Invert negative values in a list

Question

I am trying to convert a list that contains negative values, to a list of non-negative values; inverting the negative ones. I have tried abs but it didn\'t work

Answer 1

what you want is to use the absolute value (|x| = x if x > 0, |x| = -x if x < 0)

for index in range(len(x)):
    x[index] =  x[index] if x[index] > 0 else -x[index]

Answer 2

Your attempt didn't work because abs() takes an integer, not a list. To do this, you'll have to either loop through the list:

x = [10,9,8,7,6,5,4,3,2,1,0,-1,-2,-3,-4,-5,-6,-7,-8,-9,-10]
for i in range(len(x)):
     x[i] = abs(x[i])

Or you can use list comprehension, which is shorter:

x = [abs(i) for i in x]

Or simply use the built-in map function, which is even shorter :)

x = list(map(abs, x))

Hope this helps!

Answer 3

Try a list comprehension:

x2 = [abs(k) for k in x]

Answer 4

The simple pythonic way is the list comprehension above but if you're using Numpy for anything else you could do:

x2 = numpy.abs(x)

with no need to convert or do any looping.

Answer 5

This is a wrong answer to your question, but this is what I came here looking for. This is how to invert all the numbers in your list using operator.neg; i.e. also positives to negatives.

import operator
x = [10,9,8,7,6,5,4,3,2,1,0,-1,-2,-3,-4,-5,-6,-7,-8,-9,-10]
x = list(map(operator.neg, x))

It returns:

[-10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Or you can do a list comprehension of course:

x = [-xi for xi in x]