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Loops(2)
do-while, for
Computer Programming
Topics
• Repeating Statements (review)
• while loop (review)
• do-while loop
• for loop
• Infinite loops
• Examples
Repeating Statements In many cases same operations are repeated for different
data.
For example, finding average of a student is repeated for other students too.
In these cases, we can use a statement called a loop
The statements inside a loop are repeated until its condition becomes false.
C uses three types of loops:
While loop (discussed last week)
Do-while loop
For loop
While Loop While loop repeats a group of C statements until its
condition becomes false
While loop has the following syntax:
while(condition)
[statements];
Condition is a logical expression
Statements inside the while loop are called body of while loop
If more than one statement is written inside body of a while loop, they should be put in {}
Example 1
Write a program to print all positive even integers less than
100.
The output should be like:
2
4
6
…..
96
98
#include<stdio.h>
void main()
{
int count;
count = 2;
while( count < 100 )
{
printf(“%d\n”, count);
count = count + 2;
}
}
Example 2
Write a program to read a series of positive integers and find their
sum until -1 is entered.
Example input can be:
3 9 11 -1
Output: 23
Example input can also be:
-1
Output: 0
In all cases the input has at least one number (at lease -1 is entered)
#include<stdio.h>
void main()
{
int num, sum;
sum = 0;
printf(“Enter an integer:\n”);
scanf(“%d”,&num);
while( num != -1)
{
sum = sum + num;
printf(“Enter an integer:\n”);
scanf(“%d”,&num);
}
printf(“The sum of the numbers is %d \n”, sum);
}
Do While Loop
do-while loop is the same as while loop except that the
condition is checked at the end of the loop.
do-while syntax is:
do
{
[Loop Body]
}while( condition);
The body is execute until the condition becomes false
As the condition is checked at the end, the loop is executed
at least once.
Example 3
Re-write previous example with do-while loop
[Notice that scanf is written only once]
#include<stdio.h>
void main()
{
int num, sum;
sum = 0;
do
{
printf(“Enter an integer:\n”);
scanf(“%d”,&num);
if( sum != -1 )
sum = sum + num;
} while( num != -1);
printf(“The sum of the numbers is %d \n”, sum);
}
For Loop (1)
Whenever you know the number of repetitions in a loop, for
loop is used.
In a loop which counts the repetitions, you need a counter ( a
variable defined in your program)
The counter is initialized before the loop.
The counter is updated inside the body of the loop
For loop does:
Gives initial value to counter
Updates the counter
Checks loop condition
For Loop (2)
For loop syntax:
for ( initial value assignment ; condition ; update counter )
{
[loop body statements]
}
Example 4 Write a program to find sum of the integer numbers from 1 to 10.
#include<stdio.h>
void main()
{
int count, sum;
sum = 0;
for( count = 1; count <=10 ; count ++ )
sum = sum+count;
printf(“The sum of positive integer up to 10 is %d\n”,sum);
}
Example 5 Write a program to read a positive integer and find its factorial.
#include<stdio.h>
void main()
{ int count, num, fact;
fact = 1;
scanf(“%d”,&num);
if( num > 0 )
{
for( count = 1; count <=10 ; count ++ )
fact = fact*count;
printf(“The factorial of %d is %d\n”,num,fact);
}
else
printf(“You entered a non-positive number\n”);
}
For Loop Without Initialization
You may have no initialization in a for loop.
In this case you can leave the first part empty however, you
need to put the ;
Therefore the syntax of for loop without initialization will
be
for( ; condition ; update counter )
[loop body]
Example 6
Write a C program to compute the discount offered to a
customer by an insurance company. The company has the
following rules:
A customer gets a discount for each year of being registered with
the company.
For each year, the age of the customer is multiplied by 0.05
The discount is the sum of the discounts given for each year
For example, if the customer registers when he is 30, and his
current age is 34, he gets a discount as below:
Discount = (30*0.05 + 31*0.05 + 32*0.05 + 33*0.05) %
Discount = 6.3 %
#include<stdio.h>
void main()
{
int startAge, currentAge;
float discount ;
discount = 0;
printf(“Enter the age you started insurance, and your current age: \n”);
scanf(“%d%d”, &startAge, ¤tAge);
for( ; startAge < currentAge ; startAge ++)
discount = discount + startAge*0.05;
printf(“The total discount is %f\n”, discount );
}
Note: The initialization part of the loop is empty here
For Loop without Condition Part
The condition part is optional in for loops
A loop without a condition should still include ; characters to
separate three parts of the loop
for( initialization ; ; update )
[body statements]
A loop without a condition will not terminate. These loops
are called infinite loops
You can terminate a for loop without a condition using
break statement.
Example 7
Assume 3 people labeled as ‘A’, ‘B’, and ‘C’ are playing a game. The user enters an integer number.
If the number is divisible by 7 ‘A’ gets 1 point.
If the number is divisible by 11 ‘B’ gets 2 point,
If the number is divisible by 13 ‘C’ will get 3 point.
The program ends when -1 is entered.
The score of each player is the total point he gets divided by the number of times user enters a number
Example: with data 3, 6, 7, 11, 33, 21, 39 scores are:
A=2/7
B=4/7
C=3/7
Write a C program to compute the scores in this game
#include<stdio.h>
void main()
{
float scoreA, scoreB, scoreC;
int nextNum, count ;
scoreA = scoreB = scoreC = 0;
for( count = 0 ; ; count ++ )
{
printf(“Enter next number: \n”);
scanf(“%d”, &nextNum);
if( nextNum == -1 ) break ;
if( nextNum % 7 == 0 )
scoreA = scoreA + 1;
if( nextNum % 11 == 0 )
scoreB = scoreB + 2;
if( nextNum % 13 == 0 )
scoreC = scoreC + 3;
}
scoreA = scoreA / count;
scoreB = scoreB / count;
scoreC = scoreC / count;
printf(“The scores are: A=%f B=%f C=%f\n”, scoreA, scoreB, scoreC );
}
No condition part in for
loop
For Loop without update counter Part
The update counter part of a for loop is also optional
If you decide not to have update counter part in a for loop you
should:
Update the variable inside the body of the loop, otherwise the
condition will not change and the loop will not terminate.
Include ; even if update part is omitted.
A for loop without initialization and update parts is the
same as a while loop
Example 8
Assume we want to compare the probability of getting a total of
10 when we roll two dice with the probability of getting a total
of 9. For instance, the number of times you get (4,6) or (5,5)
compared to the probability of getting (3,6) or (4,5), etc.
Write a C program to read two positive integer and find out
their sum. The program should count the number of times that
you get 10, and the number of times you get 9. The program
terminates when you get twenty 10s or 9s.
Hint: Make sure that the numbers are between 1 and 6
#include<stdio.h>
void main()
{
int dice1, dice2, tens, nines;
nines = tens = 0;
for( ; tens < 20 && nines < 20 ; )
{
printf(“Enter dice numbers \n”);
scanf(“%d%d”, &dice1, &dice2);
if( dice1 < 1 || dice1 > 6 || dice1 < 1 || dice2 > 6)
printf(“ You entered invalid numbers\n”);
else
{
if( dice1 + dice 2 == 10 )
tens++;
if( dice1 + dice 2 == 9 )
nines ++;
}
} /* end of for loop */
if( nines == 20 )
printf(“The total of 9 is more probable\n”);
else
printf(“The total of 10 is more probable\n”);
}
Summary
If parts of a program are repeated with different values, we can use a loop
Do-While loop repeats executing the statements until the condition becomes false. Do-while is executed at least once
For loops have initialization, condition, and update parts
All parts of a for loop are optional
Questions?
