CS7: Programming Constructs
Understanding variables, constants, sequence, selection (IF/CASE), and iteration (FOR/WHILE/REPEAT) - the building blocks of all programs.
📦 Variables and Constants
Variable: A named storage location in memory whose value can change during program execution. Constant: A named value that cannot be changed once it has been set.
| Feature | Variable | Constant |
|---|---|---|
| Value can change? | Yes | No |
| When to use | Values that need updating (score, counter) | Fixed values (PI, tax rate, max attempts) |
| Example | score ← 0, then score ← score + 1 | CONST PI ← 3.14159 |
| Advantage | Flexible - can be updated | Prevents accidental changes, makes code readable |
Variables vs Constants
CONST VAT_RATE ← 0.20 // Constant - won't change CONST MAX_SCORE ← 100 // Constant - won't change score ← 0 // Variable - will change name ← "" // Variable - will change score ← score + 10 // OK - variable can change // VAT_RATE ← 0.25 // ERROR - constant cannot change
📝 Assignment
Assignment means storing a value in a variable. The variable name goes on the left, the value or expression goes on the right. The previous value is overwritten.
Assignment Examples
age ← 16 // Assign the value 16 name ← "Bob" // Assign the string "Bob" total ← price * quantity // Assign the result of an expression counter ← counter + 1 // Increment: add 1 to current value
Key Rule: Assignment is NOT the same as mathematical equality.
counter ← counter + 1 means "take the current value of counter, add 1, and store the result back in counter". In maths, x = x + 1 has no solution!
counter ← counter + 1 means "take the current value of counter, add 1, and store the result back in counter". In maths, x = x + 1 has no solution!
➡️ Sequence
Sequence means executing instructions one after another in order, from top to bottom. This is the simplest programming construct - every program uses sequence.
Sequence Example
INPUT "Enter length: ", length INPUT "Enter width: ", width area ← length * width OUTPUT "Area = ", area
Each line executes in order: input, input, calculate, output. No decisions, no repetition.
🔀 Selection
Selection allows a program to choose between different paths based on a condition. The three forms are: IF...THEN, IF...THEN...ELSE, and CASE/SWITCH.
IF...THEN
Simple IF
IF temperature > 30 THEN
OUTPUT "It is hot today"
ENDIF
The body only executes if the condition is TRUE.
IF...THEN...ELSE
IF with ELSE
IF age >= 17 THEN
OUTPUT "You can learn to drive"
ELSE
OUTPUT "You cannot learn to drive yet"
ENDIF
One path if TRUE, another path if FALSE.
ELSEIF for Multiple Conditions
ELSEIF Chain
IF grade >= 90 THEN
OUTPUT "A*"
ELSEIF grade >= 80 THEN
OUTPUT "A"
ELSEIF grade >= 70 THEN
OUTPUT "B"
ELSEIF grade >= 60 THEN
OUTPUT "C"
ELSE
OUTPUT "Below C"
ENDIF
CASE / SWITCH
CASE Statement
CASE OF day
1 : OUTPUT "Monday"
2 : OUTPUT "Tuesday"
3 : OUTPUT "Wednesday"
4 : OUTPUT "Thursday"
5 : OUTPUT "Friday"
OTHERWISE : OUTPUT "Weekend"
ENDCASE
Use CASE when checking a single variable against several specific values.
| Feature | IF...ELSE | CASE |
|---|---|---|
| Best for | Range checks, complex conditions | Checking specific values of one variable |
| Conditions | Can combine multiple variables | One variable only |
| Example | IF age >= 18 AND hasID THEN | CASE OF menuChoice |
🔄 Iteration
Iteration means repeating a block of code. There are three types: count-controlled (FOR), condition-controlled (WHILE), and post-condition (REPEAT...UNTIL).
FOR Loop (Count-Controlled)
FOR Loop
FOR i ← 1 TO 10
OUTPUT i
NEXT i
// Outputs: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Use FOR when you know exactly how many times you need to repeat.
FOR Loop with STEP
FOR i ← 10 TO 1 STEP -1
OUTPUT i
NEXT i
// Outputs: 10, 9, 8, 7, 6, 5, 4, 3, 2, 1
WHILE Loop (Pre-condition)
WHILE Loop
password ← ""
WHILE password <> "secret"
INPUT "Enter password: ", password
ENDWHILE
OUTPUT "Access granted"
Use WHILE when you don't know how many times to repeat, and the loop may need to execute zero times.
REPEAT...UNTIL (Post-condition)
REPEAT UNTIL Loop
REPEAT
INPUT "Enter a number 1-10: ", num
UNTIL num >= 1 AND num <= 10
Use REPEAT when the loop body must execute at least once.
| Feature | FOR | WHILE | REPEAT...UNTIL |
|---|---|---|---|
| When to use | Known number of repetitions | Unknown, may run 0 times | Unknown, must run at least once |
| Condition checked | Automatically (counter) | Before each iteration | After each iteration |
| Can run zero times? | No (if range is valid) | Yes | No |
| Infinite loop risk | Low (counter handles it) | High (must update condition) | High (must update condition) |
🪆 Nesting Constructs
Nesting means placing one construct inside another. You can put IF statements inside loops, loops inside IF statements, or loops inside other loops. This allows complex logic.
IF inside a FOR loop
FOR i ← 1 TO 20
IF i MOD 2 = 0 THEN
OUTPUT i, " is even"
ELSE
OUTPUT i, " is odd"
ENDIF
NEXT i
WHILE loop inside an IF statement
INPUT difficulty
IF difficulty = "hard" THEN
attempts ← 0
WHILE attempts < 3
INPUT "Guess the number: ", guess
IF guess = secretNumber THEN
OUTPUT "Correct!"
ELSE
OUTPUT "Wrong!"
ENDIF
attempts ← attempts + 1
ENDWHILE
ENDIF
Exam Tip: When nesting, always match your opening and closing keywords. Every IF needs an ENDIF, every WHILE needs an ENDWHILE, and every FOR needs a NEXT. Indent your code to make the structure clear.
❓ Practice Questions
Q1: Explain the difference between a variable and a constant.
Q2: Write pseudo-code that asks for a number and outputs "Positive", "Negative", or "Zero" using selection.
Q3: When would you use a REPEAT...UNTIL loop instead of a WHILE loop?
Q4: Write pseudo-code using a FOR loop to output all multiples of 3 from 3 to 30.
Q5: Write pseudo-code that keeps asking for a password until the user enters "open123", then outputs "Welcome".
✅ Answers
- A variable is a named memory location whose value can change during execution. A constant is a named value that cannot be changed once set. Constants prevent accidental changes and make code more readable.
-
INPUT number IF number > 0 THEN OUTPUT "Positive" ELSEIF number < 0 THEN OUTPUT "Negative" ELSE OUTPUT "Zero" ENDIF - Use REPEAT...UNTIL when the loop body must execute at least once (e.g. input validation). Use WHILE when the loop may need to execute zero times (e.g. searching a list that might be empty).
-
FOR i ← 3 TO 30 STEP 3 OUTPUT i NEXT i -
password ← "" WHILE password <> "open123" INPUT "Enter password: ", password ENDWHILE OUTPUT "Welcome"
🎯 Exam Tips
- Always close your constructs: ENDIF, ENDWHILE, NEXT, ENDCASE
- Use constants (CONST) for values that should never change
- Choose the right loop: FOR for counted, WHILE for pre-condition, REPEAT for post-condition
- WHILE checks before running; REPEAT checks after - know the difference
- Indent nested constructs to show structure clearly
- Remember: sequence, selection, and iteration are the THREE basic constructs
⚠️ Common Errors
✗ Using IF instead of WHILE for repeated actions ✓ IF executes once based on a condition; WHILE repeats as long as the condition is true. Use IF for decisions, WHILE for repetition.
✗ Thinking FOR and WHILE loops are interchangeable in all situations ✓ FOR loops are for counted iteration (known number of repeats); WHILE loops are for conditional iteration (unknown number of repeats until a condition is met).
✗ Forgetting that WHILE checks the condition BEFORE the loop body, so it may never execute ✓ A WHILE loop is pre-tested: if the condition is FALSE initially, the loop body never executes. A REPEAT-UNTIL loop is post-tested and always executes at least once.
✗ Confusing ELSE and ELSEIF/ELIF ✓ ELSE is the default case when no IF or ELSEIF condition is true. ELSEIF/ELIF allows additional conditions to be checked in sequence after the initial IF.
✍️ Model Answer
Full-Mark Response
Write pseudo-code for a program that asks a user to enter a password. The password must be at least 8 characters long and contain at least one digit. The program should keep asking until a valid password is entered, then output 'Password accepted'. [5 marks]
valid ← FALSE WHILE valid = FALSE INPUT 'Enter password: ', password IF LENGTH(password) >= 8 THEN found_digit ← FALSE FOR i ← 0 TO LENGTH(password) - 1 IF password[i] >= '0' AND password[i] <= '9' THEN found_digit ← TRUE ENDIF NEXT i IF found_digit = TRUE THEN valid ← TRUE ELSE OUTPUT 'Password must contain at least one digit' ENDIF ELSE OUTPUT 'Password must be at least 8 characters' ENDIF ENDWHILE OUTPUT 'Password accepted'
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Computational Thinking — 40%): Demonstrate knowledge and understanding of the principles and concepts of computer science, including programming constructs: sequence, selection and iteration for AQA 8525, OCR J277 & Edexcel 1CP2.
AO2 (Application — 40%): Apply knowledge and understanding of computer science, including programming constructs: sequence, selection and iteration to analyse problems in computational terms and to design, write and evaluate solutions.
AO3 (Evaluation — 20%): Evaluate the effectiveness, correctness and efficiency of computational solutions, including programming constructs: sequence, selection and iteration, and make reasoned judgements about trade-offs.
📝 Exam Technique
GCSE Computer Science Exam Tips:
Choose the correct construct: IF for decisions, FOR for counted repetition (known number), WHILE for conditional repetition (unknown count). Always indent nested constructs clearly. For validation, use a WHILE loop with a Boolean flag. Show initialisation of variables BEFORE the loop. Remember: WHILE = pre-tested (may not execute), REPEAT-UNTIL = post-tested (always executes at least once). Use ENDIF, ENDWHILE to close each construct.
Choose the correct construct: IF for decisions, FOR for counted repetition (known number), WHILE for conditional repetition (unknown count). Always indent nested constructs clearly. For validation, use a WHILE loop with a Boolean flag. Show initialisation of variables BEFORE the loop. Remember: WHILE = pre-tested (may not execute), REPEAT-UNTIL = post-tested (always executes at least once). Use ENDIF, ENDWHILE to close each construct.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Computer Science exams?
Get the best revision books and guides to boost your grades.