SYLLABUS 8.1.1–8.1.3 · PAPER 2 · ALSO FOR 0984 AND 2210

Variables, constants, data types, input and output

Every program stores values while it runs. This topic covers naming them, choosing the right data type, and getting values in and out of a program.

Variables and constants

A variable is a named store whose value can change while the program runs. A constant is a named value that is set once and can’t change. Using a constant, such as a tax rate, means the value is written in one place: to change it you edit one line, and the program can’t change it by mistake.

CONSTANT TaxRate <- 0.2
DECLARE Price : REAL
DECLARE Item : STRING
OUTPUT "Enter the item and its price"
INPUT Item
INPUT Price
OUTPUT Item, "costs", Price * (1 + TaxRate), "with tax"

The data types

INTEGER: a whole number, such as 42 or −7.
REAL: a number with a fractional part, such as 3.75.
CHAR: a single character, such as 'Y'.
STRING: text of any length, such as "Redpen".
BOOLEAN: TRUE or FALSE.

Input and output

INPUT reads a value into a variable, and OUTPUT shows values on screen. Several items can be output together by separating them with commas. A prompt before each INPUT tells the user what to type.

Where marks go

Choosing INTEGER for something that can have a fractional part, like a price.
Storing a phone number as INTEGER: it isn’t used in calculations and may start with 0, so STRING fits.
Explaining a constant as “a value that doesn’t change” without saying why that helps.
Writing an INPUT with no variable to store the value in.
Mixing up CHAR (one character) with STRING.

Try a question

4 MARKS · MARKED ON THIS PAGE

A stationery shop adds tax to every sale. Write pseudocode that: - declares a constant TaxRate with the value 0.2 - declares a REAL variable Price, an INTEGER variable Quantity and a REAL variable Total - inputs Price (the price of one item) and then Quantity (the number of items bought) - calculates Total as Price multiplied by Quantity, with tax added using TaxRate (Total = Price x Quantity x (1 + TaxRate)) - outputs "Total: " followed by Total rounded to 2 decimal places using ROUND Do not output any prompt messages.

solutionPseudocode

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