lines (2d) and a set of points P, how to compute distance from each point j (P[j]) to each line i (P0[i],P1[i])? (★★★)
No hints provided...
80. Consider an arbitrary array, write a function that extract a subpart with a fixed shape and centered on a given element (pad with a fill value when necessary) (★★★)
hint: minimum maximum
81. Consider an array Z = [1,2,3,4,5,6,7,8,9,10,11,12,13,14], how to generate an array R = 1,2,3,4], [2,3,4,5], [3,4,5,6], ..., [11,12,13,14? (★★★)
hint: stride_tricks.as_strided, from numpy.lib.stride_tricks import sliding_window_view (np>=1.20.0)
82. Compute a matrix rank (★★★)
hint: np.linalg.svd, np.linalg.matrix_rank
83. How to find the most frequent value in an array?
hint: np.bincount, argmax
84. Extract all the contiguous 3x3 blocks from a random 10x10 matrix (★★★)
hint: stride_tricks.as_strided, from numpy.lib.stride_tricks import sliding_window_view (np>=1.20.0)
85. Create a 2D array subclass such that Z[i,j] == Z[j,i] (★★★)
hint: class method
86. Consider a set of p matrices with shape (n,n) and a set of p vectors with shape (n,1). How to compute the sum of of the p matrix products at once? (result has shape (n,1)) (★★★)
hint: np.tensordot
87. Consider a 16x16 array, how to get the block-sum (block size is 4x4)? (★★★)
hint: np.add.reduceat, from numpy.lib.stride_tricks import sliding_window_view (np>=1.20.0)
88. How to implement the Game of Life using numpy arrays? (★★★)
No hints provided...
89. How to get the n largest values of an array (★★★)
hint: np.argsort | np.argpartition
90. Given an arbitrary number of vectors, build the cartesian product (every combinations of every item) (★★★)
hint: np.indices
91. How to create a record array from a regular array? (★★★)
hint: np.core.records.fromarrays
92. Consider a large vector Z, compute Z to the power of 3 using 3 different methods (★★★)
hint: np.power, *, np.einsum
93. Consider two arrays A and B of shape (8,3) and (2,2). How to find rows of A that contain elements of each row of B regardless of the order of the elements in B? (★★★)
hint: np.where
94. Considering a 10x3 matrix, extract rows with unequal values (e.g. [2,2,3]) (★★★)
No hints provided...
95. Convert a vector of ints into a matrix binary representation (★★★)
hint: np.unpackbits
96. Given a two dimensional array, how to extract unique rows? (★★★)
hint: np.ascontiguousarray | np.unique
97. Considering 2 vectors A & B, write the einsum equivalent of inner, outer, sum, and mul function (★★★)
hint: np.einsum
98. Considering a path described by two vectors (X,Y), how to sample it using equidistant samples (★★★)?
hint: np.cumsum, np.interp
99. Given an integer n and a 2D array X, select from X the rows which can be interpreted as draws from a multinomial distribution with n degrees, i.e., the rows which only contain integers and which sum to n. (★★★)
hint: np.logical_and.reduce, np.mod
100. Compute bootstrapped 95% confidence intervals for the mean of a 1D array X (i.e., resample the elements of an array with replacement N times, compute the mean of each sample, and then compute percentiles over the means). (★★★)
hint: np.percentile