Variables, constants & data types

A program stores data in named locations:

• Variable — a named store whose value can change while the program runs (e.g. score).
• Constant — a named store whose value is fixed and never changes (e.g. VAT = 0.20).

Every value has a data type:

• Integer — whole number (7)
• Real / float — decimal number (3.14)
• Boolean — True or False
• Character — a single symbol ('A')
• String — text ("hello")

Nail it: a variable changes; a constant stays the same. Choosing the right data type saves memory and prevents errors.

Which data type?

Operators: arithmetic, comparison & Boolean

Operators act on values:

• Arithmetic — + − * /, plus MOD (remainder), DIV (integer/whole division) and ^ (exponent). e.g. 7 MOD 3 = 1; 7 DIV 3 = 2.
• Comparison — == equal, != not equal, <, >, <=, >=. These give a Boolean result.
• Boolean (logical) — AND, OR, NOT to combine conditions.

MOD & DIV: MOD gives the remainder, DIV gives the whole-number quotient. e.g. 17 MOD 5 = 2 and 17 DIV 5 = 3.

Your turn — MOD

Sequence, selection & iteration

All programs are built from three constructs:

• Sequence — instructions carried out one after another in order.
• Selection — a decision: run different code depending on a condition (if…then…else, switch/case).
• Iteration — repeating code. A for loop repeats a set number of times (count-controlled); a while loop repeats while a condition is true (condition-controlled).

Loops: use a for loop when you know how many times to repeat; use a while loop when you repeat until a condition changes.

Which loop?

String manipulation & arrays

String manipulation lets you work with text: find its length, take a substring, change case, and join (concatenate) strings.

• e.g. length of "cat" = 3; "cat"+"dog" = "catdog".
• Arrays store many values under one name, accessed by an index (usually starting at 0). e.g. scores[0] is the first item.
• Arrays can be 1D (a list) or 2D (a grid/table).

Watch the index: most arrays start at index 0, so an array of 5 items has indexes 0–4.

Your turn — array index

Which construct?

Tap a code idea, then tap which construct it uses.

File handling, SQL & subprograms

Three more essentials:

• File handling — programs can open, read, write and close text files to store data permanently.
• SQL — used to search a database. SELECT columns FROM a table WHERE a condition is true (e.g. SELECT name FROM pupils WHERE age > 15).
• Subprograms — named blocks of reusable code. A procedure carries out a task; a function also returns a value. They make code shorter, clearer and reusable.

Procedure vs function: a function returns a value; a procedure just does a job. SQL SELECT…FROM…WHERE retrieves matching records.

Read the SQL

Procedure vs function

Match term to meaning

Tap a term on the left, then its matching description on the right.

The big ideas to know

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