The use of the three basic programming constructs used to control the flow…

The idea

Program flow is controlled by three constructs. Sequence executes statements one after another in written order, for example reading a sensor then storing its value then displaying it. Selection chooses between paths using a condition, such as IF score >= 50 THEN grade = "pass" ELSE grade = "fail". Iteration repeats code: count-controlled loops repeat a known number of times, for example FOR i = 1 TO 10, while condition-controlled loops repeat while or until a condition changes, for example WHILE answer != "yes" or REPEAT ... UNTIL valid. Combining constructs lets a programmer express any algorithm: a menu uses selection inside a condition-controlled loop, and processing a list uses a count-controlled loop containing selection. Choosing the right construct makes code readable, correct and efficient, and exam questions ask you to trace, write or correct code using them.

What earns the marks

• Sequence: statements execute in the order written, so swapping two lines can change the result; tracing follows that order line by line.
• Selection: a condition is evaluated and exactly one branch runs, for example IF ... THEN ... ELSE ... or a CASE statement; the condition must be Boolean.
• Count-controlled iteration: the number of repetitions is known or calculated before the loop starts, for example FOR i = 1 TO 5 or a loop over a fixed list.
• Condition-controlled iteration: repetition continues while or until a condition is tested, for example WHILE lives > 0 or REPEAT ... UNTIL password = "secret".
• Nesting: constructs can be placed inside one another, so a selection can sit inside a loop and a loop can sit inside a selection.
• Tracing: track variable values after each iteration or branch to predict output and to find logic errors.

Common mistakes and how to put them right

• Confusing count-controlled and condition-controlled loops.

  Put it right: A FOR loop is used when the number of repeats is known, while a WHILE loop is used when repetition depends on a condition; choose the construct that matches the problem.

• Writing a condition-controlled loop whose condition never becomes false, producing an infinite loop.

  Put it right: Ensure the loop body changes a variable used in the condition, for example incrementing a counter.

• Forgetting that an IF without ELSE simply skips its body when the condition is false, so no alternative action occurs.

  Put it right: Add ELSE when a different path is required.

• Using = instead of a comparison operator in a condition, or mixing up < and >, which changes which branch runs.

  Put it right: Check the operator against the intended meaning.

Exam tips

• Read the question's wording for clues: 'repeat until' or 'while' signals condition-controlled iteration, while 'for each' or a fixed number signals count-controlled iteration.
• When tracing, draw a table of variables and update it after every line or iteration so the final output is easy to justify.
• When writing code, indent nested constructs clearly; this helps you and the examiner see which statements belong to which branch or loop.
• Check loop conditions by asking what value makes the loop stop and whether the body moves the variable towards that value.