Python - swapping lists?

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lindsey

We have an assignment in my beginning Python class and I am completely lost on it. I have asked some students and most of them were lost and the teacher gave me a very vague answer that did not help me at all.

THE ASSIGNMENT

"Write a function that is passed in a list and 2 index numbers (your function can assume these index numbers are between 0 and len() - 1, inclusive). Your function should swap the values at those index locations.

For example:

stuff = [10, 20, 30, 40]

swap(stuff, 0, 2)

print stuff

Would display:

[30, 20, 10, 40]"

I have no idea what i'm doing wrong but I'm getting

[10,10,30,40]

MY CODE:

def swap(list, indexA, indexB):

result =[]

sub=len(list)-1

for item in list:

if item == list[indexA]:

item=list[indexB]

if item == list[indexB]:

item=list[indexA]

result.append(item)

return result

def main():

test=[10,20,30,40]

print swap(test,0,1)

main()

----

I'm not trying to cheat and get my homework done by someone else, so can someone just explain what I am doing wrong or give me a much needed hint to point me in the right direction?

Thank you.

Dave

The problem is:

Both of your "if item == list[index...]:" lines run with each trip through the for loop.

On the first trip through the loop you get this:

for item in list:     ## item = 10

  if item == list[indexA]:     ## item == 10, list[indexA] == list[0] == 10, True

    item=list[indexB]     ## item = list[indexB] = 20

  if item == list[indexB]:     ## item == 20, list[indexB] == list[1] == 20, True

    item=list[indexA]     ## item = list[indexA] = list[0] = 10

  result.append(item)     ## item == 10, result == [10]

The second trip looks like this...

for item in list:     ## item = 20

  if item == list[indexA]:     ## item == 20, list[indexA] == list[0] == 10, False

    item=list[indexB]     ## does not execute

  if item == list[indexB]:     ## item == 20, list[indexB] == list[1] == 20, True

    item=list[indexA]     ## item = list[indexA] = list[0] = 10

  result.append(item)     ## item == 10, result == [10, 10]

So you need to come up with a way to keep that 2nd "if item==..." line from running in the first loop.

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