Lecture 13 Loops1 with C++ programming.PPT

Repetition (1)
C Programming
AAST
CCIT

 What is a Loop?
 Count-Controlled Loops
 Event-Controlled Loops
 While Statement Syntax
 Using a While Statement for Summing
and Counting
 Nested While Loops
 Loop Testing and Debugging

 A loop is a repetition control structure.
 That is, you can execute particular
statements more than once in a
controlled fashion
 Statements are executed as long as
some condition remains true

count controlled loops
repeat a specified number of times
event-controlled loops
some condition within the loop body
changes and this causes the repeating to
stop

SYNTAX
while ( Expression )
{ .
. /*loop body */
.
}
NOTE: Loop body can be a single
statement, a null statement, or a block.

When the expression is tested and found to be
false, the loop is exited and control passes to
the statement which follows the loop body.
WHILE LOOP
FALSE
TRUE
body
statement
Expression

 Every while loop will always contain three main
elements:
 Priming: initialize your variables.
 Testing: test against some known condition.
 Updating: update the variable that is tested.
Parts of a While Loop

int count ;
count = 4; /* initialize loop variable */
while (count > 0) /* test expression */
{
printf(“ %d n ”,count ) ; /* repeated action */
count -- ; /*update loop variable */
}
printf( “Done” ) ;
1. Priming
1. Priming
2. Test Condition
2. Test Condition
3. Update
3. Update

int count ;
count = 4;
while (count > 0)
{
printf( " %d n " , count ) ;
count -- ;
}
printf( " Done " ) ;
OUTPUT
count

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
count
4

int count ;
count = 4;
while (count > 0) TRUE
{
count -- ;
}
OUTPUT
count
4

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
count
4

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
count
3

int count ;
count = 4;
{
count -- ;
}
OUTPUT
4
count
3

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
3
count
3

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
3
count
2

int count ;
count = 4;
{
count -- ;
}
OUTPUT
4
3
count
2

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
3
2
count
2

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
3
2
count
1

int count ;
count = 4;
{
count -- ;
}
OUTPUT
4
3
2
count
1

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
3
2
1
count
1

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
3
2
1
count
0

int count ;
count = 4;
while (count > 0) FALSE
{
count -- ;
}
OUTPUT
4
3
2
1
count
0

int count ;
count = 4;
while (count > 0)
{
count -- ;
}
OUTPUT
4
3
2
1
Done
count
0

#include <stdio.h>
#define MAX 10
main ()
{
int index =1;
while (index <= MAX)
{
printf ("Index: %dn", index);
index = index + 1;
}
}
Simple While Loop
1. Priming
1. Priming
2. Test Condition
2. Test Condition
3. Update
3. Update
OUTPUT:
Index: 1
Index: 2
Index: 3
Index: 4
Index: 5
Index: 6
Index: 7
Index: 8
Index: 9
Index: 10

•Infinite Loop: A loop that never ends.
–Generally, you want to avoid these!
–There are special cases, however, when
you do want to create infinite loops on
purpose.
•Common Exam Questions:
–Given a piece of code, identify the bug in
the code.
–You may need to identify infinite loops.
Infinite loop

#include <stdio.h>
#define MAX 10
main ()
{
int index =1;
while (index <= MAX)
{
}
}
Infinite While Loop
1. Priming
1. Priming
2. Test Condition
2. Test Condition
3. Where is the
3. Where is the
update part
update part
Index: 1
Index: 1
Index: 1
Index: 1
Index: 1
… [forever]

#include <stdio.h>
/* no MAX here */
main ()
{
int index =1;
while (index > 0)
{
index = index + 1;
}
}
Infinite While Loop
1. Priming
1. Priming
2. Test Condition
2. Test Condition
3. Update
3. Update
Index: 1
Index: 2
Index: 3
Index: 4
Index: 5
… [forever]

Use a while loop to read 100
grades in an exam and find their
total.
Count-Controlled Loop Example

int thisGrade ;
int total=0 ;
int count ;
count = 1 ; /* initialize */
while ( count < 101 ) /* test expression */
{
printf(“ Student %d Grade : “, count );
scanf(“ %d “ , &thisGrade ) ;
total = total + thisGrade ;
count++ ; /* update */
}
printf(“The total = %d n“ , total ) ;

 Signals the end of data entry
 Also known as signal value, dummy value,
or flag value
 Also known as indefinite repetition
because the number of repetitions the
code will perform is unknown before the
loop

 How are they used?
 Programmer picks a value that would never be
encountered for normal data
 User enters normal data and then when done,
enters the unusual value
 The loop would stop when seeing the unusual
value

total = 0;
printf(“ Grade ( -1 to stop): “ );
while (thisGrade != -1)
{
total = total + thisGrade;
printf(“ Grade ( -1 to stop): “ );
}
printf(“ The total is %d n” ,total );

count = 0;
total = 0;
GoOn = 1; /* initialize flag */
while ( GoOn == 1 )
{
printf(“ Enter Grade “);
scanf(“ %d “ , &thisGrade );
if ( thisGrade == -1 )
GoOn = 0 ; /* change flag value */
else {
count++;
}
}
printf( “ Total is %d n “, total ) ;

 write a program that reads in the
grades of 50 students in a course (out
of 100 points each ) and then count the
number of A students ( grade > 85 ) and
the number of B students (grade > 75 ).

 Write a program that does a survey on a certain
question. The question has 3 possible answers.
Run the survey on 20 people and then display
the number of people who chose each answer.
 Example:
What is your favorite subject?
A. Mathematics
B. Economics
C. Programming

Is a looping control structure in which the loop
condition is tested after each iteration of the
loop.
SYNTAX
do
{
Statements
} while ( Expression ) ;
Loop body statement can be a single statement or a block.

char more ;
int thisGrade , total;
total = 0 ;
do
{
printf(“ Grade : “ ) ;
printf(“ Any more students ? (Y/N) “ ) ;
scanf(“ %c “ , &more ) ;
} while ( more == ‘y’ || more == ‘Y’ ) ;
printf ( “ Total = %d “ , total );
Grades Example

 POST-TEST loop
(exit-condition)
 The looping condition
is tested after
executing the loop
body.
 Loop body is always
executed at least
once.
 PRE-TEST loop
(entry-condition)
 The looping condition
is tested before
executing the loop
body.
 Loop body may not
be executed at all.

When the expression is tested and found to be false,
the loop is exited and control passes to the
statement that follows the do-while statement.
Statement
Expression
DO
WHILE
FALSE
TRUE

 Assume you put 1000 pounds in a projects that
returns a profit of about 5% per year. How long
will it take for your money to double ?
 Assume you put 5000 pounds in a projects that
returns a profit of about 10% per year. How
much money will you have in 5 years ?

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