PERFORM VARYING and PERFORM UNTIL – Loops with COBOL

In COBOL, loops play a crucial role in program flow control, allowing for the repetition of a set of instructions until a specific condition is met. The PERFORM VARYING command is used to establish a loop controlled by an iteration variable, which is incremented or decremented with each iteration. This provides flexibility to execute a block of code a defined number of times, depending on the value of the control variable. On the other hand, the PERFORM UNTIL command allows for repeated execution of a code block until a specific condition is met, making it useful when a set of instructions needs to be repeated until a desired state is reached. These two approaches, PERFORM VARYING and PERFORM UNTIL, offer powerful tools for the effective implementation of loops in COBOL, facilitating the creation of structured and efficient programs.

PERFORM VARYING

In COBOL, the PERFORM VARYING loop is used to carry out a series of repetitive actions, where a control variable is incremented or decremented with each iteration. Here is a simple example of a PERFORM VARYING loop:

IDENTIFICATION DIVISION.
PROGRAM-ID. LoopExample.
AUTHOR. YourName.
DATE-WRITTEN. DD/MM/YYYY.

DATA DIVISION.
WORKING-STORAGE SECTION.
01 Counter PIC 9(3) VALUE 1.

PROCEDURE DIVISION.
    DISPLAY 'Start of loop'.

    PERFORM VARYING Counter FROM 1 BY 1 UNTIL Counter > 5
        DISPLAY 'Iteration ' Counter
    END-PERFORM.

    DISPLAY 'End of loop'.
    STOP RUN.

WORKING – STORAGE SECTION

Counter: Control variable of the loop, initialized to 1.

PROCEDURE DIVISION

• PERFORM VARYING Counter FROM 1 BY 1 UNTIL Counter > 5: Starts the PERFORM VARYING loop, which will execute the code block until the UNTIL condition is met.
  • FROM 1 BY 1: Indicates that the control variable (Counter) starts at 1 and increments by 1 with each iteration.
  • UNTIL Counter > 5: This is the loop’s termination condition.
• DISPLAY 'Iteration ' Counter: Displays the iteration number in the console for each cycle.
• END-PERFORM: Marks the end of the loop.
• DISPLAY 'End of loop': Shows a message indicating that the loop has finished.
• STOP RUN: Indicates the end of the program.

In this example, the PERFORM VARYING loop will execute five times, displaying the message “Iteration” followed by the iteration number. You can adjust the UNTIL condition as needed for the specific logic of your program.

PERFORM UNTIL

As a COBOL programmer, you could design a simple program that uses the PERFORM UNTIL command to perform a specific task. Suppose you want to create a program that prints the numbers from 1 to 10. Here’s an example of how the program could be structured in COBOL using PERFORM UNTIL:

IDENTIFICATION DIVISION.
PROGRAM-ID. PrintNumbers.
AUTHOR. YourName.
DATE-WRITTEN. DD/MM/YYYY.

DATA DIVISION.
WORKING-STORAGE SECTION.
01 Number PIC 9(2) VALUE 1.

PROCEDURE DIVISION.
DISPLAY 'Printing numbers from 1 to 10'.
PERFORM UNTIL Number > 10
    DISPLAY Number
    ADD 1 TO Number
END-PERFORM.

DISPLAY 'Finished printing numbers'.
STOP RUN.

Explanation:

• IDENTIFICATION DIVISION: Contains metadata about the program.
• DATA DIVISION: Defines the data used in the program, including the Number variable, initialized to 1.
• PROCEDURE DIVISION: Contains the executable code.
  • The DISPLAY statement shows the starting message.
  • The PERFORM UNTIL loop continues executing until Number is greater than 10.
  • Inside the loop, the current value of Number is displayed, and then 1 is added to Number.
  • After the loop, a message indicating completion is displayed, and the program ends with STOP RUN.

Differences between PERFORM VARYING and PERFORM UNTIL.

The PERFORM VARYING command and the PERFORM UNTIL command in COBOL are loop control structures, but they differ in how they control the repetition of a block of code.

PERFORM VARYING

Usage: It is used to perform loops based on a control variable that changes its value in each iteration.

PERFORM [section-name]
VARYING [variable-name] FROM [initial-value]
BY [increment-or-decrement]
UNTIL [termination-condition].

Description: In the example, the loop executes while the Counter variable is less than or equal to 10, incrementing by 1 in each iteration.

PERFORM UNTIL

Usage: It is used to perform loops based on a condition that must evaluate to true to continue executing the block of code.

PERFORM [section-name]
UNTIL [termination-condition].

Description: In the example, the loop executes until the condition Counter > 10 becomes true, incrementing by 1 in each iteration.

In summary, PERFORM VARYING is used when you want to loop based on the changing value of a variable, while PERFORM UNTIL is used when you want to loop based on a specific condition that must evaluate to true to continue. The choice between them depends on the specific logic you are implementing in your COBOL program.