# 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.

Run the initialize.py module, then for each question you can query the answer or an hint with hint(n) or answer(n) for n question number.

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%run initialise.py


#### 1. Import the numpy package under the name np (★☆☆)¶

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#### 2. Print the numpy version and the configuration (★☆☆)¶

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#### 3. Create a null vector of size 10 (★☆☆)¶

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#### 4. How to find the memory size of any array (★☆☆)¶

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#### 5. How to get the documentation of the numpy add function from the command line? (★☆☆)¶

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#### 6. Create a null vector of size 10 but the fifth value which is 1 (★☆☆)¶

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#### 7. Create a vector with values ranging from 10 to 49 (★☆☆)¶

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#### 8. Reverse a vector (first element becomes last) (★☆☆)¶

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#### 9. Create a 3x3 matrix with values ranging from 0 to 8 (★☆☆)¶

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#### 10. Find indices of non-zero elements from [1,2,0,0,4,0] (★☆☆)¶

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#### 11. Create a 3x3 identity matrix (★☆☆)¶

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#### 12. Create a 3x3x3 array with random values (★☆☆)¶

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#### 13. Create a 10x10 array with random values and find the minimum and maximum values (★☆☆)¶

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#### 14. Create a random vector of size 30 and find the mean value (★☆☆)¶

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#### 15. Create a 2d array with 1 on the border and 0 inside (★☆☆)¶

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#### 16. How to add a border (filled with 0's) around an existing array? (★☆☆)¶

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#### 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

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#### 18. Create a 5x5 matrix with values 1,2,3,4 just below the diagonal (★☆☆)¶

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#### 19. Create a 8x8 matrix and fill it with a checkerboard pattern (★☆☆)¶

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#### 20. Consider a (6,7,8) shape array, what is the index (x,y,z) of the 100th element? (★☆☆)¶

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#### 21. Create a checkerboard 8x8 matrix using the tile function (★☆☆)¶

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#### 22. Normalize a 5x5 random matrix (★☆☆)¶

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#### 23. Create a custom dtype that describes a color as four unsigned bytes (RGBA) (★☆☆)¶

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#### 24. Multiply a 5x3 matrix by a 3x2 matrix (real matrix product) (★☆☆)¶

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#### 25. Given a 1D array, negate all elements which are between 3 and 8, in place. (★☆☆)¶

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#### 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))

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#### 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

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#### 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)

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#### 29. How to round away from zero a float array ? (★☆☆)¶

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#### 30. How to find common values between two arrays? (★☆☆)¶

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#### 31. How to ignore all numpy warnings (not recommended)? (★☆☆)¶

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#### 32. Is the following expressions true? (★☆☆)¶

np.sqrt(-1) == np.emath.sqrt(-1)

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#### 33. How to get the dates of yesterday, today and tomorrow? (★☆☆)¶

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#### 34. How to get all the dates corresponding to the month of July 2016? (★★☆)¶

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#### 35. How to compute ((A+B)*(-A/2)) in place (without copy)? (★★☆)¶

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#### 36. Extract the integer part of a random array of positive numbers using 4 different methods (★★☆)¶

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#### 37. Create a 5x5 matrix with row values ranging from 0 to 4 (★★☆)¶

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#### 38. Consider a generator function that generates 10 integers and use it to build an array (★☆☆)¶

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#### 39. Create a vector of size 10 with values ranging from 0 to 1, both excluded (★★☆)¶

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#### 40. Create a random vector of size 10 and sort it (★★☆)¶

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#### 41. How to sum a small array faster than np.sum? (★★☆)¶

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#### 42. Consider two random array A and B, check if they are equal (★★☆)¶

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#### 43. Make an array immutable (read-only) (★★☆)¶

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#### 44. Consider a random 10x2 matrix representing cartesian coordinates, convert them to polar coordinates (★★☆)¶

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#### 45. Create random vector of size 10 and replace the maximum value by 0 (★★☆)¶

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#### 46. Create a structured array with x and y coordinates covering the [0,1]x[0,1] area (★★☆)¶

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#### 47. Given two arrays, X and Y, construct the Cauchy matrix C (Cij =1/(xi - yj)) (★★☆)¶

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#### 48. Print the minimum and maximum representable value for each numpy scalar type (★★☆)¶

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#### 49. How to print all the values of an array? (★★☆)¶

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#### 50. How to find the closest value (to a given scalar) in a vector? (★★☆)¶

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#### 51. Create a structured array representing a position (x,y) and a color (r,g,b) (★★☆)¶

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#### 52. Consider a random vector with shape (100,2) representing coordinates, find point by point distances (★★☆)¶

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#### 53. How to convert a float (32 bits) array into an integer (32 bits) in place?¶

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#### 54. How to read the following file? (★★☆)¶

1, 2, 3, 4, 5
6,  ,  , 7, 8
,  , 9,10,11
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#### 55. What is the equivalent of enumerate for numpy arrays? (★★☆)¶

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#### 56. Generate a generic 2D Gaussian-like array (★★☆)¶

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#### 57. How to randomly place p elements in a 2D array? (★★☆)¶

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#### 58. Subtract the mean of each row of a matrix (★★☆)¶

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#### 59. How to sort an array by the nth column? (★★☆)¶

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#### 60. How to tell if a given 2D array has null columns? (★★☆)¶

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#### 61. Find the nearest value from a given value in an array (★★☆)¶

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#### 62. Considering two arrays with shape (1,3) and (3,1), how to compute their sum using an iterator? (★★☆)¶

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#### 63. Create an array class that has a name attribute (★★☆)¶

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#### 64. Consider a given vector, how to add 1 to each element indexed by a second vector (be careful with repeated indices)? (★★★)¶

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#### 65. How to accumulate elements of a vector (X) to an array (F) based on an index list (I)? (★★★)¶

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#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★☆)¶

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#### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)¶

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#### 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? (★★★)¶

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#### 69. How to get the diagonal of a dot product? (★★★)¶

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#### 70. Consider the vector [1, 2, 3, 4, 5], how to build a new vector with 3 consecutive zeros interleaved between each value? (★★★)¶

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#### 71. Consider an array of dimension (5,5,3), how to mulitply it by an array with dimensions (5,5)? (★★★)¶

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#### 72. How to swap two rows of an array? (★★★)¶

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#### 73. Consider a set of 10 triplets describing 10 triangles (with shared vertices), find the set of unique line segments composing all the triangles (★★★)¶

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#### 74. Given a sorted array C that corresponds to a bincount, how to produce an array A such that np.bincount(A) == C? (★★★)¶

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#### 75. How to compute averages using a sliding window over an array? (★★★)¶

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#### 76. Consider a one-dimensional array Z, build a two-dimensional array whose first row is (Z,Z,Z) and each subsequent row is shifted by 1 (last row should be (Z[-3],Z[-2],Z[-1]) (★★★)¶

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#### 77. How to negate a boolean, or to change the sign of a float inplace? (★★★)¶

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#### 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])? (★★★)¶

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#### 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])? (★★★)¶

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#### 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) (★★★)¶

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#### 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]]? (★★★)¶

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#### 82. Compute a matrix rank (★★★)¶

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#### 83. How to find the most frequent value in an array?¶

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#### 84. Extract all the contiguous 3x3 blocks from a random 10x10 matrix (★★★)¶

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#### 85. Create a 2D array subclass such that Z[i,j] == Z[j,i] (★★★)¶

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#### 86. Consider a set of p matrices wich 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)) (★★★)¶

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#### 87. Consider a 16x16 array, how to get the block-sum (block size is 4x4)? (★★★)¶

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#### 88. How to implement the Game of Life using numpy arrays? (★★★)¶

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#### 89. How to get the n largest values of an array (★★★)¶

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#### 90. Given an arbitrary number of vectors, build the cartesian product (every combinations of every item) (★★★)¶

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#### 91. How to create a record array from a regular array? (★★★)¶

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#### 92. Consider a large vector Z, compute Z to the power of 3 using 3 different methods (★★★)¶

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#### 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? (★★★)¶

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#### 94. Considering a 10x3 matrix, extract rows with unequal values (e.g. [2,2,3]) (★★★)¶

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#### 95. Convert a vector of ints into a matrix binary representation (★★★)¶

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#### 96. Given a two dimensional array, how to extract unique rows? (★★★)¶

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#### 97. Considering 2 vectors A & B, write the einsum equivalent of inner, outer, sum, and mul function (★★★)¶

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#### 98. Considering a path described by two vectors (X,Y), how to sample it using equidistant samples (★★★)?¶

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#### 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. (★★★)¶

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#### 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). (★★★)¶

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