Level 3, lesson 12

# 3.12 The Return Stack and the DO..LOOP

We've already told you that the limit and the counter of a DO..LOOP (or DO..+LOOP) are stored on the Return Stack. But how does this affect saving values in the middle of a loop? Well, this example will make that quite clear:

         1                    ( n)
         10 0 do              ( n)
               >r             ( --)
               i .            ( --)
               r>             ( n)
         loop                 ( n)
         cr                   ( n)
         drop                 ( --)

You might expect that it will show you the value of the counter ten times. In fact, it doesn't. Let's take a look at the Return Stack:

         1                    ( --)
         10 0 do              ( l c)
               >r             ( l c n)
               i .            ( l c n)
               r>             ( l c)
         loop                 ( --)
         cr                   ( --)
         drop                 ( --)

You might have noticed (unless you're blind) that it prints ten times the number "1". Where does it come from? Usually 'I' prints the value of the counter, which is on top of the Return Stack.

This time it isn't: the number "1" is there. So 'I' thinks that "1" is actually the counter and displays it. Since that value is removed from the Return Stack when 'LOOP' is encountered, it doesn't do much harm.

We see that we can safely store temporary values on the Return Stack inside a DO..LOOP, but we have to clean up the mess, before we encounter 'LOOP'. So, this rule applies here too:

"Clean up your mess inside a DO..LOOP"

But we still have to be prepared that the word 'I' will not provide the expected result (which is the current value of the counter). In fact, 'I' does simply copy the topmost value on the Return Stack. Which is usually correct, unless you've manipulated the Return Stack yourself.

Note that there are other words beside 'I', which do exactly the same thing: copy the top of the Return Stack. But they are intended to be used outside a DO..LOOP. We'll see an example of that in the following section.