Programming concepts: selection

The idea

Selection lets a program choose between alternative paths depending on whether a condition is true or false. The common forms are IF...THEN (do something only when the condition holds), IF...THEN...ELSE (choose between two paths), and ELSEIF or nested IF structures for more than two possibilities. Conditions are built from comparison operators such as equals, less than and greater than, combined with Boolean operators AND, OR and NOT. Selection differs from iteration because it runs a branch at most once per evaluation rather than repeating code. Choosing the right structure keeps a program readable and correct.

What earns the marks

• Selection executes different code paths depending on whether a condition evaluates to true or false.
• IF...THEN runs a block only when the condition is true; IF...THEN...ELSE chooses between two alternative blocks.
• ELSEIF or nested IF structures handle more than two possible outcomes.
• Conditions use comparison operators and may combine them with AND, OR and NOT.
• Selection differs from iteration because it does not repeat a block; it chooses a path.

Common mistakes and how to put them right

• Treating selection as repetition.

  Put it right: Selection chooses a path once per evaluation, whereas iteration repeats code.

• Assuming IF...THEN without ELSE does nothing when the condition is false.

  Put it right: It simply skips the block, which is the intended behaviour.

• Combining conditions with AND when OR is needed.

  Put it right: AND requires both parts true, while OR requires at least one true.

Exam tips

• State clearly which condition is tested and what happens on each outcome.
• Use ELSEIF or nesting when a question describes three or more distinct cases.
• Check combined conditions carefully, deciding whether AND or OR matches the described logic.