Algorithms — flowcharts and converting between representations

By the end of this lesson

By the end you can draw a flowchart for a given algorithm, convert a flowchart to pseudocode, and trace execution using a trace table.

Side-by-side comparison of structured English, pseudocode, and flowchart for the same algorithm

Flowchart to pseudocode — systematic conversion

Follow the flow lines from Start to End, translating each symbol:

Flowchart symbol	Pseudocode equivalent
Oval (Start/End)	(implied by program bounds)
Parallelogram (Input)	`READ variable`
Parallelogram (Output)	`PRINT expression`
Rectangle (Process)	`variable ← expression`
Diamond (Decision)	`IF condition THEN ... ELSE ... ENDIF`
Predefined process	`CALL procedure()`
Connector	(labels/GOTO in code — avoid; use structured loops)

Key rules:

Worked example: Flowchart → Pseudocode

Convert the 'find max of 3' flowchart to pseudocode

Have a go first?

Identify the three sections (Input, Process, Output)
Draw Start oval
For each INPUT line: add parallelogram with READ
For each PROCESS line: add rectangle with assignment
For IF/ELSE: add diamond, draw Yes/No branches, join after ENDIF
For FOR loop:
- Initialize counter (rectangle)
- Decision diamond: counter ≤ limit?
- Yes → body → increment counter → back to diamond
- No → exit
For WHILE: decision diamond at top, body on Yes, back to diamond
For REPEAT: body first, then decision diamond on No (continue), Yes (exit)
For OUTPUT: parallelogram with PRINT
End oval

Worked example: Pseudocode → Flowchart

Draw flowchart for: READ n; sum ← 0; FOR i ← 1 TO n DO sum ← sum + i ENDFOR; PRINT sum

Have a go first?

A trace table shows variable values at each step. Columns = variables + output; rows = execution steps.

Example: READ n; sum ← 0; FOR i ← 1 TO n DO sum ← sum + i ENDFOR; PRINT sum with n=3

Step	n	sum	i	Output
1	3	—	—
2	3	0	—
3	3	0	1
4	3	1	1
5	3	1	2
6	3	3	2
7	3	3	3
8	3	6	3
9	3	6	4
10	3	6	4	6

Stop when loop condition fails (i=4, 4≤3 is false). Final output is 6.

Quick check — flowcharts and tracing

Multiple choiceQuestion 1 · Recall

In your own wordsQuestion 2 · Apply

Fill in the blanksQuestion 3 · Recall

Answer the 3 questions above to continue.