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Cambridge IGCSE Computer Science · 0478

Topic 7: Algorithm Design and Problem-Solving — Part 3

Errors & Flowchart Symbols

Types of programming errors

Programs can contain three main types of error. Each is detected at a different stage:

Error type Description When detected Example
Syntax error Breaks the grammar rules of the programming language At compile/translate time — program will not run Typing PRNT instead of PRINT
Logic error Code runs but produces the wrong output due to flawed logic During testing — program runs but results are incorrect Using > instead of >= in a boundary condition
Runtime error Causes the program to crash while it is running During execution — program starts but stops unexpectedly Divide by zero or accessing invalid memory

Flowchart symbols

Flowcharts use standard symbols to represent different parts of an algorithm. Learn the shape and purpose of each:

Symbol Shape Purpose
Terminator Oval Start or end of the algorithm
Process Rectangle A calculation or instruction (e.g. assign a value, perform arithmetic)
Input / Output Parallelogram Data entering or leaving the system (e.g. INPUT, PRINT)
Decision Diamond A question with two paths — Yes/No or True/False
Subroutine Rectangle with double border A pre-written procedure or function that is called
Flow line Arrow Shows the direction of flow between symbols
Flowchart symbols: oval terminator, rectangle process, parallelogram input/output, diamond decision, double-border subroutine, and arrow flow lines
Diagram 1: Standard flowchart symbols — terminator (oval), process (rectangle), input/output (parallelogram), decision (diamond), subroutine (double-border rectangle), and flow lines (arrows).

Exam Traps

  • Do not confuse process (rectangle) with input/output (parallelogram) or decision (diamond) — exam questions often test symbol shapes.

Explaining, finding and amending algorithms

To explain the purpose of an algorithm, describe its inputs, processes, and outputs in plain language. For example: "This algorithm reads three numbers, counts how many are greater than 10, and outputs the count."

Exam pseudocode must use precise symbols — write > or <=, not English phrases like "is greater than". Use the assignment arrow ←, not "=" for storing values.

Finding logic errors: Trace the algorithm step by step or use a trace table. Common mistakes include wrong comparison operators, incorrect loop bounds, and forgetting to initialise counters.

Amending an algorithm — before (logic error):

Count ← 0
FOR i ← 1 TO 3
    INPUT Number
    IF Number > 10 THEN
        Count ← Count + 1
    ENDIF
NEXT i
OUTPUT Count

The loop runs for i ← 1 TO 2 in the faulty version below, so the third input is never processed:

// Faulty version — only reads two numbers
FOR i ← 1 TO 2

After (corrected): change the upper bound to 3 so all three inputs are processed: FOR i ← 1 TO 3.

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