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141 lines
1.2 KiB
Markdown
141 lines
1.2 KiB
Markdown
# 对角线
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这里，使用与之前不同的导入方法：
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In [1]:
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```py
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import numpy as np
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```
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使用numpy中的函数前，需要加上 `np.`：
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In [2]:
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```py
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a = np.array([11,21,31,12,22,32,13,23,33])
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a.shape = 3,3
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a
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```
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Out[2]:
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```py
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array([[11, 21, 31],
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[12, 22, 32],
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[13, 23, 33]])
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```
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查看它的对角线元素：
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In [3]:
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```py
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a.diagonal()
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```
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Out[3]:
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```py
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array([11, 22, 33])
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```
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可以使用偏移来查看它的次对角线，正数表示右移，负数表示左移：
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In [4]:
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```py
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a.diagonal(offset=1)
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```
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Out[4]:
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```py
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array([21, 32])
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```
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In [5]:
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```py
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a.diagonal(offset=-1)
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```
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Out[5]:
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```py
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array([12, 23])
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```
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可以使用花式索引来得到对角线：
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In [6]:
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```py
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i = [0,1,2]
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a[i, i]
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```
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Out[6]:
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```py
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array([11, 22, 33])
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```
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可以更新对角线的值：
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In [7]:
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```py
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a[i, i] = 2
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a
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```
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Out[7]:
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```py
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array([[ 2, 21, 31],
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[12, 2, 32],
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[13, 23, 2]])
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```
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修改次对角线的值：
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In [8]:
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```py
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i = np.array([0,1])
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a[i, i + 1] = 1
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a
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```
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Out[8]:
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```py
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array([[ 2, 1, 31],
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[12, 2, 1],
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[13, 23, 2]])
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```
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In [9]:
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```py
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a[i + 1, i] = -1
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a
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```
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Out[9]:
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```py
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array([[ 2, 1, 31],
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[-1, 2, 1],
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[13, -1, 2]])
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``` |