FORTRAN / CONTROL FLOW
if, select case, and logical masks
Learn how if, select case, and logical masks works in Fortran, why the underlying model matters, and how to apply it in a small program without hiding important trade-offs.
What you will learn
- Explain if in Fortran using the correct mental model
- Trace a focused Fortran example and predict its result before execution
- Recognize a boundary case involving select case and handle it deliberately
Understanding if, select case, and logical masks
if, select case, and logical masks belongs to the practical core of Fortran. 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 if 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.
if, select case, and logical masks is a defining part of practical Fortran work. Start by identifying the data or state involved, then trace the operation that changes or interprets it. Pay attention to the rules Fortran 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 do loops, array syntax, and iteration so the ideas form a coherent progression rather than a list of isolated syntax facts.
Worked examples
if, select case, and logical masks example
A focused Fortran example for if.
program hello
print *, 'Hello from Fortran'
end program hello
! Lesson 6: if. Change one value and predict the result before running it.Example explained
Line 1Identify where if appears in the Fortran example and name the data it operates on.
Line 2Trace the relevant Fortran 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 if example small enough to trace completely.
Use the normal Fortran toolchain or browser workspace to compare the actual result with your prediction.
Common mistakes
Treating if as punctuation to memorize instead of a Fortran behavior to reason about.
Ignoring select case until it appears in production data or a larger program.
Try it yourself
Change, predict, then run
Create a small Fortran example that demonstrates if. Add a normal case and a boundary case, write down the expected result for each, then explain which Fortran rule produces that result. Lesson 6 should remain small enough to trace without guessing.
Open Fortran workspaceCheck your understanding
What is the best first step when working with if?
- 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.