How can I swap items in a Python list?

Question

The problem was to write the python code to generate all the permutations of the numbers {1,2,3,...,n}. So, I write this code:

def permute(n):

  if n==len(a):
    print a
    return

  for i in range(n,len(a)):
    swap(i,n)
    permute(n+1)
    swap(i,n)


def swap(x,y):
  a[x],a[y]=a[y],a[x]

a=[1,2,3,4]       #any list 
permute(0)

And it worked perfectly. But then thanks to the free time that I did have, I modified it a bit and wrote this code:

def permute(n):

  if n==len(a):
    print a
    return

  for i in range(n,len(a)):
    swap(a[i],a[n])           #modification
    permute(n+1)
    swap(a[i],a[n])           #modification


def swap(x,y):
  x,y=y,x                     #modification

a=[1,2,3,4]
permute(0)

This time it didn't work. But after then, I read a bit about how the assigning of variables to values is different in python.

But still I would like to know what, according to you, is wrong with the second code, so that I can cross-check and discuss what I think is going wrong! That's my first question.

My second question is how the swapping of values takes place in a python list? Is it something different from what will happen with simple values? Because both the codes above seem to apply so. But I cannot figure out it a way I can make myself understand, plus it further confused me that then how python manipulates its lists.

I am sure there is something in the design of Python language that I don't know about leading to all these confusions. Help me sort them out and if possible, use somewhat picture visualizations. It would be very easy then for me to understand what's going on!!

Surely you can think of a title for this post which actually summarizes the problem you're facing. — Matt Ball, Jan 06 '15 at 17:34
Please see e.g. http://stackoverflow.com/q/986006/3001761 and http://nedbatchelder.com/text/names.html — jonrsharpe, Jan 06 '15 at 17:37
But, with the simple modification, just replace the call to the `swap` function entirely: `swap(a[i],a[n])` -> `a[n], a[i] = a[i], a[n]`. — , Jan 06 '15 at 17:37

score 12 · Answer 1 · edited Jan 06 '15 at 18:14

12

First question:

This is the reason:

def swap(x,y):
    x, y = y, x

This just swaps the local names. x will then equal y, and vice versa, but only inside that function. Outside that function (global scope), nothing will change.

Second question (you should really avoid asking multiple questions in one question):

Assuming:

x = a[i]
y = a[n]

To swap values in a list, you must set them:

a[n] = x
a[i] = y

Which is that same as:

a[i], a[n] = y, x

And since y = a[n] and x = a[i], it is the same as:

a[i], a[n] = a[n], a[i]

edited Jan 06 '15 at 18:14

thumbtackthief

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answered Jan 06 '15 at 17:36

matsjoyce

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For the second answer, shouldn't you use some other variable other than y at one of the two places? – thisisashwani Jan 06 '15 at 17:48

score 1 · Answer 2 · answered Jan 06 '15 at 17:36

1

The swap function is swapping the local variables x and y and not the global array.

def swap(x,y):
  x,y=y,x

This is completely wrong

You can instead do the following

Change the function body to

return y,x and call the function as a[n],a[i] = swap(a[i],a[n])
Or directly swap in-place a[n],a[i] = a[i],a[n]
Or do it as you had done earlier

answered Jan 06 '15 at 17:36

Bhargav Rao

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1

In which case, of course, you can remove the `swap()` call entirely, just keep the arguments. – Jan 06 '15 at 17:38
@Evert Hope you did see the present Edit. Thanks – Bhargav Rao Jan 06 '15 at 17:46

score 0 · Answer 3 · answered Nov 24 '18 at 20:00

0

If the list is composed only by numbers:

A[i] = A[i] + A[n]
A[n] = A[i] - A[n]
A[i] = A[i] - A[n]

answered Nov 24 '18 at 20:00

How can I swap items in a Python list?

3 Answers3

First question:

Second question (you should really avoid asking multiple questions in one question):