如何将数组的前N个元素设置为零? [英] How to set first N elements of array to zero?

问题描述

我的数组很长:

x= ([2, 5, 4, 7, ...])

我需要将

的第一个N元素设置为0.因此，对于N = 2，所需的输出将是:

for which I need to set the first N elements to 0. So for N = 2, the desired output would be:

x = ([0, 0, 4, 7, ...])

在Python中有一种简单的方法吗?一些numpy功能?

Is there an easy way to do this in Python? Some numpy function?

推荐答案

纯python:

x[:n] = [0] * n

和numpy:

y = numpy.array(x)
y[:n] = 0

还请注意，如果x是python列表(而不是numpy数组)，则x[:n] = 0不会不起作用.

also note that x[:n] = 0 does not work if x is a python list (instead of a numpy array).

对任何可变的东西使用[{some object here}] * n也是一个坏主意，因为列表将不包含n个不同的对象，而是n个对同一对象的引用:

It is also a bad idea to use [{some object here}] * n for anything mutable, because the list will not contain n different objects but n references to the same object:

>>> a = [[],[],[],[]]
>>> a[0:2] = [["a"]] * 2
>>> a
[['a'], ['a'], [], []]
>>> a[0].append("b")
>>> a
[['a', 'b'], ['a', 'b'], [], []]

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