numpy 100题练习 <一>

100 numpy exercises

numpy 100题练习

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.
这是在stackoverflow和numpy文档里汇总的numpy练习题，目的是为新老用户提供快速参考。

github地址

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

导入numpy包，命名为np

import numpy as np

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

打印numpy版本和配置

print(np.__version__)
np.show_config()

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

创建一个10*10的0数组

Z = np.zeros(10)
print(Z)

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

如何查看数组占内存大小

Z = np.zeros((10,10))
print("%d bytes" % (Z.size * Z.itemsize))

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

如何在命令行下查看numpy add函数文档

%run `python -c "import numpy; numpy.info(numpy.add)"`

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

创建一个长度为10的0数组，第5个值为1

Z = np.zeros(10)
Z[4] = 1
print(Z)

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

创建一个值从10到49的数组

Z = np.arange(10,50)
print(Z)

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

反转数组（第一个元素变成最后一个）

Z = np.arange(50)
Z = Z[::-1]
print(Z)

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

创建一个从0~8的3*3矩阵

Z = np.arange(9).reshape(3,3)
print(Z)

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

从[1,2,0,0,4,0]中找到非0元素的索引

nz = np.nonzero([1,2,0,0,4,0])
print(nz)

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

生成一个3*3的对角矩阵

Z = np.eye(3)
print(Z)

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

创建一个333的随机值数组

Z = np.random.random((3,3,3))
print(Z)

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

创建一个10*10的随机值数组，并找到最大最小值

Z = np.random.random((10,10))
Zmin, Zmax = Z.min(), Z.max()
print(Zmin, Zmax)

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

创建一个长度为30的随机值数组，并找到平均值

Z = np.random.random(30)
m = Z.mean()
print(m)

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

创建一个四边为1，中间为0的二维数组，

Z = np.ones((10,10))
Z[1:-1,1:-1] = 0
print(Z)

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

如何给一个已经存在的数组添加边（填充0）

Z = np.ones((5,5))
Z = np.pad(Z, pad_width=1, mode='constant', constant_values=0)
print(Z)

17. What is the result of the following expression? (★☆☆)

看看下面表达式的结果是什么？

print(0 * np.nan)
print(np.nan == np.nan)
print(np.inf > np.nan)
print(np.nan - np.nan)
print(np.nan in set([np.nan]))
print(0.3 == 3 * 0.1)

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

创建一个5*5矩阵，对角线下方值为1，2，3，4

Z = np.diag(1+np.arange(4),k=-1)
print(Z)

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

创建一个8*8矩阵，并用棋盘图案填充

Z = np.zeros((8,8),dtype=int)
Z[1::2,::2] = 1
Z[::2,1::2] = 1
print(Z)

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

给定一个678的三维矩阵，求100个元素的索引是什么？

print(np.unravel_index(99,(6,7,8)))

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

使用tile函数创建8*8的棋盘矩阵

Z = np.tile( np.array([[0,1],[1,0]]), (4,4))
print(Z)

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

对一个5*5矩阵标准化处理

Z = np.random.random((5,5))
Z = (Z - np.mean (Z)) / (np.std (Z))
print(Z)

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

新建一个dtype类型用来描述一个颜色（RGBA）

color = np.dtype([("r", np.ubyte, 1),
                  ("g", np.ubyte, 1),
                  ("b", np.ubyte, 1),
                  ("a", np.ubyte, 1)])

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

5*3矩阵和3*2矩阵相乘

Z = np.dot(np.ones((5,3)), np.ones((3,2)))
print(Z)

# Alternative solution, in Python 3.5 and above
Z = np.ones((5,3)) @ np.ones((3,2))
print(Z)

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

给定一个一维数组，将第3~8个元素取反

# Author: Evgeni Burovski

Z = np.arange(11)
Z[(3 < Z) & (Z <= 8)] *= -1
print(Z)

26. What is the output of the following script? (★☆☆)

看看下面脚本的输出是什么？

# Author: Jake VanderPlas

print(sum(range(5),-1))
from numpy import *
print(sum(range(5),-1))

27. Consider an integer vector Z, which of these expressions are legal? (★☆☆)

给定一个整数数组Z，看看下面哪个表达式是合法的？

Z**Z
2 << Z >> 2
Z <- Z
1j*Z
Z/1/1
Z<Z>Z

28. What are the result of the following expressions?

下面表达式的结果是什么？

print(np.array(0) / np.array(0))
print(np.array(0) // np.array(0))
print(np.array([np.nan]).astype(int).astype(float))

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

如何对数组进行四舍五入操作？

# Author: Charles R Harris

Z = np.random.uniform(-10,+10,10)
print (np.copysign(np.ceil(np.abs(Z)), Z))

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

如何找出两个数组的共同值？

Z1 = np.random.randint(0,10,10)
Z2 = np.random.randint(0,10,10)
print(np.intersect1d(Z1,Z2))

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

如何忽略所有numpy警告？

# Suicide mode on
defaults = np.seterr(all="ignore")
Z = np.ones(1) / 0

# Back to sanity
_ = np.seterr(**defaults)

An equivalent way, with a context manager:

with np.errstate(divide='ignore'):
    Z = np.ones(1) / 0

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

下面表达式正确吗？

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

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

如何获得昨天、今天、明天的日期？

yesterday = np.datetime64('today', 'D') - np.timedelta64(1, 'D')
today     = np.datetime64('today', 'D')
tomorrow  = np.datetime64('today', 'D') + np.timedelta64(1, 'D')

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

如何获得2016年7月对应的所有日期？

Z = np.arange('2016-07', '2016-08', dtype='datetime64[D]')
print(Z)

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

如何计算((A+B)*(-A/2)) ？

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 (★★☆)

提取随机数列整数部分的五种方法

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 (★★☆)

创建一个5*5的矩阵，每一行值为1~4

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 (★☆☆)

给定一个生成器函数，可以生成10个整数，使用它来创建一个数组

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 (★★☆)

创建一个长度为10的数组，值为0~1之间，不包含首尾

Z = np.linspace(0,1,11,endpoint=False)[1:]
print(Z)

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

创建一个长度为10的数组，并做排序操作

Z = np.random.random(10)
Z.sort()
print(Z)

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

如何对一个数组进行相加操作，并且速度快于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 (★★☆)

给定两个随机数组A和B，验证它们是否相等

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) (★★☆)

使一个数组不变（只读）

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 (★★☆)

给定表示笛卡尔坐标的一个10*2的随机矩阵，将其转换为极坐标

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 (★★☆)

创建一个长度为10的随机矩阵，并将最大值替换为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 (★★☆)

创建具有x和y坐标的结构化数组，它们覆盖[0,1] x [0,1]区域

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))

给定两个数组X和Y，构造柯西矩阵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 (★★☆)

打印每种numpy标量类型的最小和最大可表示值

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? (★★☆)

如何打印数组中所有值

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? (★★☆)

如何在数组中找到最接近给定值的值

Z = np.arange(100)
v = np.random.uniform(0,100)
index = (np.abs(Z-v)).argmin()
print(Z[index])

备注：目前只翻译前50题，后面50题下期呈现

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