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索引
1、索引
- 索引的时候要用中括号,即 x[0]
# Indexing
x = np.array([1, 2, 3])
print ("x[0]: ", x[0])
x[0] = 0
print ("x: ", x)
输出结果:
x[0]: 1
x: [0 2 3]
2、Slicing
# Slicing
x = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]])
print (x)
print ("x column 1: ", x[:, 1])
print ("x row 0: ", x[0, :])
print ("x rows 0,1,2 & cols 1,2: \n", x[:3, 1:3])
输出结果:
[[ 1 2 3 4]
[ 5 6 7 8]
[ 9 10 11 12]]
x column 1: [ 2 6 10]
x row 0: [1 2 3 4]
x rows 0,1,2 & cols 1,2:
[[ 2 3]
[ 6 7]
[10 11]]
3、整数数组索引
# Integer array indexing
x = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]])
print (x)
rows_to_get = np.arange(len(x))
print ("rows_to_get: ", rows_to_get)
cols_to_get = np.array([0, 2, 1])
print ("cols_to_get: ", cols_to_get)
print ("indexed values: ", x[rows_to_get, cols_to_get])
输出结果:
[[ 1 2 3 4]
[ 5 6 7 8]
[ 9 10 11 12]]
rows_to_get: [0 1 2]
cols_to_get: [0 2 1]
indexed values: [ 1 7 10]
解释:(0,0)位置是1;(1,2)位置是7;(2,1)位置是10
4、布尔数组索引
# Boolean array indexing
x = np.array([[1,2], [3, 4], [5, 6]])
print ("x:\n", x)
print ("x > 2:\n", x > 2)
print ("x[x > 2]:\n", x[x > 2])
输出结果:
x:
[[1 2]
[3 4]
[5 6]]
x > 2:
[[False False]
[ True True]
[ True True]]
x[x > 2]:
[3 4 5 6]