Flowchart symbols
A flowchart is a diagram that shows the steps of an algorithm using standard shapes joined by arrows. Using the correct shape makes the logic easy to read.
| Oval | Start / Stop (terminal) |
| Parallelogram | Input / Output |
| Rectangle | Process |
| Diamond | Decision |
Key idea
A decision (diamond) has one way in and two ways out, labelled Yes/No (or True/False). It is what makes both selection and repetition possible.
Selection
Selection chooses between paths using a decision. For example, check whether a mark is 50 or more: if Yes, print "Pass"; if No, print "Fail". The two arrows leaving the diamond join together again afterwards, so only one of the paths actually runs. Selection is how a program makes a choice based on a condition.
Repetition
Repetition (a loop) repeats a group of steps until a condition is met. It uses a decision plus an arrow that flows back to an earlier point in the diagram. Each time round the loop the condition is tested again; when it is finally satisfied, the flow leaves the loop and continues. This lets a short flowchart describe work that is repeated many times.
Example
To output the numbers 1 to 5: set count = 1; output count; add 1 to count; a diamond asks "count > 5?". While the answer is No, the arrow loops back to output; when it becomes Yes, the flow goes to Stop.
Remember
- Oval = start/stop, parallelogram = input/output, rectangle = process, diamond = decision.
- A decision box always has exactly two exits.
- Repetition uses an arrow that loops backwards.