1. Flowcharts and Pseudo Code

2. Overview
We will explore simple algorithms using flowcharts and pseudo code
We will look at:
Sequencing – putting things in the right order
Conditions – doing different things depending on some rules
Iteration (Looping) – repeating something
Flowcharts and Pseudo Code

3. What is Pseudo code?
Pseudo code is an informal, high-level, natural-language description of an algorithm. E.g.
Start
get value
result = value * 100
display result
Stop
Flowcharts and Pseudo Code

4. What is a Flowchart?
A flowchart is a graphical representation of an algorithm
It uses standard shapes including:
Tools: MS Visio, Word etc.
Terminator
Input or output
Flow line
Process
Flowcharts and Pseudo Code

5. Why use Pseudo Code or Flowcharts?
They are more concise and clear than a paragraph of writing
They show the logical branching with indentation or flow lines
They are not specific to a particular programming language
They can be used to plan a solution before coding it
They can be checked by or discussed with someone who cannot program
Computers and Programming

6. Pseudo Code
Pseudo code is informal but we will use this basic structure:
Start
get value
result = value * 2
display result
Stop
The indentation is important…
To begin
To ask the user for some data (input)
To do some operation
To show something on the screen (output)
To finish
Flowcharts and Pseudo Code

7. Flowchart To begin To ask the user for some data (input)
To do some operation
To show something on the screen (output)
To finish
Flowcharts and Pseudo Code

8. Simple Sequencing
Problem: calculate the average of three input numbers.
How do we do this?
Start
get n1
get n2
get n3
average = (n1 + n2 + n3)/3
display average
Stop
n1 represents the first number entered
average will contain the result of the sum:
(n1 + n2 + n3) / 3
Flowcharts and Pseudo Code
8 of 33

9. Simple Sequencing Here is the flowchart for the same problem
Remember to use the right shapes
for input or output
for a process or calculation
Flowcharts and Pseudo Code

10. Conditions
Conditions are the way we make a choice between two or more alternatives.
Doing something different depending on a condition is called altering the flow of control.
Start
get mark
if mark >= 50
result = “Pass”
else
result = “Fail”
end-if
display result
Stop
1. Evaluate the condition getting True or False
2. If the result is True then perform
these operation(s)
3. If it is False then perform
these operation(s)
Flowcharts and Pseudo Code

11. A Note on Meta-Symbols
Note that the < and > characters are called meta-symbols
What lies between then indicates the kind of thing which is allowed in its place.
if <condition> then
<operations for then-part>
else
<operations for else-part>
end-if
<variable> = <expression>
A condition must go here. E.g. number < 10
A variable must go here. E.g. salary
An expression must go here. E.g.
basic + bonus
Flowcharts and Pseudo Code
11 of 33

12. Variable? What’s That?
Variables are used to represent some data that might change, e.g.
name = “Fred”
age = 16
Constants are similar but their values don’t change, e.g.
PI = 3.14
FILE_NAME = “mydata.txt”
Variables start with a lower case letter
Constants are always in UPPERCASE
12 of 33

13. Conditions Problem: Work out whether the student has passed or failed
Start
get mark
if mark >= 50
result = “Pass”
else
result = “Fail”
end-if
display result
Stop
Flowcharts and Pseudo Code
13 of 33

14. Conditions
There can be more than one expression in the <then> or <else> parts. E.g.
Start
Get mark
if mark >= 50 then
result = “Pass”
message = “Well done!”
else
result = “Fail”
message = “See me!”
end-if
display result
display message
Stop
Flowcharts and Pseudo Code
14 of 33

15. Conditions It is also possible to have multiple conditions. Start
get mark
if mark < 50 then
result = “Fail”
else if mark < 60 then
result = “Pass”
else if mark < 70 then
result = “Merit”
else
result = “Distinction”
end-if
display result
Stop
Flowcharts and Pseudo Code
15 of 33

16. Condition Flowchart
16 of 33

17. What does this do? What value is printed if: What is this doing?
a=5, b=7, c=2
a=8, b=2, c=4
a=3, b=5, c=8
What is this doing? b a c
Flowcharts and Pseudo Code
17 of 33

18. Your Turn
Write some pseudo code and sketch a flow chart for the following cinema ticketing system:
If customer is aged under 12 then the ticket £5
Otherwise, if they are over 65 the ticket is £7
Otherwise the ticket is £10
Flowcharts and Pseudo Code
18 of 33

19. Pseudo Code Start get age if age < 12: price = 5
else if age > 65:
price = 7
else
price = 10
end-if
display price
Stop
Flowcharts and Pseudo Code
19 of 33

20. Cinema Ticket Flow Chart
20 of 33

21. Iteration
If you wanted to display all the numbers from 1 to 1000, you wouldn’t want to do this, would you?
Start
display 1
display 2
etc.
Stop
Wouldn’t it be better to say something like this?
number = 1
while the number <= 1000…
display the number
increase the number
Flowcharts and Pseudo Code
21 of 33

22. Iteration: Display Numbers up to 1000
We can write this in pseudo code:
Start
number = 1
while number <= 1000
display number
number = number + 1
end-while
Stop
Why do we need to do this?
Flowcharts and Pseudo Code
22 of 33

23. Iteration: Display Numbers up to 1000
What is going on here?
Flowcharts and Pseudo Code
23 of 33

24. Iteration: Get Five Numbers
Problem: Allow the user to enter 5 numbers
How do we know when we have 5 numbers?
Start
count = 0
while count < 5
get number
count = count + 1
end-while
Stop
Why less than?
Why do we need to do this?
Flowcharts and Pseudo Code
24 of 33

25. Iteration: Get Five Numbers
25 of 33

26. Iteration: Total Five Numbers
Now let’s also display the total of those numbers
Start
count = 0
total = 0
while count < 5
get number
count = count + 1
total = total + number
end-while
display total
Stop
Here is the pseudo code to ask the user for five numbers
We need a total, initially set to zero
We need to add the number to that total
At the end, we need to display the final total
Flowcharts and Pseudo Code
26 of 33

27. Iteration: Total Five Numbers
Flowcharts and Pseudo Code
27 of 33

28. Your Turn Write some pseudo code and sketch a flow chart to:
Ask the user for ten numbers
Display the total and average of these numbers
Flowcharts and Pseudo Code
28 of 33

29. Average Ten Numbers Pseudo Code
Start count = 0 total = 0 while count < 10 get number count = count + 1 total = total + number end-while average = total / count display total display average Stop
Flowcharts and Pseudo Code
29 of 33

30. Average Ten Numbers Flow Chart
30 of 33

31. Iteration Types
There are different ways of expressing iteration using pseudo code
Which you use depends on what you are trying to do
The programming language you intend to use is also likely to affect your choice
We will use the first one because it translates most directly to Python
while <condition>
<operation(s)
end-while do
<operation(s)
while <condition>
repeat
<operation(s)>
until <condition>
Flowcharts and Pseudo Code
Flowcharts and Pseudo Code
31 of 33

32. Tracing a Flowchart
Use a table of possible values and expected outcomes
mark
Expected result
Actual result 49
Fail
Fail  50
Pass
Pass  59 60
Merit
Merit  69 70
Distinction
Distinction 
You must carefully consider the test values
you need to validate your algorithm
Flowcharts and Pseudo Code

33. Summary We have looked at why pseudo code and flowcharts are useful
We have looked at pseudo code and flowcharts for sequencing, conditions and iteration
On the VLE (Pseudocode and Flowcharts):
FlowchartSymbols.pdf gives a summary of shapes
PseudocodeNotes.pdf gives a summary of keywords
Homework: Hodder’s OCR Computing for GCSE, page 156, questions 1 – 4 (due 17th October)
Flowcharts and Pseudo Code
33 of 33