How to raise all values to the power of 2, but keep their original sign-CodePudding

How do I raise all values for the vector v = [4,7,-2,9,3,-6,-4,1]to the power of 2, but keep their original sign ?

CodePudding user response：

>>> from math import copysign
>>> v = [4,7,-2,9,3,-6,-4,1]
>>> [int(copysign(x*x,x)) for x in v]
[16, 49, -4, 81, 9, -36, -16, 1]

The copysign function basically does what you want: Return the first argument, but with the sign of the second. Except that copysign always returns a float, so if you want ints, you cast them back. (Beware of overflow!)

CodePudding user response：

I like @Sören's method better, but if you prefer not to have to import math for some reason, this will work.

>>> v = [4,7,-2,9,3,-6,-4,1]
>>> result = [x*abs(x) for x in v]
>>> print(result)
[16, 49, -4, 81, 9, -36, -16, 1]

We do x * abs(x) and to keep the sign. If the original number is positive, both x and abs(x) will be positive, yielding a positive answer. If the original is negative, x will retain the original sign, meaning we get a negative.

I use a list comprehension, and then wrap it in int(...) to make it an int again (from a float).