Sequence, Selection and Iteration

Sequence is the simplest construct: statements execute one after another, in the exact order they appear. There is no branching, no repetition - just a straight-line execution from top to bottom. It is the default behaviour of any algorithm unless selection or iteration keywords appear.

Even the most complex programs ultimately reduce to sequences of instructions at the hardware level. Everything else is just a way of controlling which sequences run in which order.

// A simple sequential calculation (AQA)
OUTPUT "Enter length: "
length ← USERINPUT
OUTPUT "Enter width: "
width ← USERINPUT
area ← length * width
perimeter ← 2 * (length + width)
OUTPUT "Area: " + area
OUTPUT "Perimeter: " + perimeter

# A simple sequential calculation (OCR)
print("Enter length: ")
length = input()
print("Enter width: ")
width = input()
area = length * width
perimeter = 2 * (length + width)
print("Area: " + area)
print("Perimeter: " + perimeter)

Selection allows an algorithm to choose between different paths based on whether a condition is true or false. Without selection, every user would get the same output regardless of their input, which makes most real programs impossible.

Selection is implemented with IF/ELSE or CASE (switch) statements. The condition is always a Boolean expression - one that evaluates to either True or False.

Basic IF/ELSE:

score ← USERINPUT
IF score >= 50 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

score = input()
if score >= 50 then
    print("Pass")
else
    print("Fail")
end if

Nested IF (ELSEIF) for multiple grades:

score ← USERINPUT
IF score >= 90 THEN
    OUTPUT "Grade A"
ELSEIF score >= 70 THEN
    OUTPUT "Grade B"
ELSEIF score >= 50 THEN
    OUTPUT "Grade C"
ELSE
    OUTPUT "Fail"
ENDIF

score = input()
if score >= 90 then
    print("Grade A")
elseif score >= 70 then
    print("Grade B")
elseif score >= 50 then
    print("Grade C")
else
    print("Fail")
end if

Iteration repeats a block of code. This is essential for processing lists, validating input, searching, sorting - almost every non-trivial algorithm needs at least one loop. There are two types: count-controlled (FOR) and condition-controlled (WHILE).

Use a FOR loop when you know exactly how many times the loop should run before it starts. The loop counter is set to a start value, compared to an end value, and incremented automatically each iteration.

// Print 5 to 1 countdown (AQA)
FOR i ← 5 TO 1 STEP -1
    OUTPUT i
ENDFOR
OUTPUT "Blast off!"

# Print 5 to 1 countdown (OCR)
for i = 5 to 1 step -1
    print(i)
next i
print("Blast off!")

Use a WHILE loop when you do not know in advance how many iterations are needed - the loop continues until a condition becomes false. The condition is checked before the loop body executes, so if the condition is already false, the loop body never runs.

// Find the first power of 2 greater than 1000 (AQA)
n ← 1
WHILE n <= 1000
    n ← n * 2
ENDWHILE
OUTPUT n    // outputs 1024

# Find the first power of 2 greater than 1000 (OCR)
n = 1
while n <= 1000
    n = n * 2
end while
print(n)    # outputs 1024

Real algorithms combine all three constructs. This example counts how many numbers entered by the user are above average. It uses sequence (to store running totals), iteration (to process each input), and selection (to compare each value to the average).

// Count how many of 5 numbers are above the average (AQA)
total ← 0
FOR i ← 1 TO 5            // ITERATION: repeat 5 times
    OUTPUT "Enter number " + i
    num ← USERINPUT
    total ← total + num    // SEQUENCE: update running total
ENDFOR
average ← total / 5
OUTPUT "Average: " + average
aboveCount ← 0
FOR i ← 1 TO 5            // Second pass to count above-average values
    OUTPUT "Enter number " + i
    num ← USERINPUT
    IF num > average THEN    // SELECTION: compare to average
        aboveCount ← aboveCount + 1
    ENDIF
ENDFOR
OUTPUT aboveCount + " numbers above average"

# Count how many of 5 numbers are above the average (OCR)
total = 0
for i = 1 to 5
    print("Enter number " + i)
    num = input()
    total = total + num
next i
average = total / 5
print("Average: " + average)
aboveCount = 0
for i = 1 to 5
    print("Enter number " + i)
    num = input()
    if num > average then
        aboveCount = aboveCount + 1
    end if
next i
print(aboveCount + " numbers above average")