Cambridge O Level Computer Science · Syllabus 2210 · Algorithm Design and Problem-Solving
Flowchart
What is Flowchart?
A control-flow diagram in which a terminator marks Start and Stop, a rectangle is a process, a parallelogram is input or output, a diamond is a decision with two labelled outcomes, a rectangle with double side bars is a subroutine call, and arrows show the direction control passes.
This definition is part of the Algorithm Design and Problem-Solving chapter in Cambridge O Level Computer Science.
Flowchart in context
Topic 7 is about the work that happens before and around writing code. You analyse a problem (identify it, strip away what does not matter, break it into parts, and name the inputs, processes, outputs and storage), you design a solution (structure diagram, flowchart, pseudocode), you code it, and you test it with data you chose on purpose. Along the way you use a small fixed set of standard methods — linear search, bubble sort, totalling, counting, maximum, minimum, average — you protect the input with validation and verification, and you prove behaviour with trace tables and with normal, abnormal, extreme and boundary test data. Nothing in this topic requires you to be fluent in a programming language; it requires you to be exact.
A flowchart is a diagram that shows the order in which the steps of an algorithm are carried out, using a fixed set of symbols joined by flow lines with arrowheads. Section 4 of the syllabus defines exactly six symbols, and each has one meaning. Using the wrong shape is not a presentation slip — it changes what the diagram claims.
The three design notations are not three solutions to compare. They are three views of the same solution, each showing something the others hide. A structure diagram shows what the parts are; a flowchart shows the paths through them; pseudocode states each step exactly. In an exam you will normally be asked for one — but being able to move between them is what makes a flowchart question answerable when you thought in pseudocode, and the other way round.
This mixes every kind of demand Topic 7 makes: definitions, ordering a process, reading a flowchart, tracing an algorithm, choosing a standard method, classifying test data and diagnosing a defect. Answer all fifteen before checking anything. Every question explains why the right answer is right, so a wrong answer is worth more than a lucky one.
Common mistakes with Flowchart
- M5. “A structure diagram is a kind of flowchart.” Correct A structure diagram shows hierarchy and containment. A flowchart shows control flow. Only the flowchart has arrowheads, decisions and terminators.
- M6. “An algorithm has to be written in a programming language.” Correct An algorithm is a finite, ordered, unambiguous sequence of steps. It can be a flowchart, pseudocode, a structure diagram or ordinary written steps.
- M7. “Flowchart shapes are interchangeable as long as the text is right.” Correct Each of the six symbols has one meaning. A parallelogram is input or output; a rectangle is a process; a diamond is a decision. Using the wrong shape changes what the diagram claims.
Examiner tips on Flowchart
- Which one will the exam ask for? Read the command. “Draw a flowchart” means shapes and arrows. “Write an algorithm”, “write pseudocode” or “complete the pseudocode” means Cambridge pseudocode. “Complete the structure diagram” means fill in boxes in a hierarchy. Answering in the wrong notation is one of the few ways to write a completely correct solution and score zero.
- Check the loop count before you check anything else. A flowchart that inputs five values but tests Count >= 5 instead of Count > 5 reads only four. The fastest way to be certain is to trace the first and the last iteration by hand: what is Count when the first score is read, and what is it when the loop is left? Here the answers are 1 and 6, which is five readings.
- Where the decision sits tells you which loop to write. A decision at the bottom of the loop, reached after the body has run, becomes REPEAT … UNTIL <exit condition>. A decision at the top, reached before the body, becomes WHILE <continue condition> DO … ENDWHILE — and note that the condition flips: the flowchart’s exit test becomes the WHILE loop’s continue test. When the number of repetitions is fixed and known, either can be replaced by FOR Count ← 1 TO 5.
Questions students ask about Flowchart
Can I draw a flowchart when the question asks for pseudocode, or the other way round?
No. The command tells you the notation and answering in another one risks scoring nothing, however correct the logic is. What you can do is sketch a flowchart in rough to work out the structure, and then write the pseudocode that the question asked for.