In this section, we will explore the various looping constructs available in COBOL. Looping constructs are essential for executing a block of code multiple times, which is a common requirement in programming. We will cover the following topics:
- PERFORM UNTIL
- PERFORM VARYING
- PERFORM TIMES
- PERFORM UNTIL
The PERFORM UNTIL loop executes a block of code repeatedly until a specified condition is met.
Syntax
Example
IDENTIFICATION DIVISION.
PROGRAM-ID. PerformUntilExample.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 COUNTER PIC 9(2) VALUE 1.
PROCEDURE DIVISION.
PERFORM UNTIL COUNTER > 10
DISPLAY 'Counter: ' COUNTER
ADD 1 TO COUNTER
END-PERFORM.
STOP RUN.Explanation
- COUNTER is initialized to 1.
- The
PERFORM UNTILloop will execute the block of code untilCOUNTERis greater than 10. - Inside the loop, the current value of
COUNTERis displayed, and thenCOUNTERis incremented by 1.
- PERFORM VARYING
The PERFORM VARYING loop is used to iterate over a range of values, similar to a for loop in other programming languages.
Syntax
PERFORM
[paragraph-name]
VARYING [index-variable] FROM [start-value] BY [increment-value]
UNTIL [condition]Example
IDENTIFICATION DIVISION.
PROGRAM-ID. PerformVaryingExample.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 INDEX PIC 9(2).
PROCEDURE DIVISION.
PERFORM VARYING INDEX FROM 1 BY 1 UNTIL INDEX > 10
DISPLAY 'Index: ' INDEX
END-PERFORM.
STOP RUN.Explanation
- INDEX is used as the loop variable.
- The
PERFORM VARYINGloop starts withINDEXset to 1 and increments it by 1 each iteration. - The loop continues until
INDEXis greater than 10. - Inside the loop, the current value of
INDEXis displayed.
- PERFORM TIMES
The PERFORM TIMES loop executes a block of code a specified number of times.
Syntax
Example
IDENTIFICATION DIVISION.
PROGRAM-ID. PerformTimesExample.
DATA DIVISION.
WORKING-STORAGE SECTION.
PROCEDURE DIVISION.
PERFORM 5 TIMES
DISPLAY 'Hello, COBOL!'
END-PERFORM.
STOP RUN.Explanation
- The
PERFORM TIMESloop will execute the block of code 5 times. - Each iteration, the message "Hello, COBOL!" is displayed.
Practical Exercises
Exercise 1: Using PERFORM UNTIL
Write a COBOL program that uses a PERFORM UNTIL loop to display the numbers from 1 to 20.
Solution
IDENTIFICATION DIVISION.
PROGRAM-ID. Exercise1.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 COUNTER PIC 9(2) VALUE 1.
PROCEDURE DIVISION.
PERFORM UNTIL COUNTER > 20
DISPLAY 'Number: ' COUNTER
ADD 1 TO COUNTER
END-PERFORM.
STOP RUN.Exercise 2: Using PERFORM VARYING
Write a COBOL program that uses a PERFORM VARYING loop to display even numbers from 2 to 20.
Solution
IDENTIFICATION DIVISION.
PROGRAM-ID. Exercise2.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 INDEX PIC 9(2).
PROCEDURE DIVISION.
PERFORM VARYING INDEX FROM 2 BY 2 UNTIL INDEX > 20
DISPLAY 'Even Number: ' INDEX
END-PERFORM.
STOP RUN.Exercise 3: Using PERFORM TIMES
Write a COBOL program that uses a PERFORM TIMES loop to display "Learning COBOL!" 10 times.
Solution
IDENTIFICATION DIVISION.
PROGRAM-ID. Exercise3.
DATA DIVISION.
WORKING-STORAGE SECTION.
PROCEDURE DIVISION.
PERFORM 10 TIMES
DISPLAY 'Learning COBOL!'
END-PERFORM.
STOP RUN.Common Mistakes and Tips
- Off-by-One Errors: Ensure that your loop conditions are correctly set to avoid off-by-one errors.
- Infinite Loops: Always make sure that the loop condition will eventually be met to avoid infinite loops.
- Variable Initialization: Initialize your loop variables properly before entering the loop.
Conclusion
In this section, we covered the three primary looping constructs in COBOL: PERFORM UNTIL, PERFORM VARYING, and PERFORM TIMES. These constructs allow you to execute blocks of code multiple times under different conditions. By practicing the provided exercises, you should now have a solid understanding of how to implement loops in COBOL. In the next section, we will delve into nested control structures, which will build upon the concepts learned here.