Question

Tuples

I can't understand what the author means in this passage of "How to think like a computer scientist":

"Functions can return tuples as return values. For example, we could write a function that swaps two parameters:

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

Then we can assign the return value to a tuple with two variables:

a, b = swap(a, b)

In this case, there is no great advantage in making swap a function. In fact, there is a danger in trying to encapsulate swap, which is the following tempting mistake:

def swap(x, y): # incorrect version
x, y = y, x

Answer 1

If we call this function like this:

swap(a, b)

then a and x are aliases for the same value. Changing x inside swap makes x refer to a different value, but it has no effect on a in __main__. Similarly, changing y has no effect on b.

This function runs without producing an error message, but it doesn't do what we intended. This is an example of a semantic error."

Answer 2

When I run the following code:

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

a=1
b=2
a,b = swap(a,b)

a becomes 2 and b becomes 1

Can anyone explain what the author is trying to say plz?

Answer 3

the second - incorrect version - does not have a return statement which although it may look like it swaps x and y, it only affects variables local to the function and does not actually swap x and y in __main__.

Answer 4

Oh ok. I assumed that the return was still there. Without the return is quite obvious. Thank you.