Variables, constants, data types, and input/output

Variables are labelled boxes

A variable is a named location in memory that stores a value. The value can change while the program runs. Think of it as a labelled box: the name is on the label, the value is inside.

In pseudocode, declare each variable with its data type, then use <- to put a value into it. Assignment replaces whatever was in the box before.

DECLARE Score : INTEGER
Score <- 10
Score <- Score + 5
OUTPUT Score

Constants

A constant is a named value that cannot change while the program runs. Using a constant means the value is written in one place, so it is easy to update and impossible to change by accident.

CONSTANT VatRate <- 0.2
DECLARE Price : REAL
INPUT Price
OUTPUT "Price with VAT: ", Price * (1 + VatRate)

Data types

Every variable has a data type that sets what it can hold:

Input and output

INPUT waits for the user to type a value and stores it in a variable. OUTPUT displays values. Separate several values with commas.

In PseudoEditor, the terminal asks for each INPUT while the program runs, so you can test interactive programs in the browser.

DECLARE Name : STRING
DECLARE Age : INTEGER
OUTPUT "What is your name?"
INPUT Name
OUTPUT "How old are you?"
INPUT Age
OUTPUT "Hello ", Name, ", next year you will be ", Age + 1

Sequence

Sequence means statements run one after another, in order, from top to bottom. Changing the order changes the result: in the program above, the age must be input before it is used.

Key terms

Variable

A named memory location whose value can change while the program runs.

Constant

A named value that cannot change while the program runs.

Data type

The kind of data a variable holds, such as INTEGER or STRING.

Sequence

Instructions executed one after another in order.

Check yourself