COBOL program logic

Program logic is how you organize the steps and decisions in your COBOL program. It boils down to three ideas: sequence (do steps in order), selection (choose one path with IF or EVALUATE), and iteration (repeat with PERFORM UNTIL or PERFORM VARYING). This page shows how these appear in COBOL and how to structure them clearly. For where execution starts and how PERFORM returns, see Program flow.

Three kinds of logic

Sequence

Sequence means statements run one after another. The first sentence runs, then the next, then the next. PERFORM calls are part of sequence: PERFORM A then PERFORM B means “run A, then run B.” So your main paragraph often looks like a list of PERFORMs and other statements in the order you want them to run.

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       MAIN.
           PERFORM INIT-FILES
           PERFORM READ-AND-PROCESS UNTIL WS-EOF
           PERFORM CLOSE-AND-FINISH
           GOBACK.
       *> Sequence: init, then loop, then close

Selection: IF and EVALUATE

Selection means “do one thing or another” based on a condition. IF condition ... ELSE ... END-IF picks between two paths. EVALUATE ... WHEN ... WHEN OTHER ... END-EVALUATE picks among many values (like a switch/case). Use IF for simple true/false; use EVALUATE when you have several distinct values to test.

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       *> Selection: two-way branch
       IF WS-STATUS = "OK"
           PERFORM NORMAL-PROCESS
       ELSE
           PERFORM ERROR-PROCESS
       END-IF.
 
       *> Selection: multi-way
       EVALUATE WS-CODE
           WHEN "A"  PERFORM HANDLE-A
           WHEN "B"  PERFORM HANDLE-B
           WHEN OTHER PERFORM HANDLE-OTHER
       END-EVALUATE.

Iteration: PERFORM UNTIL and VARYING

Iteration means repeating a block. PERFORM paragraph-name UNTIL condition runs the paragraph until the condition is true (tested after each execution). PERFORM VARYING idx FROM 1 BY 1 UNTIL idx > 10 ... END-PERFORM runs the in-line statements with idx 1, 2, ... 10. So loops are written with PERFORM, not with a separate “DO” or “WHILE” verb.

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       *> Iteration: repeat until end-of-file
       PERFORM READ-RECORD
       PERFORM PROCESS-LOOP UNTIL WS-EOF.
 
       *> Iteration: repeat with a counter
       PERFORM VARYING WS-I FROM 1 BY 1 UNTIL WS-I > 10
           ADD WS-I TO WS-TOTAL
       END-PERFORM.

Organizing logic into paragraphs

Give each logical task its own paragraph and PERFORM it by name. The main flow then reads like a table of contents: INIT, then READ-AND-PROCESS until done, then CLOSE. Keep paragraphs short and focused. Use IF/EVALUATE inside a paragraph for small decisions; use PERFORM to jump to another paragraph for a larger step. For more on structure see Algorithm design and Program control.

Step-by-step: design the logic first

• List the main steps in order (sequence).
• Mark any “if this then that” with IF or EVALUATE (selection).
• Mark any “repeat until” or “do N times” with PERFORM UNTIL or VARYING (iteration).
• Name a paragraph for each step and write PERFORM in the main paragraph.
• Implement each paragraph with the right mix of sequence, IF/EVALUATE, and PERFORM.

Related concepts