How to turn all numbers in a list into their negative counterparts?

Question

I am trying to turn a list of positive numbers into a list of negative numbers with the same value in python 3.3.3

For example turning [1,2,3] into

Answer 1

If you want to modify a list in place:

mylist = [ 1, 2, 3, -7]
print(mylist)
for i in range(len(mylist)):
    mylist[i] = -mylist[i]
print(mylist)