创建一个具有元素作为索引函数的numpy矩阵 [英] Create a numpy matrix with elements as a function of indices

问题描述

如何创建一个numpy矩阵，其元素是其索引的函数? 例如，一个乘法表:a[i,j] = i*j

How can I create a numpy matrix with its elements being a function of its indices? For example, a multiplication table: a[i,j] = i*j

Un-numpy和un-pythonic将创建一个零数组，然后循环遍历.

An Un-numpy and un-pythonic would be to create an array of zeros and then loop through.

毫无疑问，有更好的方法可以做到这一点，而无需循环.

There is no doubt that there is a better way to do this, without a loop.

但是，更好的方法是直接创建矩阵.

However, even better would be to create the matrix straight-away.

推荐答案

一个通用的解决方案是使用

A generic solution would be to use np.fromfunction()

来自文档:

numpy.fromfunction(function, shape, **kwargs)

通过在每个坐标上执行一个函数来构造数组.这 因此，结果数组在坐标(x，y， z).

Construct an array by executing a function over each coordinate. The resulting array therefore has a value fn(x, y, z) at coordinate (x, y, z).

下面的行应提供所需的矩阵.

The below line should provide the required matrix.

numpy.fromfunction(lambda i, j: i*j, (5,5))

输出:

array([[  0.,   0.,   0.,   0.,   0.],
       [  0.,   1.,   2.,   3.,   4.],
       [  0.,   2.,   4.,   6.,   8.],
       [  0.,   3.,   6.,   9.,  12.],
       [  0.,   4.,   8.,  12.,  16.]])

该函数的第一个参数是可调用的，可为每个坐标执行.如果foo是您作为第一个参数传递的函数，则foo(i,j)将是(i,j)的值.这也适用于更高的尺寸.可以使用shape参数修改坐标数组的形状.

The first parameter to the function is a callable which is executed for each of the coordinates. If foo is a function that you pass as the first argument, foo(i,j) will be the value at (i,j). This holds for higher dimensions too. The shape of the coordinate array can be modified using the shape parameter.

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