1. Civil Engineering
Skills
Computer
Programming
CIV1900
Dr Sam Clarke
Control of Program Flow

2. What is Program flow?
Learning outcomes & Aims
◦ What program flow is
◦ Why it may be useful to control the
flow
◦ How to control program flow using
sequential logic
“Making Decisions”

3. Program Flow
Process
Decision
Data

4. Program Flow – simple algorithm
Data in A=[1,2;3,4]
Data processing A=A*2
B=A
Data out B=[2,4;6,8]
Pure logic

5. Program Flow – another algorithm
Data in A=[1,2;3,4]
if A(1,1)==1
A=A*2
else
Statement
A=A*3
True False end
Process A Process B B=A
1st time B=[2,4;6,8]
Data out 2nd time B=[6,12;18,24]
Decisions give flexibility

6. Making Decisions– why?
• When two or more possible
outcomes are required
• Decisions are made based on a
conditional tests
• if-then
• if-then-else....

7. if-then
If this lecture is too boring then I will fall
asleep
if
expression (this lecture is too boring)
then
statement (I will fall asleep)
Let’s code this up

8. if-then
If this lecture is too boring then I will fall
asleep
relational operator
if boredom > tiredness
student = ‘asleep’
end
then is implied
by the next line

9. if-then-else
If I fall asleep in lectures then I will not
understand the lab class otherwise I will
be able to complete the class quickly.
if
expression (I fall asleep in lectures)
then
statement (I will not understand the lab)
else
statement (I will be able to complete the
class quickly)

10. if-then-else
If I fall asleep in lectures then I will not
understand the lab class otherwise I will
be able to complete the class quickly.
relational operator
if boredom > tiredness
student = ‘confused in lab’
else
student = ‘going home early’
end

11. Relational operators
Operator Description
< Less than
<= Less than or equal to
> Greater than
>= Greater than or equal to
== Equal to
~= Not equal to

12. Relational operators
Can be used independently of if-else
A = 12;
B = 18;
C = A > B
Remember = is an assignment
C = 0
False = 0, True = 1

13. Logical operators
Operator Description
& True if all relations are
(AND) true
| True if at least one
(OR) relation is true
~ True if all relations are
(NOT) false

14. More if-then-else
Conditionals with more than 2 cases can
be built as well:
if (expression1)
statement1
elseif (expression2)
statement2
else
statement3
end

15. More if-then-else (Practical 1)
m=randn(100,1); loops next week
for loop1=1:100
if m(loop1)>2
n(loop1,1)=2;
elseif m(loop1)<-2
n(loop1,1)=0;
else
n(loop1,1)=1;
end
end
o=find(n==0);
p=find(n==2);
q=length(o)+length(p);