Data types

Every value has a data type that tells the computer how to store and use it:

• Integer — a whole number, e.g. 42, -7.
• Real / float — a number with a decimal point, e.g. 3.14.
• Boolean — only True or False.
• Character — a single symbol, e.g. 'A'.
• String — a sequence of characters, e.g. "hello".

Watch out: "42" (in quotes) is a string, not an integer — you can't do arithmetic on it until you convert it with something like int("42"). Choosing the right type is called casting when you convert.

Which data type?

Variables & constants

• A variable is a named store whose value can change while the program runs, e.g. score = score + 1.
• A constant is a named value that must not change, e.g. VAT = 0.20 or PI = 3.14159. Using a constant makes code clearer and easier to update.

Assignment: total = price + 10 means "work out the right-hand side, then store it in total". The = sign is assignment, not "equals" in the maths sense.

Operators

• Arithmetic: + − * /, plus ** (power), // (integer division / DIV) and % (remainder / MOD).
• Comparison: == (equal to), != (not equal), <, >, <=, >=.
• Logical: AND, OR, NOT — combine conditions.

Common trap: = assigns a value; == tests whether two values are equal. Mixing them up is a classic bug.

Your turn — remainder (MOD)

Sequence, selection & iteration

Every program is built from three constructs:

• Sequence — statements run in order, one after another.
• Selection — choose a path using IF / ELIF / ELSE.
• Iteration (loops) — repeat code. A FOR loop repeats a set number of times; a WHILE loop repeats while a condition is true (a condition-controlled loop).

FOR vs WHILE: use FOR when you know how many repeats; use WHILE when you repeat until something happens (e.g. until the user types "quit").

Your turn — how many times?

Read the code

Arrays & lists

An array (a list in Python) stores many values under one name. Each value has a position number called its index, starting at 0.

Off-by-one trap: a list of 4 items has indexes 0 to 3. Asking for index 4 gives an out-of-range error.

What data type is each value?

Tap a value, then tap its data type.

Your turn — the last index

Subprograms: functions & procedures

A subprogram is a named block of code you can call whenever you need it. This supports decomposition and avoids repeating code.

• A function returns a value, e.g. area(w, h) returns w × h.
• A procedure does a job but doesn't return a value (in Python both use def).
• Values passed in are parameters/arguments; the answer sent back uses return.

Benefits: subprograms make code easier to read, test and reuse, and let a team work on different parts.

String manipulation

Strings can be measured and sliced:

• Length: len("cat") → 3.
• Index a character: "cat"[0] → "c".
• Slice/substring: "computer"[0:4] → "comp" (positions 0,1,2,3).
• Join (concatenate): "AB" + "CD" → "ABCD".
• Change case: "hi".upper() → "HI".

Slicing rule: [start:end] includes start but stops before end. So [0:4] gives 4 characters, not 5.

Read the string code

File handling

Programs can read from and write to text files so data is saved permanently (not lost when the program closes). Typical steps:

• Open the file, choosing a mode: read ("r"), write ("w", overwrites) or append ("a", adds to the end).
• Read a line or write a line of text.
• Close the file to save changes and free it.

Careful: opening in "w" mode deletes the old contents. Use "a" (append) if you want to keep what's already there.

Which file mode?

Match the description to the term

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

Which loop?

Function or procedure?

The big ideas to know

You've covered the whole of Edexcel Programming. Press Finish to see your score.