COBOL by Example: PERFORM Loops
Understanding iteration with the versatile PERFORM statement handling all loop types in COBOL, implementing simple fixed repetition with PERFORM TIMES, creating counter-based loops with PERFORM VARYING for initialization increment and termination, using PERFORM UNTIL for conditional iteration like while loops, and utilizing inline perform blocks with END-PERFORM for localized logic.
Code
IDENTIFICATION DIVISION.
PROGRAM-ID. LOOPS.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 I PIC 99.
PROCEDURE DIVISION.
*> Simple loop
PERFORM 3 TIMES
DISPLAY "Hello"
END-PERFORM.
*> Loop with counter
PERFORM VARYING I FROM 1 BY 1 UNTIL I > 5
DISPLAY "Count: " I
END-PERFORM.
STOP RUN.Explanation
Loops in COBOL are handled by the PERFORM statement. Unlike other languages with distinct for, while, and do-while loops, COBOL uses variations of PERFORM to handle all iteration needs. It is one of the most versatile verbs in the language.
The VARYING phrase is particularly powerful. It allows you to initialize a counter (FROM), specify the increment (BY), and define the termination condition (UNTIL) all in one readable sentence. You can even nest loops by adding AFTER clauses to the VARYING statement.
Modern COBOL encourages the use of inline PERFORM blocks (terminated by END-PERFORM) rather than performing a separate paragraph. This keeps the logic localized and easier to read, similar to code blocks in C-style languages.
- Simple Repetition:
PERFORM 3 TIMESexecutes a block a fixed number of times. - Conditional Loop:
PERFORM UNTIL conditionacts like a while loop, checking the condition before each iteration. - Inline Loop:
PERFORM VARYING...is the equivalent of a for-loop, managing a counter variable automatically.