Programming Algorithms

Lessons

Senior 4 · ICT

Programming Algorithms

5 lessons

Lesson 3 of 5

Representing Algorithms: Pseudocode and Flowcharts

25 min

Drawing a program before writing it

In the 1960s and 1970s, programs were punched on cards and every run of the computer was booked in advance. A mistake could cost a day. So programmers planned on paper first, drawing their algorithms with plastic stencils like the one below, and only then wrote the code. Computers are now fast and cheap, but the habit survives because it works: an algorithm is easier to check, and to explain to others, in a form that does not depend on any programming language.

Definition

Pseudocode: an algorithm written in structured, precise English using a small set of keywords (INPUT, OUTPUT, IF, WHILE…) and indentation, close to a program but not tied to any language.

Flowchart: a diagram of an algorithm in which standard shapes stand for kinds of step and arrows (flow lines) show the order.

Control structures: the three ways steps can be combined: sequence (one after another), selection (choosing a path by a condition) and iteration (repeating). In 1966 Corrado Böhm and Giuseppe Jacopini showed that these three are enough to express any algorithm.

Flowchart symbols

Pseudocode conventions

There is no single official pseudocode; examiners accept any clear, consistent style. This unit uses the following.

Purpose	Pseudocode
Start and end	BEGIN … END
Input and output	INPUT x OUTPUT "text", x
Assignment	x ← expression
Selection (lesson 4)	IF condition THEN … ELSE … ENDIF
Iteration (lesson 5)	WHILE condition DO … ENDWHILE; REPEAT … UNTIL condition; FOR i ← 1 TO n … ENDFOR
Comment	// explanation for the reader

Keywords are in capitals, variable names are meaningful, and the lines inside a structure are indented so its beginning and end are obvious.

Worked example 1: one algorithm, two forms

Problem: input three marks and output their average. This is pure sequence: each step runs once, in order. Each shape in the flowchart matches one line of pseudocode.

Trace with 64, 71 and 87: total = 222, average = $\frac{222}{3} = 74$ .

Worked example 2: a flowchart with a decision

A school library charges 100 Frw for each day a book is late (an illustrative rate). Read the flowchart below and write the pseudocode.

plaintext

BEGIN
    INPUT daysLate
    IF daysLate > 0 THEN
        fine ← daysLate * 100
    ELSE
        fine ← 0
    ENDIF
    OUTPUT fine
END

Code Explanation

The diamond becomes IF … THEN … ELSE … ENDIF: the Yes path goes after THEN, the No path after ELSE, and the point where the two paths join is ENDIF. Trace: daysLate = 3 gives fine = 300; daysLate = 0 gives fine = 0.

Stuff Made Easy: Flowcharts and Pseudocode, part 1. Watch how each flowchart shape is used and how the same algorithm is written in pseudocode; check that the shapes match the table above.

Warning

Every decision has two labelled exits. A diamond with one arrow out, or with arrows not marked Yes and No (or True and False), cannot be followed.

One Start, one Stop, and no dead ends. Every path through the chart must reach Stop. Use arrows on every flow line; a line without an arrowhead leaves the order unclear.

Pseudocode is not "almost Python". Do not mix in a language's special symbols (such as // for division or : after IF); keep to the agreed keywords.

Tip

Draw it in Google Drawings. In Google Docs, Insert → Drawing → New (Unit 1, lesson 4) offers the flowchart shapes under Shape → Shapes, and elbow connectors that stay attached when you move a shape.

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Practice

Name the flowchart symbol for: (a) INPUT price; (b) total ← total + price; (c) Is total > 5000?; (d) Stop.
Name the three control structures and give a one-line example of each in pseudocode.
Write pseudocode that inputs the base and height of a triangle and outputs its area. Then draw the flowchart.
Convert to pseudocode: Start → INPUT celsius → fahrenheit ← celsius * 9 / 5 + 32 → OUTPUT fahrenheit → Stop. Trace it for 25.
Draw a flowchart for: input a number; if it is greater than or equal to 0, output "positive or zero", otherwise output "negative".
A student's flowchart has a diamond "age ≥ 18?" with only one arrow leaving it, labelled Yes. What is wrong, and how would you fix it?
Give two advantages of a flowchart over pseudocode, and one advantage of pseudocode over a flowchart.
Evaluate: "A flowchart is only useful for beginners; real programmers never plan."

Show answersHide answers

(a) Input/output parallelogram; (b) process rectangle; (c) decision diamond; (d) terminal.
Sequence: total ← a + b then OUTPUT total. Selection: IF mark ≥ 50 THEN OUTPUT "Pass" ENDIF. Iteration: WHILE n > 0 DO n ← n - 1 ENDWHILE.
BEGIN; INPUT base, height; area ← base * height / 2; OUTPUT area; END. Flowchart: Start → parallelogram (INPUT base, height) → rectangle (area ← base * height / 2) → parallelogram (OUTPUT area) → Stop.
BEGIN; INPUT celsius; fahrenheit ← celsius * 9 / 5 + 32; OUTPUT fahrenheit; END. For 25: $25 \times 9 = 225$ , $\frac{225}{5} = 45$ , $45 + 32 = 77$ .
Start → INPUT n → diamond "n ≥ 0?" → Yes: OUTPUT "positive or zero"; No: OUTPUT "negative" → paths join → Stop.
A decision must have two exits. Add a No arrow from the diamond to the steps for people under 18, and make both paths lead to Stop.
Flowchart: the paths and branches can be seen at a glance; it is easy for non-programmers to follow. Pseudocode: it is quicker to write and edit, and it converts more directly into program code.
Not true. Professional developers plan with diagrams (flowcharts, activity diagrams) and pseudocode, especially for complex or safety-critical systems, because finding a mistake in a plan is cheaper than finding it in running code.

Quick check

Match each step to its flowchart symbol.

Tap one on the left, then its partner on the right.

Key points

Pseudocode uses capital keywords, ← for assignment and indentation; a flowchart uses standard shapes joined by arrows.
Shapes: terminal, input/output (parallelogram), process (rectangle), decision (diamond, two labelled exits), connector, predefined process.
Sequence, selection and iteration are enough for any algorithm.
Each flowchart shape matches a pseudocode line; a decision becomes IF … THEN … ELSE … ENDIF.

Sources and credits

Flowchart, Pseudocode and Structured program theorem, Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Photo: Flowcharting template, Spideog, CC BY-SA 4.0, via Wikimedia Commons.
Video: Flowcharts and Pseudocode #1, Stuff Made Easy, embedded from YouTube.
The symbol table, flowcharts, worked examples (library fine illustrative), pseudocode and practice items were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Drawing a program before writing it

2Flowchart symbols

3Pseudocode conventions

Drawing a program before writing it

Flowchart symbols

Pseudocode conventions