Control Structures

Don't use GO TO

GO TO is both widely used and widely deplored. Here we will merely summarize a broader debate:
  1. Don't use GO TO if you don't have to.

  2. You (almost) never have to.

Don't use fall-through logic

If you don't use GO TO this rule mostly takes care of itself, except when fall-through logic starts at the initial entry point of a module.

Don't use the SORT verb.

Among other reasons, this rule eliminates one possible reason to use GO TO.

Don't PERFORM THRU

If you don't use GO TO or fall-through logic, a THRU clause on a PERFORM statement is a useless bug-breeder.

Don't use sections in the PROCEDURE DIVISION.

Except when the compiler requires them, the use of sections invites problems without solving any.

Avoid segmentation.

You may have to use segmentation to get a large program to fit into a small machine. However, careless use of segmentation invites bizarre and mysterious bugs.

Use CONTINUE instead of NEXT SENTENCE.

These two constructs are not quite the same, and CONTINUE is safer.

Always include a default clause in an case structure.

If you code an EVALUATE statement, include a WHEN OTHER clause. If you use a series of ELSE IF clauses instead, include a final ELSE clause.

If you follow this rule, you will at least think about what to do in the default case. If there is nothing to do, then code a CONTINUE statement, just to show that you did think about it.

It does not follow that you should code an ELSE for every IF, as sometimes recommended. All those pointless CONTINUEs (or NEXT SENTENCEs) are an annoyance to type and a distraction to read.

For short loops, use an in-line PERFORM.

Before COBOL II, a PERFORM UNTIL required a separate paragraph, even if it merely decremented a counter. An in-line PERFORM can be more readable because you don't have to look somewhere else to see what the loop is doing.

Don't overdo it. Most non-trivial loops -- more than about five lines -- deserve paragraphs of their own. Otherwise the PERFORMing paragraph may become too big and complicated. Main exception: when the loop makes up almost the entire paragraph anyway.

Use 88-levels whenever possible for flags and switches.

In most cases the name of a flag or switch variable shouldn't even appear in the PROCEDURE DIVISION. Test it with the 88-levels, and use the SET verb to assign a value to it.

Should you use periods?

Some people argue that you should never code a period as a statement delimiter, except at the end of a paragraph, where the compiler requires one. Instead, delimit every IF with END-IF, every READ with END-READ, and so forth.
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