Help with pure functions

[Xentrik]

I am a beginner working my way through How To Think Like A Computer Scientist. I am having a problem with an exercise that multiplies a matrix by a scalar. def scalar_mult(n, m), n being the scalar and m being the matrix.
My code works but I guess its not a pure function because if I put in

>>> m = [[1, 2], [3, 4]]
>>> scalar_mult(3, m)

my return is [[3, 6], [9, 12]] which is correct, but when i check m after I run my function I get [[3, 6], [9, 12]] instead of [[1, 2], [3, 4]]. I don't understand this because I cloned m and used mx throughout the function. This is the first time I have posted on a forum so please excuse me if my format is incorrect.

def scalar_mult(n, m):
    mx = m[:]
  (my code is here)
 

[Glenton]

Hi

Welcome to the forum. Please go ahead and post the rest of your code.

Thanks

Glenton

[Xentrik]

@Glenton
Glenton, here is my code.

def scalar_mult(n, m):
    mx = m[:]
    x = 0
    while x < len(mx):
        for index, value in enumerate(mx[x]):
            mx[x][index] = mx[x][index] * n
        x += 1
    return mx 
 

[bvdet]

The problem is that you have a list of lists. You made a copy of the original list, but you did not make copies of the internal lists. Do this:

def scalar_mult(n, m):
    mx = [item[:] for item in m]
    x = 0
    while x < len(mx):
        for index, value in enumerate(mx[x]):
            mx[x][index] = mx[x][index] * n
        x += 1
    return mx 

Following is an example that makes a new list. It uses recursion to get at nested lists also.

def scalar_mult(seq, n):
    results = []
    for elem in seq:
        if isinstance(elem, (list, tuple)):
            results.append(scalar_mult(elem, n))
        else:
            results.append(elem * n)
    return results
 
seq = [[1, 2], [3, 4]]
seq1 = scalar_mult(seq, 3)
print seq
print seq1
 
seq2 = ((1, 2), (3, (4, 5, 6), ((6, 7, 8), (9, 20))))
seq3 = scalar_mult(seq2, 3)
print seq2
print seq3
 
''' The output:
>>> [[1, 2], [3, 4]]
[[3, 6], [9, 12]]
((1, 2), (3, (4, 5, 6), ((6, 7, 8), (9, 20))))
[[3, 6], [9, [12, 15, 18], [[18, 21, 24], [27, 60]]]]
'''