Question

in this program I’m trying to shuffle a list by randomly choosing two items from a list and swapping them round, and then repeating this process several times.

The problem I have encountered is I don’t know how I can swap the items round and print the shuffled list.

For instance if my two random values were a and b, if I were to just put:

a = b
b = a

then that would change the value of a to b, but when it tries to change b to a, no change would occur as a has already been changed to b.

The only way I can think that this would work is swapping them at the same time, but I do not know of a function/ way to swap them round.

Also if a, b were items of a list L, after I swapped them round if I used

print L

should it print the altered version? I only ask because from what I have tried it is not doing that.

NB I am trying to shuffle this list stage by stage by swapping, instead of using the shuffle function imported from random.

Answer 1

Use a temp variable for your first problem:

temp = a
a = b
b = temp

In Python you can also do this:

a, b = b, a

I suspect your second problem is because you’re changing things you got out of the list, instead of changing the list. Try this:

i, j = # two indexes to swap in the list
L[i], L[j] = L[j], L[i]