How do you swap two local variables within a function in python

Question

I am trying to swap local variables within a function to rotate a 2 by 2 matrix by 90 degrees.

def twobytwo(m):
    last = len(m)-1
    for i in range(0, last):
        swap(m[i][i], m[i][last])
        swap(m[i][i], m[last][last])
        swap(m[i][i], m[last][i])
    return m


def swap(i, j):
    temp = i
    i = j
    j = temp

print(twobytwo([[0, 1], [2, 3]]))

Currently, I am returned the original matrix but I want to see

[[2,0],[3,1]]

`swap` isn't changing anything in `twobytwo`, because `int`'s aren't mutable. — iz_, Jan 18 '19 at 00:15
@Tomothy32: It wouldn't change anything even if ints were mutable, or if you passed in lists instead of ints. Local variable assignment doesn't mutate objects. — user2357112, Jan 18 '19 at 00:21

keineahnung2345 · Accepted Answer · 2019-01-18T00:35:21.510

1

I think this is what you want to achieve:

def twobytwo(m):
    last = len(m)-1
    for i in range(0, last):
        m[i][i], m[i][last] = m[i][last], m[i][i]
        m[i][i], m[last][last] = m[last][last], m[i][i]
        m[i][i], m[last][i] = m[last][i], m[i][i]
    return m

print(twobytwo([[0, 1], [2, 3]]))

EDIT: If you still want to maintain the function swap:

def swap(i, j):
    return j, i

a, b = swap(a, b)

But I think a, b = b, a is good enough.

edited Jan 18 '19 at 00:35

answered Jan 18 '19 at 00:24

keineahnung2345

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Thanks so much! Do you by chance know how I would do this if I wanted to use the swap function as well? – stillearning Jan 18 '19 at 00:30
1

@user9476376 Added additional answer above. – keineahnung2345 Jan 18 '19 at 00:36

Greg Glockner · Answer 2 · 2019-01-18T01:05:10.427

0

Your assumption is that the swap() parameters are modified in place (i.e. pass-by-reference). Python does not do that in your code: when you call something like swap(m[i][i], m[i][last]), it dereferences the values of m and calls swap(0,1). So swap() only modifies i,j; it does not modify your "matrix" (which technically is a list of lists).

Try rewriting your swap() function to take the "matrix" plus two pairs of indices: (i,j) and (k,l).

edited Jan 18 '19 at 01:05

answered Jan 18 '19 at 00:35

Greg Glockner

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Python does also not use "pass by value". See https://stackoverflow.com/questions/986006/how-do-i-pass-a-variable-by-reference/986145#986145 – mkrieger1 Jan 18 '19 at 00:43
@mkrieger1 : That would be true if you pass an object like m, but the sample code dereferences to int values, which are not objects. Nevertheless, I updated my answer to clear this up. – Greg Glockner Jan 18 '19 at 01:06

How do you swap two local variables within a function in python

2 Answers2