Python-Swap Function-New to Programming

Question

I'm new to programming and I'm learning Python. I have to make a function that will swap 2 indices within a list. The list is given but the indices to be swapped need to be input by the user. So far I've been able to come up with this...

def listSwap():
    print("This program will swap two indices within a list")
    myList = [3, 4, 2, 5, 1, 14, 23, 1]
    print("Here is your list... ", myList)
    index1 = eval(input("Pick an index from the list you would like to swap... "))
    index2 = eval(input("Now pick another index to swap with the first... "))

    tempIndex = index1
    myList[index1] = myList[index2]
    myList[index2] = myList[tempIndex]
    print("Here is your new list with swapped indices", myList)


def main():
    listSwap()

main()

but it doesn't work like I need it to. It will swap 1 index but not the other. Could I get some help? Maybe explain what I'm doing wrong?

score 7 · Answer 1 · answered Sep 11 '15 at 07:50

7

The problem is that your code essentially equals to:

myList[index1] = myList[index2]
myList[index2] = myList[index1]

and the usage of the third 'temporary' variable for the index does not help. A correct version with a temp variable would look like:

temp = myList[index1]
myList[index1] = myList[index2]
myList[index2] = temp

But, luckily, Python has more elegant way of swapping values:

myList[index1], myList[index2] = myList[index2], myList[index1]

answered Sep 11 '15 at 07:50

bereal

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Thank you so much! I can't believe it was such an easy fix.. But I understand now what I was doing wrong. Thanks again! – J.Riley Sep 11 '15 at 16:09

score 1 · Answer 2 · edited May 23 '17 at 10:26

First, do not use eval, it is not recommended.

Instead, use:

index1 = int(input("Pick an index from the list you would like to swap... ")) index2 = int(input("Now pick another index to swap with the first... "))

Secondly there is no need to temporarily save the index value, just the list item with that index:

index_value_1 = MyList[index1]

And then perform the switch.

myList[index1] = myList[index2]
myList[index2] = index_value_1

score 0 · Answer 3 · answered Sep 11 '15 at 07:57

def your_swap(the_list, A, B):
    assert A > 0 and A < len(the_list)
    assert B > 0 and B < len(the_list)
    if A == B: return the_list

    object_at_position_A = the_list[A]
    the_list[A]          = the_list[B]
    the_list[B]          = object_at_position_A
    return the_list

score 0 · Answer 4 · edited May 23 '17 at 11:58

You're not swapping "indexes" here, you're swapping the values stored in the array, de-referenced using the index. Given a list:

a = [1, 2, 5, 3]
print(a[0])

prints "1". When you do:

a[0] = a[1]

you are replacing the value at a[0] with the value at a[1], losing the value at a[0]. You will now have a = [2, 2, 5, 3].

What you're trying to achieve is to swap numbers at position 0 and 1. To do this, you need to make a copy of the value at a[0] so you don't lose it when you write over it with a[1].

Do this:

a = [1, 2, 5, 3]
index1 = 0
index2 = 1
temp = a[index1] # stores "1" in temp
a[index1] = a[index2]  # changes a to [2, 2, 5, 3] by replacing a[0] with value in a[1]
a[index2] = temp # stores "1" inside temp into a[1], changing a to [2, 1, 5, 3]

This is a common problem, related to reference vs value.

Python-Swap Function-New to Programming

4 Answers4