Pseudocode Command Set - GCSE Computers

Pseudocode Command Set

Questions in the written examination that involve code will use this pseudocode for clarity and consistency. However, students may answer questions using any valid method.

Data types

INTEGER
REAL
BOOLEAN
CHARACTER

Type coercion

Type coercion is automatic if indicated by context. For example 3 + 8.25 = 11.25 (integer + real = real)

Arithmetic Coercion

Mixed mode arithmetic coercion

	INTEGER	REAL
INTEGER	INTEGER	REAL
REAL	REAL	REAL

Explicit Coercion & Constants

Explicit Coercion

Coercion can be made explicit. For example, RECEIVE age FROM (INTEGER) KEYBOARD assumes that the input from the keyboard is interpreted as an INTEGER, not a STRING.

Constants

The value of constants can only ever be set once. They are identified by the keyword CONST. Two examples of using a constant are shown.

CONST REAL PI
SET PI TO 3.14159
SET circumference TO radius * PI * 2

Data structures

ARRAY
STRING
Indices start at zero (0) for all data structures.
All data structures have an append operator, indicated by &.
Using & with a STRING and a non-STRING will coerce to STRING. For example, SEND 'Fred' & age TO DISPLAY, will display a single STRING of 'Fred18'.

Identifiers

Identifiers are sequences of letters, digits and '_', starting with a letter, for example: MyValue, myValue, My_Value, Counter2

Functions

LENGTH(): For data structures consisting of an array or string.
RANDOM(n): This generates a random number from 0 to n.

Comments

Comments are indicated by the # symbol, followed by any text.
A comment can be on a line by itself or at the end of a line.

Devices

Use of KEYBOARD and DISPLAY are suitable for input and output.
Additional devices may be required, but their function will be obvious from the context. For example, CARD_READER and MOTOR are two such devices.

Notes

In the following pseudocode, the < > indicates where expressions or values need to be supplied. The < > symbols are not part of the pseudocode.

Pseudocode Syntax Reference

Variables and arrays

Syntax	Explanation of syntax	Example
`SET Variable TO <value>`	Assigns a value to a variable.	`SET Counter TO 0 SET MyString TO 'Hello world'`
`SET Variable TO <expression>`	Computes the value of an expression and assigns to a variable.	`SET Sum TO Score + 10 SET Size TO LENGTH(Word)`
`SET Array[index] TO <value>`	Assigns a value to an element of a one-dimensional array.	`SET ArrayClass[1] TO 'Ann' SET ArrayMarks[3] TO 56`
`SET Array TO [<value>, ...]`	Initialises a one-dimensional array with a set of values.	`SET ArrayValues TO [1, 2, 3, 4, 5]`
`SET Array[RowIndex, ColumnIndex] TO <value>`	Assigns a value to an element of a two-dimensional array.	`SET ArrayClassMarks[2,4] TO 92`

Selection

Syntax Explanation of syntax Example

IF <expression> THEN
  <command>
END IF

If <expression> is true then command is executed.

IF Answer = 10 THEN
  SET Score TO Score + 1
END IF

IF <expression> THEN
  <command>
ELSE
  <command>
END IF

If <expression> is true then first <command> is executed, otherwise second <command> is executed.

IF Answer = 'correct' THEN
  SEND 'Well done' TO DISPLAY
ELSE
  SEND 'Try again' TO DISPLAY
END IF

Repetition

Syntax	Explanation of syntax	Example
`WHILE <condition> DO <command> END WHILE`	Pre-conditioned loop. Executes <command> whilst <condition> is true.	`WHILE Flag = 0 DO SEND 'All well' TO DISPLAY END WHILE`
`REPEAT <command> UNTIL <expression>`	Post-conditioned loop. Executes <command> until <condition> is true. The loop must execute at least once.	`REPEAT SET Go TO Go + 1 UNTIL Go = 10`
`REPEAT <expression> TIMES <command> END REPEAT`	Count controlled loop. The number of times <command> is executed is determined by the expression.	`REPEAT 100-Number TIMES SEND '*' TO DISPLAY END REPEAT`
`FOR <id> FROM <expression> TO <expression> DO <command> END FOR`	Count controlled loop. Executes <command> a fixed number of times.	`FOR Index FROM 1 TO 10 DO SEND ArrayNumbers[Index] TO DISPLAY END FOR`
`FOR <id> FROM <expression> TO <expression> STEP <expression> DO <command> END FOR`	Count controlled loop using a step.	`FOR Index FROM 1 TO 500 STEP 25 DO SEND Index TO DISPLAY END FOR`
`FOR EACH <id> FROM <expression> DO <command> END FOREACH`	Count controlled loop. Executes for each element of an array.	`SET WordsArray TO ['The', 'Sky', 'is', 'grey'] SET Sentence to '' FOR EACH Word FROM WordsUArray DO SET Sentence TO Sentence & Word & ' ' END FOREACH`

Input/output

Syntax	Explanation of syntax	Example
`SEND <expression> TO DISPLAY`	Sends output to the screen.	`SEND 'Have a good day.' TO DISPLAY`
`RECEIVE <identifier> FROM (type) <device>`	Reads input of specified type.	`RECEIVE Name FROM (STRING) KEYBOARD RECEIVE LengthOfJourney FROM (INTEGER) CARD_READER RECEIVE YesNo FROM (CHARACTER) CARD_READER`

File handling

Syntax	Explanation of syntax	Example
`READ <File> <record>`	Reads in a record from a <file> and assigns to a <variable>. Each READ statement reads a record from the file.	`READ MyFile.doc Record`
`WRITE <File> <record>`	Writes a record to a file. Each WRITE statement writes a record to the file.	`WRITE MyFile.doc Answer1, Answer2, 'xyz 01'`

Subprograms

Syntax Explanation of syntax Example

PROCEDURE <id> (<parameter>, ...)
BEGIN PROCEDURE
  <command>
END PROCEDURE

Defines a procedure.

PROCEDURE CalculateAverage (Mark1, Mark2, Mark3)
BEGIN PROCEDURE
  SET Avg to (Mark1 + Mark2 + Mark3)/3
END PROCEDURE

FUNCTION <id> (<parameter>, ...)
BEGIN FUNCTION
  <command>
  RETURN <expression>
END FUNCTION

Defines a function.

FUNCTION AddMarks (Mark1, Mark2, Mark3)
BEGIN FUNCTION
  SET Total to (Mark1 + Mark2 + Mark3)/3
  RETURN Total
END FUNCTION

<id> (<parameter>, ...) Calls a procedure or a function. Add (FirstMark, SecondMark)

Arithmetic operators

Symbol	Description
`+`	Add
`-`	Subtract
`/`	Divide
`*`	Multiply
`^`	Exponent
`MOD`	Modulo
`DIV`	Integer division

Relational operators

Symbol	Description
`=`	equal to
`<>`	not equal to
`>`	greater than
`>=`	greater than or equal to
`<`	less than
`<=`	less than or equal to

Logical operators

Symbol	Description
`AND`	Returns true if both conditions are true.
`OR`	Returns true if any of the conditions are true.
`NOT`	Reverses the outcome of the expression; true becomes false, false becomes true.