Designing and Refining Algorithms

Before writing an algorithm, identify its inputs (data it receives), processes (operations it performs), and outputs (results it produces). This IPO model defines what the algorithm must do before any code is written.

Worked example — an algorithm that calculates the area of a rectangle:

Component	Detail
Inputs	Length (number), Width (number)
Process	Multiply length by width
Output	Area (number)

A structure diagram shows how a problem is decomposed into sub-problems as a hierarchy of boxes. Each box is a sub-task; lines show which sub-tasks belong to which parent task.

Producing this diagram before coding ensures every sub-task is accounted for. OCR exams may ask you to complete or extend a partially drawn structure diagram.

Pseudocode is a structured, language-independent way to write algorithms. It is not valid code in any specific language, but it is precise enough to translate directly into one.

Common pseudocode conventions used in OCR J277:

Construct	Pseudocode example
Assignment	score = 0
Output	OUTPUT "Enter a number: "
Input	x = INPUT()
Selection	IF x > 0 THEN ... ELSE ... END IF
Count-controlled loop	FOR i = 1 TO 10 ... END FOR
Condition-controlled loop	WHILE x != 0 ... END WHILE

Worked example — algorithm to find the largest of three numbers:

largest = a
IF b > largest THEN
    largest = b
END IF
IF c > largest THEN
    largest = c
END IF
OUTPUT "Largest is " + largest

Pseudocode is read and written in exams. Practise converting between pseudocode, flowcharts, and actual code so you can move fluently between all three.

A flowchart uses standard symbols to show the sequence, decisions, and loops in an algorithm. OCR J277 requires knowledge of these six symbols:

Symbol	Shape description	Used for
Terminal	Rounded rectangle (oval/stadium)	Start and End of the algorithm
Process	Rectangle	An action or calculation
Input / Output	Parallelogram	Reading input or displaying output
Decision	Diamond (rhombus)	A yes/no or true/false branch
Sub program	Rectangle with double vertical bars	Calling a named sub-program
Line (flow)	Arrow	Direction of execution

Worked example — flowchart for "find the larger of two numbers A and B":

A decision diamond always has exactly two exit paths, labelled YES/NO or TRUE/FALSE. Every path through the flowchart must eventually reach the terminal.

A trace table records the value of every variable after each step of an algorithm. Trace tables let you verify whether an algorithm produces correct output — and pinpoint exactly where it goes wrong if it does not.

Worked example — trace the algorithm below for input n = 5:

total = 0
i = 1
WHILE i <= n
    total = total + i
    i = i + 1
END WHILE
OUTPUT total

Step	i	total	Note
Start	1	0	Initial values
Loop 1	1	1	total = 0+1; i checked: 1≤5
After inc	2	1	i = 1+1
Loop 2	2	3	total = 1+2
After inc	3	3
Loop 3	3	6	total = 3+3
After inc	4	6
Loop 4	4	10	total = 6+4
After inc	5	10
Loop 5	5	15	total = 10+5
After inc	6	15	i = 5+1; now 6>5, loop ends
Output	—	—	15

Output: 15 (which equals 1+2+3+4+5). The algorithm correctly sums integers from 1 to n.

Two categories of error arise when algorithms are written as programs:

Error type	What it is	Effect on the program
Syntax error	Breaks the grammatical rules of the programming language	The program cannot be run or translated
Logic error	The program runs but produces incorrect or unexpected output	The program runs but gives wrong results

Syntax error example — missing THEN keyword:

IF score > 10   ← missing THEN
    OUTPUT "Pass"
END IF

The translator cannot parse this. It will report an error before the program runs.

Logic error example — wrong comparison operator:

IF score > 10 THEN    ← should be >= 10 for "pass at 10"
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
END IF

A student with score exactly 10 would incorrectly be told "Fail". The program runs without errors but produces wrong output for boundary inputs.

Syntax errors are caught before the program runs. Logic errors only reveal themselves when you test with carefully chosen data.

Nesting means placing one structure inside another — an IF inside another IF, or a loop inside another loop. OCR J277 requires you to read, write, and trace nested structures.

Nested selection — award a grade based on score and bonus flag:

IF score >= 70 THEN
    IF bonus == TRUE THEN
        grade = "A*"
    ELSE
        grade = "A"
    END IF
ELSE
    IF score >= 50 THEN
        grade = "B"
    ELSE
        grade = "C"
    END IF
END IF

Nested iteration — print a multiplication table:

FOR row = 1 TO 5
    FOR col = 1 TO 5
        OUTPUT row * col
    END FOR
END FOR

The inner loop runs completely for each iteration of the outer loop. For a 5×5 table, the inner loop body executes 25 times in total.

In a trace table, track every variable — inner loop counter, outer loop counter, and any accumulator — separately. Missing a variable is the most common trace-table error.

1. Drawing flowchart decisions as rectangles

Decision diamonds must be diamond-shaped and have exactly two labelled exits (YES/NO). Using a rectangle for a decision loses marks even if the logic is correct.

2. Completing a trace table top to bottom without updating correctly

Update variables at the moment the line executes, not at the end of the loop. After i = i + 1, record the new value of i on the same row before moving on.

3. Confusing syntax and logic errors

If a program will not run, the error is a syntax error. If it runs but gives wrong output, the error is a logic error. Exam questions often ask you to classify an error — the key test is whether the program executes at all.

4. Using vague process boxes in flowcharts

A process box should contain a specific action: area = length * width, not "calculate area". The more precise the box, the closer the flowchart is to actual code.

Mistake	Correction
Diamond has three exits	Decision must have exactly two paths (YES/NO)
Trace table skips intermediate values	Record every variable change as it happens
"It runs but gives wrong answer" classified as syntax error	That is a logic error — syntax errors prevent execution