How to negative a row of a list in python

Question

I want to make a list negative among the lists of list

I tried this

X=[[1,2,3],[4,5,6]]
X[0]=-X[0]

But it produces error I want the output as

X=[[-1,-2,-3],[4,5,6]]

X[0]=[x*-1 for x in X[0]] -- [edited per luk's comment] – Thomas Kimber Jan 04 '23 at 14:27 — Thomas Kimber, Jan 04 '23 at 14:27
@ThomasKimber you could also do `[-x for x in X[0]]` – dm2 Jan 04 '23 at 14:29 — dm2, Jan 04 '23 at 14:29

score 0 · Answer 1 · answered Jan 04 '23 at 14:30

You have to iterate through the elements and multiply them with -1.

You cant use -X[0] because that is not proper python syntax for a list object
You can't use X[0]*-1 because that is a list and * operator doesn't do elementwise multiplication for a list object.

X=[[1,2,3],[4,5,6]]

X[0] = [-1*i for i in X[0]]
X

[[-1, -2, -3], [4, 5, 6]]

score 0 · Answer 2 · answered Jan 04 '23 at 14:31

0

Use list comprehension:

X[0] = [-n for n in X[0]]

Or numpy if you want to multiply arrays by a number:

import numpy as np
X = np.array([[1,2,3],[4,5,6]])
X[0] = -X[0]

answered Jan 04 '23 at 14:31

Jonathan Dauwe

2 Answers2