Control Flow: Sequence, Selection and Iteration

Variables, constants and assignment

A variable is a named place in memory that holds a value. The value can change while the program runs. A constant also has a name and a value, but the value is fixed for the whole program, for example CONST VAT ← 0.2.

Assignment means storing a value in a variable. In pseudocode we write score ← 10 (in Python: score = 10). The right side is worked out first, then stored on the left, so score ← score + 1 adds one.

Meaningful identifiers

An identifier is the name you give a variable, constant or subroutine. Good names say what the value is: totalPrice is better than tp or x. Clear names make code easier to read, fix and share. Names cannot have spaces and usually cannot start with a digit.

Why use constants? If the tax rate changes, you edit one line, and you cannot change it by mistake later in the program.

Sequence: one step after another

Sequence means instructions run in the order they are written, top to bottom. Changing the order can change the answer.

x ← 4
y ← x × 2
OUTPUT y      → 8

If you swap the first two lines, y is worked out before x has a value, so the program fails or gives a wrong answer.

Selection: IF, ELSE and CASE

Selection lets a program choose. A condition is a question that is either True or False, such as mark ≥ 50.

IF mark ≥ 50 THEN
   OUTPUT 'Pass'
ELSE
   OUTPUT 'Try again'
ENDIF

Only one branch runs. For more than two choices use ELSE IF (Python elif), or a CASE/match statement that checks one value against a list. Conditions use comparison operators (=, ≠, <, >, ≤, ≥) and can be joined with AND, OR, NOT.

Iteration: definite and indefinite loops

Iteration means repeating. It saves writing the same lines again and again.

Definite (count-controlled) loop

The number of repeats is known before the loop starts. Use FOR.

FOR i ← 1 TO 5
   OUTPUT i
ENDFOR        → 1 2 3 4 5

Indefinite (condition-controlled) loop

The loop repeats until a condition changes, so the number of turns is not known in advance.

• WHILE … ENDWHILE checks the condition at the start. If it is False at the start, the body runs zero times.
• REPEAT … UNTIL (do-while) checks at the end, so the body always runs at least once.

total ← 0
WHILE total < 20
   total ← total + 6
ENDWHILE
OUTPUT total  → 24

Typical uses: asking for a password until it is right, reading input until the user types 'stop'. Danger: if the condition never becomes False you get an infinite loop.

Nesting

Nesting means putting one control structure inside another: an IF inside a loop, a loop inside a loop, or an IF inside an IF. Indentation shows what is inside what.

FOR i ← 1 TO 4
   IF i MOD 2 = 0 THEN
      OUTPUT i, ' even'
   ELSE
      OUTPUT i, ' odd'
   ENDIF
ENDFOR

A loop inside a loop multiplies the work: an outer loop of 3 and an inner loop of 4 run the inner body 3 × 4 = 12 times. This is how you print a times-table grid or walk through every cell of a 2D array.

Try it: trace table at home

Take a sheet of paper and draw columns for each variable. Run the WHILE example from this page line by line, writing each new value in a new row. Then check your last row with the 3D at step 5. Next, change the rule to total ← total + 7: predict how many turns it takes, then test it in the 3D idea or in any free online Python editor.

Key formulas and definitions

• Assignment: name ← value (Python: name = value)
• Selection: IF condition THEN … ELSE … ENDIF
• Definite loop: FOR i ← 1 TO n … ENDFOR (runs n times)
• Indefinite loop: WHILE condition … ENDWHILE (0 or more times)
• REPEAT … UNTIL condition (1 or more times)
• Nested loops: total runs = outer count × inner count

Worked examples

Common mistakes

• Using a FOR loop when the number of repeats is unknown (e.g. until the user types 'stop'). Use WHILE or REPEAT…UNTIL.
• Forgetting to change the loop variable inside a WHILE loop, which makes an infinite loop.
• Thinking both branches of IF…ELSE run. Only one branch runs each time.
• Off-by-one counting: FOR i ← 1 TO 10 runs 10 times, but FOR i ← 0 TO 10 runs 11 times.