GeekChick - 1 year ago 74

Python Question

I'm a newbie taking Udacity's Intro to Computer Science course. We had a fairly simple question on our quiz about swapping values and I don't quite understand it. Here is the question:

Which of the following sequence of statements leaves the value of variable X the same as it was before the statements. Assume that both a and x refer to the integer values before this code.

Why is this true?

`a,x = x,a`

a,x = x,a

For example if I have:

`a,x = 4,5`

then

a = 4 and x = 5

For the second part:

`a,x = 5, 4`

then a = 5 and x = 4

So x is not equal to what it was before. Can someone explain why this is true?

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Answer Source

It's simply swapping, then swapping again.

```
>>> a = 2
>>> x = 1
>>> a,x = x,a # We swapped them, so a = 1 and x = 2 now
# During evaluation, this statement will be equivalent to "a,x = 1,2"
>>> a,x = x,a # And now we swap them again, so they're back to their original values
# During evaluation, this statement will be equivalent to "a,x = 2,1"
>>> a
2
>>> x
1
```

What it seems you're missing is that the values change in the middle. You can't go through at the beginning and replace all occurrences of `x`

and `a`

with 4 and 5 in your example because `x`

and `a`

change in the middle of the operation.

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