DO Loops and Basic Iteration in RPGLE

Learning Objective

By the end of this lesson, you will be able to read and write simple for, dow, and dou loops in free-format RPGLE, and explain when each one is a natural fit.

Simple Explanation

Everything covered so far in this lesson group runs through a program's logic once, from top to bottom. A loop lets a program repeat a block of statements multiple times. Free-format RPGLE offers a few common ways to loop:

• for, used when you know how many times you want to repeat something, or want to count through a range of values:

dcl-s i int(10);

for i = 1 to 5;
  dsply i;
endfor;

• dow (Do While), which repeats a block of statements as long as a condition remains true, checked before each repetition:

dcl-s remaining packed(5:0);

remaining = 3;
dow remaining > 0;
  dsply remaining;
  remaining = remaining - 1;
enddo;

• dou (Do Until), which repeats a block of statements until a condition becomes true, checked after each repetition, meaning the block always runs at least once:

dcl-s attempts packed(3:0);

attempts = 0;
dou attempts >= 3;
  attempts = attempts + 1;
  dsply attempts;
enddo;

for loops end with endfor; dow and dou loops both end with enddo.

Why It Matters

Loops are how a program repeats work without writing out the same statements over and over by hand. Practically any task involving "do this for each of several items" or "keep doing this until some condition is met" relies on a loop of some kind. Recognizing for, dow, and dou, and knowing roughly when each one fits naturally, is a foundational skill for writing or reading almost any real RPGLE program.

Practical Example

Imagine a simple program that needs to display a countdown from 3 down to

1. Using dow, as shown above, the condition remaining > 0 is checked before each repetition, so the loop naturally stops once remaining reaches 0, having displayed 3, 2, and then 1.

A for loop counting from 1 to 5, also shown above, instead runs a known, fixed number of times, useful when you already know exactly how many repetitions you want rather than depending on a changing condition. This is a simplified, illustrative example rather than a specific real program, but it reflects common, everyday uses of each loop style.

Common Confusions

"What is the real difference between dow and dou?" dow checks its condition before each repetition, so the loop's block may never run at all if the condition starts out false. dou checks its condition after each repetition, so the loop's block always runs at least once, even if the condition would have been true from the very start.

"When should I use for instead of dow or dou?" for is a natural fit when you already know how many times you want to repeat something, or want to count through a range of values, as in the countdown-style example. dow and dou are a better fit when the number of repetitions depends on a condition that can change while the loop runs.

"What happens if a dow loop's condition is never checked again inside the loop?" If nothing inside the loop ever changes the condition being checked, a dow or dou loop will keep repeating indefinitely. Making sure the condition can actually become false, such as decreasing remaining inside the loop in the example above, is an important, practical habit.

Quick Recap

• for loops repeat a known number of times or count through a range, ending with endfor.
• dow (Do While) checks its condition before each repetition; dou (Do Until) checks its condition after each repetition, so its block always runs at least once. Both end with enddo.
• Loops let a program repeat work without writing out the same statements multiple times.
• Something inside a dow or dou loop must be able to change the condition, or the loop will repeat indefinitely.

Try Asking the AI Tutor

Use the AI Tutor to ask follow-up questions about this lesson. For example, try asking:

• "What happens if I write a dow loop whose condition never becomes false?"
• "Can I use for to count backwards instead of counting up?"