NumPy
34. How to get all the dates corresponding to the month of July 2016? (★★☆)
Z = np.arange('2016-07', '2016-08', dtype='datetime64[D]')
print(Z)
35. How to compute ((A+B)*(-A/2)) in place (without copy)? (★★☆)
A = np.ones(3)*1
B = np.ones(3)*2
C = np.ones(3)*3
np.add(A,B,out=B)
np.divide(A,2,out=A)
np.negative(A,out=A)
np.multiply(A,B,out=A)
36. Extract the integer part of a random array using 5 different methods (★★☆)
Z = np.random.uniform(0,10,10)
print (Z - Z%1)
print (np.floor(Z))
print (np.ceil(Z)-1)
print (Z.astype(int))
print (np.trunc(Z))
37. Create a 5x5 matrix with row values ranging from 0 to 4 (★★☆)
Z = np.zeros((5,5))
Z += np.arange(5)
print(Z)
38. Consider a generator function that generates 10 integers and use it to build an array (★☆☆)
def generate():
for x in range(10):
yield x
Z = np.fromiter(generate(),dtype=float,count=-1)
print(Z)
39. Create a vector of size 10 with values ranging from 0 to 1, both excluded (★★☆)
Z = np.linspace(0,1,11,endpoint=False)[1:]
print(Z)
40. Create a random vector of size 10 and sort it (★★☆)
Z = np.random.random(10)
Z.sort()
print(Z)
41. How to sum a small array faster than np.sum? (★★☆)
# Author: Evgeni Burovski
Z = np.arange(10)
np.add.reduce(Z)
42. Consider two random array A and B, check if they are equal (★★☆)
A = np.random.randint(0,2,5)
B = np.random.randint(0,2,5)
# Assuming identical shape of the arrays and a tolerance for the comparison of values
equal = np.allclose(A,B)
print(equal)
# Checking both the shape and the element values, no tolerance (values have to be exactly equal)
equal = np.array_equal(A,B)
print(equal)
43. Make an array immutable (read-only) (★★☆)
Z = np.zeros(10)
Z.flags.writeable = False
Z[0] = 1
44. Consider a random 10x2 matrix representing cartesian coordinates, convert them to polar coordinates (★★☆)
Z = np.random.random((10,2))
X,Y = Z[:,0], Z[:,1]
R = np.sqrt(X**2+Y**2)
T = np.arctan2(Y,X)
print(R)
print(T)
45. Create random vector of size 10 and replace the maximum value by 0 (★★☆)
Z = np.random.random(10)
Z[Z.argmax()] = 0
print(Z)
46. Create a structured array with x and y coordinates covering the [0,1]x[0,1] area (★★☆)
Z = np.zeros((5,5), [('x',float),('y',float)])
Z['x'], Z['y'] = np.meshgrid(np.linspace(0,1,5),
np.linspace(0,1,5))
print(Z)
47. Given two arrays, X and Y, construct the Cauchy matrix C (Cij =1/(xi - yj))
# Author: Evgeni Burovski
X = np.arange(8)
Y = X + 0.5
C = 1.0 / np.subtract.outer(X, Y)
print(np.linalg.det(C))
48. Print the minimum and maximum representable value for each numpy scalar type (★★☆)
for dtype in [np.int8, np.int32, np.int64]:
print(np.iinfo(dtype).min)
print(np.iinfo(dtype).max)
for dtype in [np.float32, np.float64]:
print(np.finfo(dtype).min)
print(np.finfo(dtype).max)
print(np.finfo(dtype).eps)
49. How to print all the values of an array? (★★☆)
np.set_printoptions(threshold=np.nan)
Z = np.zeros((16,16))
print(Z)
50. How to find the closest value (to a given scalar) in a vector? (★★☆)
Z = np.arange(100)
v = np.random.uniform(0,100)
index = (np.abs(Z-v)).argmin()
print(Z[index])
51. Create a structured array representing a position (x,y) and a color (r,g,b) (★★☆)
Z = np.zeros(10, [ ('position', [ ('x', float, 1),
('y', float, 1)]),
('color', [ ('r', float, 1),
('g', float, 1),
('b', float, 1)])])
print(Z)
52. Consider a random vector with shape (100,2) representing coordinates, find point by point distances (★★☆)
Z = np.random.random((10,2))
X,Y = np.atleast_2d(Z[:,0], Z[:,1])
D = np.sqrt( (X-X.T)**2 + (Y-Y.T)**2)
print(D)
# Much faster with scipy
import scipy
# Thanks Gavin Heverly-Coulson (#issue 1)
import scipy.spatial
Z = np.random.random((10,2))
D = scipy.spatial.distance.cdist(Z,Z)
print(D)
53. How to convert a float (32 bits) array into an integer (32 bits) in place?
Z = np.arange(10, dtype=np.float32)
Z = Z.astype(np.int32, copy=False)
print(Z)
54. How to read the following file? (★★☆)
from io import StringIO
# Fake file
s = StringIO("""1, 2, 3, 4, 5\n
6, , , 7, 8\n
, , 9,10,11\n""")
Z = np.genfromtxt(s, delimiter=",", dtype=np.int)
print(Z)
55. What is the equivalent of enumerate for numpy arrays? (★★☆)
Z = np.arange(9).reshape(3,3)
for index, value in np.ndenumerate(Z):
print(index, value)
for index in np.ndindex(Z.shape):
print(index, Z[index])
56. Generate a generic 2D Gaussian-like array (★★☆)
X, Y = np.meshgrid(np.linspace(-1,1,10), np.linspace(-1,1,10))
D = np.sqrt(X*X+Y*Y)
sigma, mu = 1.0, 0.0
G = np.exp(-( (D-mu)**2 / ( 2.0 * sigma**2 ) ) )
print(G)
57. How to randomly place p elements in a 2D array? (★★☆)
# Author: Divakar
n = 10
p = 3
Z = np.zeros((n,n))
np.put(Z, np.random.choice(range(n*n), p, replace=False),1)
print(Z)
58. Subtract the mean of each row of a matrix (★★☆)
# Author: Warren Weckesser
X = np.random.rand(5, 10)
# Recent versions of numpy
Y = X - X.mean(axis=1, keepdims=True)
# Older versions of numpy
Y = X - X.mean(axis=1).reshape(-1, 1)
print(Y)
59. How to I sort an array by the nth column? (★★☆)
# Author: Steve Tjoa
Z = np.random.randint(0,10,(3,3))
print(Z)
print(Z[Z[:,1].argsort()])
60. How to tell if a given 2D array has null columns? (★★☆)
# Author: Warren Weckesser
Z = np.random.randint(0,3,(3,10))
print((~Z.any(axis=0)).any())
61. Find the nearest value from a given value in an array (★★☆)
Z = np.random.uniform(0,1,10)
z = 0.5
m = Z.flat[np.abs(Z - z).argmin()]
print(m)
62. Considering two arrays with shape (1,3) and (3,1), how to compute their sum using an iterator? (★★☆)
A = np.arange(3).reshape(3,1)
B = np.arange(3).reshape(1,3)
it = np.nditer([A,B,None])
for x,y,z in it: z[...] = x + y
print(it.operands[2])
63. Create an array class that has a name attribute (★★☆)
class NamedArray(np.ndarray):
def __new__(cls, array, name="no name"):
obj = np.asarray(array).view(cls)
obj.name = name
return obj
def __array_finalize__(self, obj):
if obj is None: return
self.info = getattr(obj, 'name', "no name")
Z = NamedArray(np.arange(10), "range_10")
print (Z.name)
