3. Intro-Decisions in Your Code
• Programs execute top-down
• Decisions break the top-down flow
• The if statement tests a condition
• When the condition is true, program flow is
altered.
4. Flow Control
• In computer science, control flow (or flow
of control) is the order in which individual
statements, instructions or function calls of
an imperative program are executed or
evaluated. The emphasis on explicit control
flow distinguishes an imperative
programming language from a declarative
programming language. Wikipedia
5. Flow Control
• In a program statement may be executed
sequentially, selectively or iteratively.
• Every program language provides constructs to
support sequence, selection or iteration.
6. Types of Control Structures
Flow of control through any given function is
implemented with three basic types of control
structures:
• Sequential: default mode. ... Following a
recipe
• Selection: used for decisions, branching --
choosing between 2 or more alternative
paths. ... If, if/else, switch
• Repetition: used for looping, i.e. repeating
a piece of code multiple times in a
row….while, do/while, for
7. Useful tools
Some useful tools for building programs or
program segments
• pseudocode - helps "think" out a problem
or algorithm before trying to code it.
• flowcharting - graphical way to formulate
an algorithm or a program's flow.
• stepwise refinement (top-down design) of
algorithms
9. Example of Sequential Structure
#include <stdio.h>
int main()
{
puts("I'm a computer!");
puts("Thrilled to meet you!");
return(0);
}
View code
10. SELECTION
• The Selection construct means the execution
of statement(s) depending upon a
condition-test. If a condition evaluates to
true, a course-of-action (a set of statements)
is followed otherwise another course-of-
action (a different set of statements).
• This construct(selection construct) is also
called decision construct because it helps in
making decision about which set-of-
statements is to be executed.
12. ITERATION
• Iteration construct means repetition of set of
statements depending upon a condition
test. Till the time of condition is true. ( or false
depending upon the loop). A set of
statements are repeated again and again.
As soon as the condition become false (or
true), the repetition stops. The iteration
condition is also called ”Looping Construct”.
15. THE SELECTION STATEMENT – if Statement
• An if statement tests a particular condition,
if the condition evaluated to true, a course
of action is followed, i.e., a statement or a
set of statement is executed. Otherwise if
the condition evaluated to false then the
course of action is ignored.
16. SYNTAX OF IF STATEMENT
• if (condition)
statement 1;
The statement may consist of single or
compound. If the condition evaluates non
zero value that is true then the statement 1
is executed otherwise if the condition
evaluates zero i.e., false then the statement
1 is ignored.
17. Example of if statement
Example 1:
if (age>18)
printf(“The person is eligible for vote”)
Example 2:
#include <stdio.h>
int main()
{
int a;
printf("Type an integer: ");
scanf("%d",&a);
if( a > 10 )
printf("%d is greater than 10.n",a);
return(0);
}
Notice the semi colon
18. Example of if statement
The need to evaluate the false condition would then give us the following code.
#include <stdio.h>
int main()
{
int a;
printf("Type an integer: ");
scanf("%d",&a);
if( a > 10 )
{
printf("You typed %d.n",a);
printf("%d is greater than 10.n",a);
}
if(a <= 10 )
{
printf("You typed %d.n",a);
printf("%d is less than or equal to 10.n",a);
}
return(0);
19. THE ELSE CLUASE
• An if statement can optionally include an else
clause. The else clause is included as follows:
if (expression)
statement1;
else
statement2;
• If expression evaluates to true, statement1 is
executed. If expression evaluates to false,
statement2 is executed.
• Both statement1 and statement2 can be
compound statements or blocks.
3/7/2019
20. Flow chart of if statement
if Condition ? Statement 1 Statement 2
Statement 1
Statement 2
els
e
tru
e
21. IF - ELSE FORMAT
if (condition)
{
Statement 1
Statement 2
}
else
{
Statement 1
Statement 2
}
22. Example of if-else
#include <stdio.h>
int main()
{
int a;
printf("Type an integer: ");
scanf("%d",&a);
if( a > 10 )
{
printf("You typed %d.n",a);
printf("%d is greater than 10.n",a);
}
else(a <= 10 )
{
printf("You typed %d.n",a);
printf("%d is less than or equal to 10.n",a);
}
return(0);
Run code
23. NESTED IFs
• A nested if is an if that has another if in its body or
in its else body. The nested if can have one of
the following three forms
Form 1 :
if (expression 1)
{
if (expression 2)
statement 1
else
statement 2
}
else
body of else
24. NESTED IF contd..
• Form 2:
if (expression 1)
{
if (expression 2)
statement 1
else
statement 2
……….
}
else
{
if (expression 2)
statement 1
else
statement 2
……….
}
25. NESTED IF contd..
• Form 3:
if (expression 1)
{
body of if
}
else
{
if (expression 2)
statement 1
else
statement 2
……….
}
26. THE if-else-if LADDER
• A common programming construct in C is
the if-else-if ladder, which is often also
called as the if-else-if ladder because of its
appearance. It takes the following general
form.
if (expression 1) statement 1;
else
if (expression 2) statement 2
else
if (expression 3) statement 3
……….
else
Statement 4;
27. Example
#include <stdio.h>
int main()
{
int a;
printf("Type an integer: ");
scanf("%d",&a);
if( a > 10 )
{
printf("You typed %d.n",a);
printf("%d is greater than 10.n",a);
}
else if( a < 10)
{
printf("You typed %d.n",a);
printf("%d is less than 10.n",a);
}
else
{
printf("You typed %d.n",a);
printf("%d is 10.n",a);
}
return(0);
}
Run code
28. THE ? : ALTERNATIVE TO if
• C has an operator that can be alternative to if
statement. The conditional operator ? :
• This operator can be used to replace the if
statement of C.
29. CONDITIONAL OPERATOR ? :
if (expression 2)
statement 1
else
statement 2
• The above form of if else statement can be
replaced as,
expression1?expression2:expression3;
30. CONDITIONAL OPERATOR ? :
• For example
char result;
int marks;
if (marks > 50)
{
result =‘p’;
}
Else
{
result =‘f’;
}
• This can be alternatively written as,
char result;
int marks;
Result = (marks > 50) ? ‘p’ : ‘f’;
31. COMPARISON OF if AND ? :
1. compared to if –else sequence, ?: offers more
concise, clean and compact code, but it is less
obvious as compared to if.
2. Another difference is that the conditional
operator ?: produces an expression, and hence a
single value can be assigned or incorporated into
a larger expression, where as if is more flexible. if
can have multiple statements multiple assignments
and expressions (in the form of compound
statement) in its body.
3. When ?: operator is used in its nested form it
becomes complex and difficult to understand.
Generally ?: is used to conceal (hide) the purpose
of the code.
32. Challenge- Group work
• What is the output of the following C
program fragment?
#include <stdio.h>
Int var =75
Int var2 =56
Int num;
num =sizeof(var)? (var2>23 ?)((var ==75) ? ‘A’ : 0) :0) :0;
Printf(“%d”, num);
Return o;
}
33. Multiple Decisions
• The C language lets you handle complex
decisions by stacking a bunch of if else
conditions.
• Sometimes that structure gets a bit weird.
• As an alternative you can employ the
switch-case structure which is another
decision making tool in the C-language.
34. Example
#include <stdio.h>
int main()
{
char a;
printf("Your choice (A,B,C): ");
scanf("%c",&a);
switch(a)
{
case 'A':
puts("Excellent choice!");
break;
case 'B':
puts("This is the most common choice.");
break;
case 'C':
puts("I question your decision.");
break;
default:
puts("That's not a valid choice.");
}
return(0);
}
Build and run the code lets see what it does
35. Challenge-Group Exercise
• Code a decision making structure
• Prompt for integer input
• For values1,2,or3, display the words “red”,
“green” or “blue.”
• Flag invalid input for any other values the
user inputs.
36. Solution
• Lets see
• 2
• 3
• 4
• Solutions
• Stick to one solution, groups of 5
(tengwau@must.ac.ug)
37. • THANKS FOR LISTENING
• Next iteration construct
