D / CONTROL FLOW
Ranges, foreach, and lazy iteration
Learn how ranges, foreach, and lazy iteration works in D, why the underlying model matters, and how to apply it in a small program without hiding important trade-offs.
What you will learn
- Explain ranges in D using the correct mental model
- Trace a focused D example and predict its result before execution
- Recognize a boundary case involving foreach and handle it deliberately
Understanding Ranges, foreach, and lazy iteration
Ranges, foreach, and lazy iteration belongs to the practical core of D. Start by identifying the values or state involved and the rule that connects the input to the result.
Trace the example one operation at a time. Keep ranges visible in the code rather than hiding it behind an abstraction before the behavior is understood.
Test a normal case and a boundary case. The difference between the prediction and the observed result is the most useful signal for deciding what to review next.
Ranges, foreach, and lazy iteration is a defining part of practical D work. Start by identifying the data or state involved, then trace the operation that changes or interprets it. Pay attention to the rules D applies at this boundary, because those rules explain both the useful behavior and the common failure modes. This lesson keeps the example deliberately small, then connects it to functions, delegates, templates, and constraints so the ideas form a coherent progression rather than a list of isolated syntax facts.
Worked examples
Ranges, foreach, and lazy iteration example
A focused D example for ranges.
import std.stdio;
void main() { writeln("Hello from D"); }
// Lesson 7: ranges. Change one value and predict the result before running it.Example explained
Line 1Identify where ranges appears in the D example and name the data it operates on.
Line 2Trace the relevant D rule one operation at a time, recording any state, type, or control-flow change.
Line 3Change one input or boundary condition, predict the result, and compare that prediction with the documented outcome.
Important notes
Keep the first ranges example small enough to trace completely.
Use the normal D toolchain or browser workspace to compare the actual result with your prediction.
Common mistakes
Treating ranges as punctuation to memorize instead of a D behavior to reason about.
Ignoring foreach until it appears in production data or a larger program.
Try it yourself
Change, predict, then run
Create a small D example that demonstrates ranges. Add a normal case and a boundary case, write down the expected result for each, then explain which D rule produces that result. Lesson 7 should remain small enough to trace without guessing.
Open D workspaceCheck your understanding
What is the best first step when working with ranges?
- Identify the data and predict the result
- Add more abstraction immediately
- Ignore boundary cases
- Memorize punctuation only
Show answer
A clear input, operation, and predicted result create a testable mental model.