5.8.3 Counted Loops

The basic counted loop is:

     limit start
     ?DO
       body
     LOOP

This performs one iteration for every integer, starting from start and up to, but excluding limit. The counter, or index, can be accessed with i. For example, the loop:

     10 0 ?DO
       i .
     LOOP

prints 0 1 2 3 4 5 6 7 8 9

The index of the innermost loop can be accessed with i, the index of the next loop with j, and the index of the third loop with k.

i       R:n – R:n n        core       “i”

j       R:w R:w1 R:w2 – w R:w R:w1 R:w2        core       “j”

k       R:w R:w1 R:w2 R:w3 R:w4 – w R:w R:w1 R:w2 R:w3 R:w4        gforth       “k”

The loop control data are kept on the return stack, so there are some restrictions on mixing return stack accesses and counted loop words. In particuler, if you put values on the return stack outside the loop, you cannot read them inside the loop¹. If you put values on the return stack within a loop, you have to remove them before the end of the loop and before accessing the index of the loop.

There are several variations on the counted loop:

• LEAVE leaves the innermost counted loop immediately; execution continues after the associated LOOP or NEXT. For example:

            10 0 ?DO  i DUP . 3 = IF LEAVE THEN LOOP
       

  prints 0 1 2 3

• UNLOOP prepares for an abnormal loop exit, e.g., via EXIT. UNLOOP removes the loop control parameters from the return stack so EXIT can get to its return address. For example:

            : demo 10 0 ?DO i DUP . 3 = IF UNLOOP EXIT THEN LOOP ." Done" ;
       

  prints 0 1 2 3

• If start is greater than limit, a ?DO loop is entered (and LOOP iterates until they become equal by wrap-around arithmetic). This behaviour is usually not what you want. Therefore, Gforth offers +DO and U+DO (as replacements for ?DO), which do not enter the loop if start is greater than limit; +DO is for signed loop parameters, U+DO for unsigned loop parameters.
• ?DO can be replaced by DO. DO always enters the loop, independent of the loop parameters. Do not use DO, even if you know that the loop is entered in any case. Such knowledge tends to become invalid during maintenance of a program, and then the DO will make trouble.
• LOOP can be replaced with n +LOOP; this updates the index by n instead of by 1. The loop is terminated when the border between limit-1 and limit is crossed. E.g.:

            4 0 +DO  i .  2 +LOOP
       

  prints 0 2

            4 1 +DO  i .  2 +LOOP
       

  prints 1 3

• The behaviour of n +LOOP is peculiar when n is negative:

            -1 0 ?DO  i .  -1 +LOOP
       

  prints 0 -1

            0 0 ?DO  i .  -1 +LOOP
       

  prints nothing.

  Therefore we recommend avoiding n +LOOP with negative n. One alternative is u -LOOP, which reduces the index by u each iteration. The loop is terminated when the border between limit+1 and limit is crossed. Gforth also provides -DO and U-DO for down-counting loops. E.g.:

            -2 0 -DO  i .  1 -LOOP
       

  prints 0 -1

            -1 0 -DO  i .  1 -LOOP
       

  prints 0

            0 0 -DO  i .  1 -LOOP
       

  prints nothing.

Unfortunately, +DO, U+DO, -DO, U-DO and -LOOP are not defined in ANS Forth. However, an implementation for these words that uses only standard words is provided in compat/loops.fs.

Another counted loop is:

     n
     FOR
       body
     NEXT

This is the preferred loop of native code compiler writers who are too lazy to optimize ?DO loops properly. This loop structure is not defined in ANS Forth. In Gforth, this loop iterates n+1 times; i produces values starting with n and ending with 0. Other Forth systems may behave differently, even if they support FOR loops. To avoid problems, don't use FOR loops.