"Mix 'n' Match" Control Structures

Forth is a structured language. It does not have a GOTO. It does not have a compiler option to enable GOTO's.

What it does have is a set of words that can be used to construct the regular control structures, and some more complex constructs for those times when other languages send you racing for a GOTO.

The most common control flow words are;

• IF
• ELSE
• THEN
• BEGIN
• UNTIL
• WHILE
• REPEAT

The words IF UNTIL and WHILE take a flag from the stack, and branch if the flag was FALSE (non-zero). The other words have no stack effect.

These can be used to make;

• IF ... THEN
• IF ... ELSE ... THEN
• BEGIN ... UNTIL
• BEGIN ... WHILE ... REPEAT

As well as such esoteria as;

• BEGIN ... WHILE ... WHILE ... REPEAT THEN

Which is a loop with two exit points.

As there is a simple mapping between stack operators and stackFlow diagrams in Forth, so there is a simple relationship between control flow diagrams and control flow words.

I am indebted to Wil Baden for having made this relationship clear, and use his &quotflowgraphs" on this page to illustrate that mapping.

In the next section we look at how stackFlow diagrams can be combined with flowgraphs, and see some applications of control flow words.