Program Control Structures

What Program Control Structures covers

Program Control Structures is examined in Unit AS 1, Introduction to Object Oriented Development — a written examination. It covers sequence and the order of execution, methods and passing control between them, selection with IF and IF ELSE, nested IF and multi-way selection, the switch statement, count-controlled repetition with FOR, conditional repetition with WHILE and DO WHILE, nested loops and tracing iterations, breaking out of loops and avoiding infinite loops and choosing and evaluating control structures.

The 10 subtopics

Each one has its own page.

• Sequence and the order of executionFlow of control in a program and task that is naturally pure sequence.Open →
• Methods and passing control between themReturn value do, and what does the keyword void, three benefits of dividing a program into methods and method having a single, well-defined purpose.Open →
• Selection with IF and IF ELSEDecision, in the context of program control, null ELSE and conditional operator.Open →
• Nested IF and multi-way selectionNested IFs and else-if ladders traced through worked Java fragments, ordering the conditions correctly, the dangling else, combining conditions with AND, counting paths for testing, and when a nested IF is clearer than a switch.Open →
• The switch statementSwitch statement, expression does a switch test and fall-through in a switch.Open →
• Count-controlled repetition with FORRepetition as a control structure, unconditional repetition and off-by-one error.Open →
• Conditional repetition with WHILE and DO WHILEConditional repetition and sentinel value.Open →
• Nested loops and tracing iterationsA loop inside a loop and how many times the inner body runs, tracing the output of a nested pair, triangular patterns where the inner limit depends on the outer counter, and breaking out early.Open →
• Breaking out of loops and avoiding infinite loopsHow a flag-controlled search loop works, infinite loop and loop guard.Open →
• Choosing and evaluating control structuresThree measures of a well-chosen control structure and structured programming.Open →

What you need to know

Answer each one before you reveal it.

Take Program Control Structures further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Flow of control vs Control structure

  Flow of control: the path execution actually takes through the statements on a given run. Control structure: a construct in the language that shapes that path.

• Off-by-one error vs Infinite loop

  Off-by-one error: a fault in which a count ends after the wrong number of passes, usually one too many or too few. Infinite loop: a fault in which the test can never fail, so execution never leaves the block.

Questions from this topic

Each one with the model answer underneath.

1. 1

   What does sequence mean as a program control structure?

   Model answer

   Sequence is the control structure in which statements are carried out one after another in the order they are written, each completing before the next begins.

2. 2

   Which three basic control structures can any program be built from?

   Model answer

   The three basic control structures are sequence, selection and repetition.

3. 3

   Using a total calculated from a price as the example, why does the order of statements within a sequence matter?

   Model answer

   A statement can only use values that already exist, so calculating total = price * quantity before price has been assigned uses an unset value and gives the wrong answer or refuses to compile.

4. 4

   int a = 2; int b = 3; a = a + b; b = a - b; — what are the final values of a and b?

   Model answer

   The third statement makes a equal to 5, and the fourth then makes b equal to 5 minus 3, which is 2, so a finishes at 5 and b at 2.

5. 5

   In the fragment int p = 2; int q = 3; q = p - q; p = p + q; work through each statement and give the values of p and q at the end.

   Model answer

   The third statement stores 2 minus 3 in q, giving −1, and the fourth then stores 2 plus −1 in p, so p ends at 1 and q at −1.

6. 6

   Explain why swapping the contents of two variables needs a third variable, and give the three statements in order.

   Model answer

   Assigning one variable to the other destroys the value being overwritten, so a temporary variable must hold it first: temp = a; then a = b; then b = temp.

What is inside Levo

Six ways to work the same material.

Program Control Structures revision questions

What does Program Control Structures cover in CCEA A-Level Software Systems Development?

10 subtopics: sequence and the order of execution, methods and passing control between them, selection with if and if else, nested if and multi-way selection, the switch statement, count-controlled repetition with for, conditional repetition with while and do while, nested loops and tracing iterations, breaking out of loops and avoiding infinite loops and choosing and evaluating control structures. Each one has its own page with questions and model answers.

How is Program Control Structures organised?

By the CCEA CL4 specification, subtopic by subtopic, so it matches the order you are taught it in.