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

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.

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

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.

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.

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.

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

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.

CCEA AS-Level Software Systems Development CL4

Software and the Development Environment

Topic overview Systems software and application software The operating system and file management Source code files Binary, object and executable files Compilers, interpreters and the build process XML files and structured data Utility software and managing files on disk The integrated development environment Object oriented development platforms Open source software and licensing

Defining Data: Types, Variables and Classes

Topic overview Primitive types and their storage Reference types, objects and null Declaring variables, scope and lifetime Type casting and conversion Arithmetic operators and expressions Relational and logical operators Operator precedence and evaluation order Objects as occurrences of a class Abstract, base and inherited classes Attributes and the properties of a class Static variables in a class Interfaces and class reusability

Program Control Structures

Topic overview 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 Choosing and evaluating control structures

Objects, Classes and Data Structures

Topic overview Constructors and creating objects Methods, parameters and return values Encapsulation and access modifiers Getters, setters and validating an object's state Inheritance and the base class Overriding methods and run-time polymorphism Overloading methods Choosing and organising data structures Strings and string handling Static arrays of simple types Arrays of objects

Exception Handling and Managing Input and Output

Topic overview Why errors have to be trapped Types of error and how an exception is raised The try and catch block Catching more than one kind of exception The finally block and releasing resources Throwing exceptions and writing an exception class Checked and unchecked exceptions Reading and validating input from the command line Controlling screen output Text files Binary files Object files and serialisation

Testing an Object Oriented Application

Topic overview Why an object oriented application must be tested Choosing test data: normal, boundary and erroneous Writing a test plan for a class Testing a class method by method Testing constructors and object state Testing inheritance and overridden methods Testing error handling paths Dry runs and trace tables Test harnesses, drivers and stubs Recording and interpreting test results Evaluating a test plan and re-testing

Graphical User Interface Objects and Events

Topic overview Graphical user interfaces and console programs Forms and their common properties Labels, text boxes and text areas Buttons, check boxes and radio buttons Lists, combo boxes and other selection controls Properties, methods and events of a GUI object Containers, panels and laying out a form The event loop and the event queue Event handlers and registering a listener The event object and reading it inside a handler Button, mouse and keyboard triggers

Multiple Forms and Linking to Files

Topic overview Why an application needs more than one form Creating, showing, hiding and closing a form Modal and modeless forms Passing data between forms Menus, menu items and shortcuts Toolbars and navigation buttons Designing navigation for a multi form application Reading and writing text files Binary files and streams Object serialisation and object files Choosing between text, binary and object files

Designing, Testing and Evaluating an Event Driven Application

Topic overview Storyboards and what they show Building a storyboard from user requirements Prototyping and refining a design Error trapping in an event driven application Using try and catch inside an event handler Validating what a user types into a control Designing a test strategy for an event driven application Test plans and choosing test data Testing the paths a user takes through an interface Documenting and interpreting test results Evaluating an application against its requirements Evaluating features, functionality and usability

Program Control Structures

10 subtopics
30 things to know
AS Unit 1

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.

Where it is assessed

AS Unit 1

Introduction to Object Oriented Development.

Check the CCEA specification ↗

The 10 subtopics

Each one has its own page. CCEA AS Software Systems Development students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is recursion, and why must a recursive method have a base case?

Key term

What is a null ELSE, and how should if (raining) { } else { walk(); } be rewritten?

Key term

What is a dangling else? Explain which IF the else belongs to in: if (a > 0) if (b > 0) System.out.println("both"); else System.out.println("other");.

Key term

switch (month) { case 4: case 6: case 9: case 11: days = 30; break; } — what does this fragment achieve?

Key term

Why must case labels be constants, and why must they be distinct?

Key term

for (int i = 1; i <= 5; i++) { System.out.print(i + " "); } — what does this fragment print?

Key term

How does a pre-test loop differ from a post-test loop?

Key term

int n = 5; while (n > 0) { System.out.print(n + " "); n = n - 2; } — what does this fragment print?

Key term

int n = 0; do { System.out.print(n + " "); n++; } while (n < 0); — what does this fragment print?

Key term

Describe how a flag-controlled search loop works.

Evaluation

Evaluate the choice between a switch and an else-if ladder for a menu of six numbered options.

Key term

What is structured programming, and what does the single entry and single exit principle require?

Key term

Compare a loop that tests before the body with one that tests after, and give a case that suits each.

Process

What does sequence mean as a program control structure?

Key term

What is a nested loop?

Key term

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

Key term

What is a method?

Key term

What is selection?

Key term

What is a nested IF?

Key term

What is repetition as a control structure?

Key term

An outer loop runs three times and an inner loop four times. How many times does the inner body execute in total?

Key term

What does break do inside a loop?

Key term

What is a statement, and what is a compound statement?

Key term

What happens to the flow of control when a method is called?

Key term

What is a decision, in the context of program control?

Key term

int mark = 85; if (mark >= 40) { System.out.println("Pass"); } else if (mark >= 70) { System.out.println("Distinction"); } — what is the defect?

Key term

What kind of expression does a switch test?

Key term

What is meant by unconditional repetition, and which construct normally implements it?

Key term

Why is using the same variable name for both counters of a nested loop a mistake?

Key term

What does continue do inside a loop?

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
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
Which three basic control structures can any program be built from?
Model answer
The three basic control structures are sequence, selection and repetition.
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
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
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
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.

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Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Software and the Development Environment

Defining Data: Types, Variables and Classes

Program Control Structures

Objects, Classes and Data Structures

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Program Control Structures revision questions

Is Program Control Structures examined at AS in CCEA Software Systems Development?: Yes. It sits in AS Unit 1, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Program Control Structures cover in CCEA AS-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.

Every question in Program Control Structures, on a schedule.

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