1. Week 4 Program Control Structure
Sequence and Selection control structure
Sem1-06/07
EKT120: Computer Programming

2. EKT120: Computer Programming
Outline
Types of program control structure
Sequence control
Selection
Types of selection
One-way selection
Two-way selection
Multi-selection
Compound statement
Nested if
Conditional operator
Switch structure
Sem1-06/07
EKT120: Computer Programming

3. EKT120: Computer Programming
Discussion on Lab Exercise 3 : Introduction to LINUX Lab Exercise 4 : Getting Started with C Programming
Sem1-06/07
EKT120: Computer Programming

4. Types of Program Control Structure
Sequence
Branch or Decision
Loop
Sem1-06/07
EKT120: Computer Programming

5. Sequence Control Structure
The most common and basic structure of a program
A set of instructions in sequence from top to bottom
Statements executed one after the other in the order in which they are written-sequential execution
Sem1-06/07
EKT120: Computer Programming

6. EKT120: Computer Programming
Example
Write a program that creates a table of degrees Celsius with degrees Fahrenheit. Begins at 0 degree and proceed to 100 degree in 20 degree increments. Use no more than 2 variable in your program.
Solution
The equation F = C + 32
C = degree Celsius
F = degree Fahrenheit 9 5
Sem1-06/07
EKT120: Computer Programming

7. EKT120: Computer Programming
Specify Example
Perform the pre calculation to expect the correct output
for example…. C F 32 20 68 40
104 60
140 80
176
100
212
Sem1-06/07
EKT120: Computer Programming

8. EKT120: Computer Programming
Algorithm (pseudo code)
Begin,
Declare variables
Print headings of table
set C = 0
calculate F
print C and F
set C = 20 .
End
Sem1-06/07
EKT120: Computer Programming

9. EKT120: Computer Programming
Start
Float C,F
Algorithm (Flow chart)
C=0
F=(9/5)*C + 32
Print C,F
C=20
F=(9/5)*C + 32
Print C,F
End
Sem1-06/07
EKT120: Computer Programming

10. EKT120: Computer Programming
Pseudocode
Flowchart
C Program
Begin
Declare variable
Print heading
Set C = 0
Calculate F
Print C and F
Set C = 20
Set C = 40
Set C = 60
Set C = 80
Set C = 100
End
#include <stdio.h>
Void main (void) {
float C, F;
printf(“Table of Celcius and Fahrenheit”);
printf(“Celcius\t\tFahrenheit”);
C =20.; F =(9./5.)*C + 32.;
printf(“%8.2f\t\t%8.2f”,C, F);
C =40.; F :=(9./5.)*C + 32.;
C =60.; F :=(9./5.)*C + 32.;
C :=80.; F :=(9./5.)*C + 32.;
C :=100.; F :=(9./5.)*C + 32.; }
Sem1-06/07
EKT120: Computer Programming

11. EKT120: Computer Programming
Explanation 9 5
The equation F = C + 32
The program equation F =(9/5)*C + 32;
Command
printf(“%8.2f\t\t%8.2f”,C, F);
Print format to print float number with 8 numbers or field width and at 2 decimal points.
Semicolon ; must have at the end of C command.
Sem1-06/07
EKT120: Computer Programming

12. EKT120: Computer Programming
Selection structure
Used to choose among alternative courses of action
C has three types:
if,
if..else
switch
Sem1-06/07
EKT120: Computer Programming

13. EKT120: Computer Programming
Selection Statements
Used to control the flow of a program
Also called as decision or branches
Branches are conditions or choices used to enable selection of program flow
Sem1-06/07
EKT120: Computer Programming

14. EKT120: Computer Programming
Types of selection
One-way selection = if
Two-way selection = if..else
Multi-selection
Nested if
Switch structure = switch
Sem1-06/07
EKT120: Computer Programming

15. Review: Relational Operator
Equal To
(equality) ==
Less Than
<
Greater Than
>
Less than or equal to
<=
Greater than or equal to
>=
Not Equal to
!>
Sem1-06/07
EKT120: Computer Programming

16. The if selection structure
if structure is a single-entry/single-exit structure
Pseudo code
Flowchart
begin
if student’s grade is greater than or equal to 60
Print “Passed”
end
False
Sem1-06/07
EKT120: Computer Programming

17. EKT120: Computer Programming
One-way selection in C
Example 1:
if (grade >= 60)
printf(“Passed\n”);
…..
Comment..
(grade>=60) – This is a relation statement which result in logic TRUE or FALSE. If the statement (grade >=60) is true the the command printf(Passed\n) shall be executed.
Sem1-06/07
EKT120: Computer Programming

18. One-way Selection- example
char grade; ……
if(markah>= 90)
grade = 'A';
…...
printf(“Grade is : %c\n”, grade);
Sem1-06/07
EKT120: Computer Programming

19. EKT120: Computer Programming
One-way Selection = if
In C, a condition is represented by a logical (Boolean) expression
True and False are logical (Boolean) values
The syntax of one-way selection is:
if (expression) statement;
If the value of the expression is true, statement is executed;
if false, statement is not executed and the computer goes on to the next statement in the program.
Sem1-06/07
EKT120: Computer Programming

20. EKT120: Computer Programming
One-way Selection = if
Example 3:
if (temperature is greater than 27 degree and it is not raining)
recommended activity is golfing
bool rain=false; …
If ((temp > 27) && !(rain))
printf(“recommended activity is golfing”);
And
not
Sem1-06/07
EKT120: Computer Programming

21. EKT120: Computer Programming
One-way Selection = if
Common Errors
if score >= 90 //no parentheses
grade = 'A';
if(score >= 90); //; not here
Sem1-06/07
EKT120: Computer Programming

22. Two-way Selection = if..else
The syntax of two-way selection is:
if (expression)
statement1;
else
statement2;
If the value of the expression is true, statement1 is executed;
if false, statement2 is executed
Relational exp, or logical exp
Sem1-06/07
EKT120: Computer Programming

23. The if..else selection structure
Specifies an action to be performed both when the condition is true and when it is false
Pseudocode
Flowchart
begin
if student’s grade greater than
or equal to 60 then
Print “Passed”
else
Print “Failed”
end
Display
FAILED
PASSED
Sem1-06/07
EKT120: Computer Programming

24. Two-way Selection - if..else in C
………
if (grade >=60)
printf(“Passed\n”);
else
printf(“Failed\n”); ……
Execute if true
Execute if false
Sem1-06/07
EKT120: Computer Programming

25. Two-way Selection = if..else
Another example:
if (hours > 40.0) //Line 1
wages = 40.0*rate+1.5*rate*hours ); //Line 2
else //Line 3
wages = hours * rate; //Line 4
If hours is 50, then the statement at Line 2 executes
If hours is 30, then the statement at Line 4 executes
Sem1-06/07
EKT120: Computer Programming

26. Multi-selection = if-else if
The syntax is:
if(exp1)
statement1;
else if(exp2)
statement2;
else if(exp3)
statement3; …
else
statement n;
An if-else if control structure
shifts program control, step by
step, through a series of
statement blocks.
Sem1-06/07
EKT120: Computer Programming

27. Multi-selection if-else if Example 1:
Write a C program that can display the results and the corresponding grades as illustrated in the table
Hint : Use flowchart or pseudocode.
marks
grade
90 – 100 A
80 – 89 B
70 – 79 C
60 – 69 D
0 – 59 F
Sem1-06/07
EKT120: Computer Programming

28. EKT120: Computer Programming
Flowchart
Sem1-06/07
EKT120: Computer Programming

29. Multi-selection - if-else if in C
if(mark >= 90)
printf( “Grade = A\n”);
else if(mark >=80)
printf(“Grade = B\n”);
else if(mark >=70)
printf(“Grade = C\n”);
else if (mark >= 60)
printf( “Grade = D\n”);
else printf(“Grade = F);
Sem1-06/07
EKT120: Computer Programming

30. Compound (Block of) Statement
Several statements in a body.
A set of statements contained within a pair braces.
A compound statement (also called a block of statements) takes the form {
statement 1; statement 2; .
statement n; }
It is considered a single statement
Sem1-06/07
EKT120: Computer Programming

31. Compound (Block of) Statement
Example:
if(mark >= 90) {
printf(“Pass\n”);
printf(“Grade = A\n”);}
else if(mark >=80){
printf(“Pass”\n”);
printf(“Grade = B\n”);}
else if(mark >=70){
printf(“Grade = C\n”);}
else if (mark >= 60){
printf( “Grade = D\n”);}
else
{printf(“Pass”\n”);
printf(“Grade = F);}
A Block if statement.
If the relational expression is true, the statement in “true” block is (between 2 braces) are executed. Otherwise, the entire block of statements is ignored)
False ?
Sem1-06/07
EKT120: Computer Programming

32. EKT120: Computer Programming
Nested if
When one control statement is within another,
it is said to be nested
if(exp1)
if(exp2)
{ statement1;
statement2; } {
statement1;
statement2; }
Sem1-06/07
EKT120: Computer Programming

33. EKT120: Computer Programming
Nested if Example
if (temperature >= 50) {
if (temperature >= 80)
printf( "Good day for swimming.\n”);
else
printf( "Good day for golfing.\n“); }
printf("Good day to play tennis.\n“);
Sem1-06/07
EKT120: Computer Programming

34. The Conditional Operator
The syntax of using the conditional operator is:
expression1 ? expression2 : expression3;
This is called a conditional expression.
The statement:
if (a >= b)
max = a;
else
max = b;
Is equivalent to the statement:
max = (a >= b) ? a : b;
Sem1-06/07
EKT120: Computer Programming

35. EKT120: Computer Programming
Switch Structures
Similar to if-else if control structure
The general form (syntax):
switch (expression) {
case value1: statements1; break;
case value2: statements2; break; .
case valuen: statementsn; break;
default: statements; }
Sem1-06/07
EKT120: Computer Programming

36. EKT120: Computer Programming
Switch Structures
The break statement has a special meaning and may or may not appear after each statement.
In C, switch, case, break, and default are reserved words.
In a switch structure, first the expression is evaluated. The value of the expression is then used to perform the corresponding action.
Sem1-06/07
EKT120: Computer Programming

37. EKT120: Computer Programming
Switch Structures
The expression is usually an identifier.
The value of the expression can be only integral.
The expression is sometimes called the selector. Its value determines which statement is selected for execution.
A particular case value should appear only once.
One or more statements may follow a case label, so you do not need to use braces to turn multiple statements into a single compound statement.
The break statement may or may not appear after each statement.
Sem1-06/07
EKT120: Computer Programming

38. EKT120: Computer Programming
Switch Structures
Example:
switch (grade) {
case 'A': printf("The grade is A.“); break;
case 'B': printf("The grade is B.“); break;
case 'C': printf("The grade is C.“); break;
case 'D': printf("The grade is D.“); break;
case 'F': printf("The grade is F.“); break;
default: printf("The grade is invalid.“); }
where, grade is a variable of the type char.
If the value of grade is, say 'A', the output is
The grade is A.
Sem1-06/07
EKT120: Computer Programming

39. EKT120: Computer Programming
Switch Structures
The switch statement executes according to the following rules:
When the value of the expression is matched against a case value (also called a label), the statements execute until either a break statement is found or the end of the switch structure is reached.
If the value of the expression does not match any of the case values, the statements following the default label execute. If the switch structure has no default label, and if the value of the expression does not match any of the case values, the entire switch statement is skipped.
A break statement causes an immediate exit from the switch structure
Sem1-06/07
EKT120: Computer Programming

40. EKT120: Computer Programming
Summary
Sem1-06/07
EKT120: Computer Programming