Programming fundamentals

What you'll learn

How programs use variables, constants, inputs, outputs and assignments.
How to control the order instructions run using sequence, selection and iteration.
How to recognise and use arithmetic, comparison and Boolean operators.
How to trace small programs accurately, like you would in an exam.

The building blocks of a program

A program is a set of instructions that a computer can follow. In GCSE Computer Science, you usually write programs in a high-level language, which means a language designed to be readable by humans, such as Python or OCR-style pseudocode.

The computer still follows the instructions very literally, in the order and structure you give it.

Definition

High-level language

A high-level language is a programming language that is closer to human language than machine code. It hides many low-level hardware details, so you can focus on solving the problem.

Variables, constants and assignment

Programs need to store data while they run. That data is usually stored using named locations.

Definition

Variables, constants and assignment

A variable is a named storage location whose value can change while a program runs. A constant is a named value that should stay the same. Assignment means storing the result of a value or expression in a variable or constant name.

An expression is a piece of code that produces a value, such as score + 5 or length * width.

For example:

CONSTANT PASS_MARK = 50
score = 42
score = score + 10

After the final line, score stores 52. The old value, 42, has been overwritten.

Common Mistake

Assignment is not equality

In programming, score = score + 10 does not mean “score is equal to score plus 10” in a maths sense. It means: work out the right-hand side first, then store the result back into score.

Example

Tracing assignments

score = 10
score = score + 5
bonus = score DIV 4
OUTPUT bonus

The first assignment stores 10 in score.
In score = score + 5, the old value of score is used first: 10 plus 5 gives 15, so score becomes 15.
bonus = score DIV 4 uses whole-number quotient division: 15 DIV 4 gives 3.
The program outputs the value stored in bonus, so the output is 3.

Inputs and outputs

An input is data that enters a program, such as a value typed by a user. An output is data sent out by a program, such as text displayed on a screen.

In OCR-style pseudocode, you may see:

INPUT length
INPUT width
area = length * width
OUTPUT area

This program takes two inputs, calculates an area, and outputs the result.

Key Idea

Programs process data

Most simple programs follow this pattern: get input, store data in variables, process it using operators, then output a result.

Operators

An operator is a symbol or word that performs an action on one or more values. For example, + adds values and > compares values.

Arithmetic operators

Arithmetic operators are used for calculations.

Operator	Meaning	Example result
`+`	Addition	`7 + 3` gives 10
`-`	Subtraction	`7 - 3` gives 4
`*`	Multiplication	`7 * 3` gives 21
`/`	Division	`8 / 2` gives 4
`MOD`	Modulo: the remainder after division	`17 MOD 5` gives 2
`DIV`	Quotient: whole-number division	`17 DIV 5` gives 3
`^`	Exponentiation: to the power of	`2 ^ 3` gives 8

Tip

DIV and MOD work as a pair

For 17 DIV 5 and 17 MOD 5, ask: how many whole 5s fit into 17, and what is left over? Three 5s fit into 17, with 2 left over.

Comparison operators

A comparison operator compares two values and gives a Boolean result: either True or False.

Operator	Meaning
`==`	Equal to
`!=`	Not equal to
`<`	Less than
`<=`	Less than or equal to
`>`	Greater than
`>=`	Greater than or equal to

Common Mistake

Using = instead of ==

Use = for assignment, such as age = 15. Use == for comparison, such as age == 15. In exam questions, this difference is often important.

Boolean operators: AND, OR and NOT

A Boolean value is a value that can only be True or False.

Boolean operators combine or change Boolean values:

A	B	A AND B	A OR B
True	True	True	True
True	False	False	True
False	True	False	True
False	False	False	False

A	NOT A
True	False
False	True

AND is only True if both sides are True.
OR is True if at least one side is True.
NOT reverses a Boolean value.

Example

Evaluating Boolean logic

Given:

mark = 62
attendance = 88
lateWork = False
result = (mark >= 50 AND attendance >= 90) OR NOT lateWork

Evaluate the comparisons: mark >= 50 is True, but attendance >= 90 is False.
Combine them with AND: True AND False gives False.
Evaluate NOT lateWork: since lateWork is False, NOT lateWork gives True.
Combine the final parts with OR: False OR True gives True, so result stores True.

Control flow

Control flow means the order in which the instructions in a program are executed. GCSE programming uses three basic programming constructs: sequence, selection and iteration.

Flowchart-style schematic showing sequence, selection and iteration in programming

Sequence

Sequence means instructions run one after another, from top to bottom.

INPUT length
INPUT width
area = length * width
OUTPUT area

Here, the program must get the inputs before it can calculate the area. If the order is wrong, the program may use missing or incorrect values.

Selection

Selection means the program chooses between different paths using a condition. A condition is an expression that evaluates to True or False.

IF temperature < 18 THEN
    OUTPUT "Wear a coat"
ELSE
    OUTPUT "No coat needed"
ENDIF

The IF branch runs when the condition is True. The ELSE branch runs when the condition is False.

Example

Choosing a branch

Given:

temperature = 17

IF temperature < 18 THEN
    OUTPUT "Wear a coat"
ELSE
    OUTPUT "No coat needed"
ENDIF

Evaluate the condition temperature < 18 using the stored value 17.
Since 17 is less than 18, the condition is True.
The program follows the IF branch and outputs "Wear a coat"; the ELSE branch is skipped.

You may also see multiple branches using ELSE IF, ELIF or similar wording, depending on the language.

Iteration

Iteration means repetition. A repeated section of code is called a loop.

There are two GCSE loop types you need to know: count-controlled loops and condition-controlled loops.

Count-controlled loops

A count-controlled loop repeats a set number of times. It usually uses a counter variable.

total = 0

FOR counter = 1 TO 4
    total = total + counter
NEXT counter

OUTPUT total

Example

Tracing a count-controlled loop

On the first repetition, counter is 1, so total = total + counter changes total from 0 to 1.
On the second repetition, counter is 2, so total changes from 1 to 3.
On the third repetition, counter is 3, so total changes from 3 to 6.
On the fourth repetition, counter is 4, so total changes from 6 to 10. The loop has now completed, so the program outputs 10.

Common Mistake

Off-by-one errors

Check the start and end values of a count-controlled loop carefully. FOR counter = 1 TO 4 runs four times, but FOR counter = 0 TO 4 runs five times.

Condition-controlled loops

A condition-controlled loop repeats while, or until, a condition is met. A common form is a WHILE loop.

password = ""

WHILE password != "letmein"
    INPUT password
ENDWHILE

OUTPUT "Unlocked"

Example

Tracing a condition-controlled loop

Suppose the user enters "cat" first, then "letmein".

At the start, password is "", so password != "letmein" is True; the loop runs and the user inputs "cat".
The condition is checked again. "cat" != "letmein" is still True, so the loop runs again and the user inputs "letmein".
The condition is checked again. "letmein" != "letmein" is False, so the loop stops and the program outputs "Unlocked".

Common Mistake

Loops may not run

A WHILE loop checks its condition before running the loop body. If the condition is already False at the start, the loop body will not run at all.

Combining the fundamentals

Real programs usually combine several fundamentals together:

CONSTANT PASS_MARK = 50
total = 0

FOR counter = 1 TO 3
    INPUT mark
    total = total + mark
NEXT counter

average = total DIV 3

IF average >= PASS_MARK THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Not yet"
ENDIF

This program uses a constant, variables, input, output, assignment, arithmetic operators, comparison operators, selection and iteration.

Exam technique

In the exam

Trace variables carefully: whenever you see an assignment, update the stored value.
For conditions, work out each comparison first, then apply AND, OR and NOT.
For loops, count how many times the loop body runs and watch for off-by-one errors.

Self review

Check yourself

What is the difference between = and == in a program?
When would you use a count-controlled loop instead of a condition-controlled loop?
What are the results of 19 DIV 4 and 19 MOD 4?

Programming fundamentals

What you'll learn

How programs use variables, constants, inputs, outputs and assignments.
How to control the order instructions run using sequence, selection and iteration.
How to recognise and use arithmetic, comparison and Boolean operators.
How to trace small programs accurately, like you would in an exam.

The building blocks of a program

The computer still follows the instructions very literally, in the order and structure you give it.

Definition

High-level language

A high-level language is a programming language that is closer to human language than machine code. It hides many low-level hardware details, so you can focus on solving the problem.

Variables, constants and assignment

Programs need to store data while they run. That data is usually stored using named locations.

Definition

Variables, constants and assignment

An expression is a piece of code that produces a value, such as score + 5 or length * width.

For example:

CONSTANT PASS_MARK = 50
score = 42
score = score + 10

After the final line, score stores 52. The old value, 42, has been overwritten.

Common Mistake

Assignment is not equality

In programming, score = score + 10 does not mean “score is equal to score plus 10” in a maths sense. It means: work out the right-hand side first, then store the result back into score.

Example

Tracing assignments

score = 10
score = score + 5
bonus = score DIV 4
OUTPUT bonus

The first assignment stores 10 in score.
In score = score + 5, the old value of score is used first: 10 plus 5 gives 15, so score becomes 15.
bonus = score DIV 4 uses whole-number quotient division: 15 DIV 4 gives 3.
The program outputs the value stored in bonus, so the output is 3.

Inputs and outputs

An input is data that enters a program, such as a value typed by a user. An output is data sent out by a program, such as text displayed on a screen.

In OCR-style pseudocode, you may see:

INPUT length
INPUT width
area = length * width
OUTPUT area

This program takes two inputs, calculates an area, and outputs the result.

Key Idea

Programs process data

Most simple programs follow this pattern: get input, store data in variables, process it using operators, then output a result.

Operators

An operator is a symbol or word that performs an action on one or more values. For example, + adds values and > compares values.

Arithmetic operators

Arithmetic operators are used for calculations.

Operator	Meaning	Example result
`+`	Addition	`7 + 3` gives 10
`-`	Subtraction	`7 - 3` gives 4
`*`	Multiplication	`7 * 3` gives 21
`/`	Division	`8 / 2` gives 4
`MOD`	Modulo: the remainder after division	`17 MOD 5` gives 2
`DIV`	Quotient: whole-number division	`17 DIV 5` gives 3
`^`	Exponentiation: to the power of	`2 ^ 3` gives 8

Tip

DIV and MOD work as a pair

For 17 DIV 5 and 17 MOD 5, ask: how many whole 5s fit into 17, and what is left over? Three 5s fit into 17, with 2 left over.

Comparison operators

A comparison operator compares two values and gives a Boolean result: either True or False.

Operator	Meaning
`==`	Equal to
`!=`	Not equal to
`<`	Less than
`<=`	Less than or equal to
`>`	Greater than
`>=`	Greater than or equal to

Common Mistake

Using = instead of ==

Use = for assignment, such as age = 15. Use == for comparison, such as age == 15. In exam questions, this difference is often important.

Boolean operators: AND, OR and NOT

A Boolean value is a value that can only be True or False.

Boolean operators combine or change Boolean values:

A	B	A AND B	A OR B
True	True	True	True
True	False	False	True
False	True	False	True
False	False	False	False

A	NOT A
True	False
False	True

AND is only True if both sides are True.
OR is True if at least one side is True.
NOT reverses a Boolean value.

Example

Evaluating Boolean logic

Given:

mark = 62
attendance = 88
lateWork = False
result = (mark >= 50 AND attendance >= 90) OR NOT lateWork

Evaluate the comparisons: mark >= 50 is True, but attendance >= 90 is False.
Combine them with AND: True AND False gives False.
Evaluate NOT lateWork: since lateWork is False, NOT lateWork gives True.
Combine the final parts with OR: False OR True gives True, so result stores True.

Control flow

Control flow means the order in which the instructions in a program are executed. GCSE programming uses three basic programming constructs: sequence, selection and iteration.

Flowchart-style schematic showing sequence, selection and iteration in programming

Sequence

Sequence means instructions run one after another, from top to bottom.

INPUT length
INPUT width
area = length * width
OUTPUT area

Here, the program must get the inputs before it can calculate the area. If the order is wrong, the program may use missing or incorrect values.

Selection

Selection means the program chooses between different paths using a condition. A condition is an expression that evaluates to True or False.

IF temperature < 18 THEN
    OUTPUT "Wear a coat"
ELSE
    OUTPUT "No coat needed"
ENDIF

The IF branch runs when the condition is True. The ELSE branch runs when the condition is False.

Example

Choosing a branch

Given:

temperature = 17

IF temperature < 18 THEN
    OUTPUT "Wear a coat"
ELSE
    OUTPUT "No coat needed"
ENDIF

Evaluate the condition temperature < 18 using the stored value 17.
Since 17 is less than 18, the condition is True.
The program follows the IF branch and outputs "Wear a coat"; the ELSE branch is skipped.

You may also see multiple branches using ELSE IF, ELIF or similar wording, depending on the language.

Iteration

Iteration means repetition. A repeated section of code is called a loop.

There are two GCSE loop types you need to know: count-controlled loops and condition-controlled loops.

Count-controlled loops

A count-controlled loop repeats a set number of times. It usually uses a counter variable.

total = 0

FOR counter = 1 TO 4
    total = total + counter
NEXT counter

OUTPUT total

Example

Tracing a count-controlled loop

On the first repetition, counter is 1, so total = total + counter changes total from 0 to 1.
On the second repetition, counter is 2, so total changes from 1 to 3.
On the third repetition, counter is 3, so total changes from 3 to 6.
On the fourth repetition, counter is 4, so total changes from 6 to 10. The loop has now completed, so the program outputs 10.

Common Mistake

Off-by-one errors

Check the start and end values of a count-controlled loop carefully. FOR counter = 1 TO 4 runs four times, but FOR counter = 0 TO 4 runs five times.

Condition-controlled loops

A condition-controlled loop repeats while, or until, a condition is met. A common form is a WHILE loop.

password = ""

WHILE password != "letmein"
    INPUT password
ENDWHILE

OUTPUT "Unlocked"

Example

Tracing a condition-controlled loop

Suppose the user enters "cat" first, then "letmein".

At the start, password is "", so password != "letmein" is True; the loop runs and the user inputs "cat".
The condition is checked again. "cat" != "letmein" is still True, so the loop runs again and the user inputs "letmein".
The condition is checked again. "letmein" != "letmein" is False, so the loop stops and the program outputs "Unlocked".

Common Mistake

Loops may not run

A WHILE loop checks its condition before running the loop body. If the condition is already False at the start, the loop body will not run at all.

Combining the fundamentals

Real programs usually combine several fundamentals together:

CONSTANT PASS_MARK = 50
total = 0

FOR counter = 1 TO 3
    INPUT mark
    total = total + mark
NEXT counter

average = total DIV 3

IF average >= PASS_MARK THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Not yet"
ENDIF

This program uses a constant, variables, input, output, assignment, arithmetic operators, comparison operators, selection and iteration.

Exam technique

In the exam

Trace variables carefully: whenever you see an assignment, update the stored value.
For conditions, work out each comparison first, then apply AND, OR and NOT.
For loops, count how many times the loop body runs and watch for off-by-one errors.

Self review

Check yourself

What is the difference between = and == in a program?
When would you use a count-controlled loop instead of a condition-controlled loop?
What are the results of 19 DIV 4 and 19 MOD 4?

Programming fundamentals

What you'll learn

The building blocks of a program

Variables, constants and assignment

Inputs and outputs

Operators

Arithmetic operators

Comparison operators

Boolean operators: AND, OR and NOT

Control flow

Sequence

Selection

Iteration

Count-controlled loops

Condition-controlled loops

Combining the fundamentals

Recap questions

You've reached the end

Programming fundamentals

What you'll learn

The building blocks of a program

Variables, constants and assignment

Inputs and outputs

Operators

Arithmetic operators

Comparison operators

Boolean operators: AND, OR and NOT

Control flow

Sequence

Selection

Iteration

Count-controlled loops

Condition-controlled loops

Combining the fundamentals

Recap questions

You've reached the end

Programming fundamentals

What you'll learn

The building blocks of a program

Variables, constants and assignment

Inputs and outputs

Operators

Arithmetic operators

Comparison operators

Boolean operators: AND, OR and NOT

Control flow

Sequence

Selection

Iteration

Count-controlled loops

Condition-controlled loops

Combining the fundamentals

Recap questions

You've reached the end

Programming fundamentals Revision Guide

Programming fundamentals

What you'll learn

The building blocks of a program

Variables, constants and assignment

Inputs and outputs

Operators

Arithmetic operators

Comparison operators

Boolean operators: AND, OR and NOT

Control flow

Sequence

Selection

Iteration

Count-controlled loops

Condition-controlled loops

Combining the fundamentals

Recap questions

You've reached the end

Programming fundamentals Revision Guide