Lesson 11
Loop Control STATEMENTS – PART I
(while, do...while and for Loops)
11.1 Loops
Besides decision control statements as described in previous Lesson-10, the other main types of control statement are the iterative statements that are used to create looping mechanism. Thus, loops allow a statement (or block of statements) to be repeated until a given criterion is fulfilled. ‘C’ facilitates three types of loop:
a) while
b) do…while
c) for
11.2 while Loop
The while loop keeps repeating an action until a condition gets false. This looping statement is useful where the programmer does not know beforehand as to how many times the loop will be executed. The general syntax is:
while (expression)
{
Statements;
}
Example 1: Programme to generate even series from 1 to 50 using while loop.
int i = 2;
while (i <= 50)
{
printf(“%d\t”, i);
i = i + 2;
}
In this example, ‘i’ considered as an integer type loop-variable, which is initialised with value, ‘0’. In the while loop, first of all the underlying criterion is checked, i.e., whether the value of ‘i’ is less than or equal to 50; if TRUE, the control will enter in the loop and keeps on updating the loop-variable ‘i’ and prints the output. Once the condition gets FALSE, the control will pass to the next statement out of the loop. The above code segment will print the following result:
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2 |
4 |
6 |
8 |
10 |
12 |
14 |
16 |
18 |
20 |
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22 |
24 |
26 |
28 |
30 |
32 |
34 |
36 |
38 |
40 |
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42 |
44 |
46 |
48 |
50 |
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Example
2: Programme to
evaluate the given series, 
; to calculate 
.
Also, it compares the calculated value with the one by using inbuilt library function.
//C programme to find sum of cosine series?
#include <stdio.h>
#include <conio.h>
#include <math.h>
void main()
{
clrscr();
float power = 2.0, numr, denr = 1.0, x1, sum;
int i = 1, n, s = -1, x;
printf("\n\n\t ENTER THE ANGLE...: ");
scanf("%d", &x);
x1 = 3.142 * (x / 180.0);
sum = 1.0;
numr = x1*x1;
printf("\n\t ENTER THE NUMBER OF TERMS...: ");
scanf("%d",&n);
while(i<=n)
{
denr = denr * power * (power - 1.0);
sum = sum + (numr / (denr * s));
s = s * (-1);
power = power + 2.0;
numr = numr * x1 * x1;
i++;
}
printf("\n\t THE SUM OF THE COSINE SERIES IS : %0.5f", sum);
printf("\n\n\t THE VALUE OF COSINE FUNCTION FOR ANGLE = %d IS :
%0.5f",x,cos(x1));
getch();
}
Let us input the angle as 5 radians and number of terms for iteration as 10. Then, the above programme produces the following output:
ENTER THE ANGLE...: 5
ENTER THE NUMBER OF TERMS...: 10
THE SUM OF THE COSINE SERIES IS : 0.99619
THE VALUE OF COSINE FUNCTION FOR ANGLE = 5 IS : 0.99619
Example 3: Programme corresponding to the algorithm given in Example-1(b) under Lesson-1 to compute average of n numbers using while loop.
#include <stdio.h>
void main()
{
float x, sum=0.0;
int n, i=1;
printf("Enter value for n: ");
scanf("%d", &n);
while (i<=n)
{
printf("Enter value for x: ");
scanf("%f", &x);
sum=sum+x;
i=i+1;
};
mean = sum/ n;
printf("\nMean =%5.2f", mean);
}
Test Output:
Enter value for n: 5
Enter value for x: 10
Enter value for x: 10.5
Enter value for x: 12.12
Enter value for x: 13.45
Enter value for x: 23.90
Mean=13.99
Example 4: Programme corresponding to the algorithm given in Example-2, Lesson-1 to input an examination’s marks of n students as well as to test each student’s marks for the award of a grade.
#include <stdio.h>
void main()
{
int n, marks, i=1;
printf("\nEnter value for n : ");
scanf("%d", &n);
while (i<=n)
{
printf("\nEnter eaxm's marks : ");
scanf("%d", &marks);
if (marks >= 80)
printf("\nDistinction");
else if (marks >= 60 && marks < 80)
printf("\nMerit");
else if (marks >= 40 && marks < 60)
printf("\nPass");
else if (marks < 40)
printf("\nFail");
i=i+1;
}
}
Test Output:
Enter value for n : 5
Enter eaxm's marks : 92
Distinction
Enter eaxm's marks : 67
Merit
Enter eaxm's marks : 56
Pass
Enter eaxm's marks : 32
Fail
Enter eaxm's marks : 71
Merit
11. 3 do…while Loop
The do…while loop is very similar to the while loop except that the condition is tested at the end of the loop body. This ensures that the loop is executed at least once. Provided that the underlying criterion is TRUE, the loop statement(s) are repeated. The general syntax is:
do
{
statements;
}
while (expression);
Note that the statements underlying a while loop might never be executed if the expression is false; however, a do…while statement will always execute the statements contained within the body of the loop at least once.
Illustration – Consider the following code segment:
int i = 1;
do
{
printf(“I like computer programming!”);
++i;
}
while (i <= 7);
In this code segment, the statements inside the loop are executed at least once without any consideration for the underlying criterion; as a result the slogan “I like computer programming” is printed during the first iteration. Thereafter, the conditional expression is evaluated; and if it is found to be FALSE, it will not execute the block of statements any more and exits the loop thereby transferring the control to the next statement.
Example
5: Programme to
calculate the resultant focal length : f
, when f1 and f2
are placed in contact; use formula;
compute for following pairs of focal lengths:
;
#include<stdio.h>
#include<conio.h>
void main()
{
float f=0.0, f1=-10.0, f2=-0.5;
clrscr();
do
{
if(f1!=0.0&&f2!=0.0)
{
f=(f1+f2)/(f1*f2);
printf("\nf1,
f2 and f values are %5.2f, %5.2f
and %7.3f",f1, f2, f);
f1=f1+2.0;
f2=f2+0.1;
}
else
{
f1=f1+2.0;
f2=f2+0.1;
f=(f1+f2)/(f1*f2);
printf("\nf1, f2 and f values are %5.2f, %5.2f
and %7.3f",f1, f2, f);
f1=f1+2.0;
f2=f2+0.1;
}
}while(f1!=12.0&&f2!=0.6);
getch();
}
Example 6: Programme equivalent to algorithm described in
Example-1(c) under Lesson-1 to compute average of n numbers using do…while
(repeat…until) loop.
#include <stdio.h>
void main()
{
float x, sum=0.0;
int n, i=1;
printf("Enter value for n: ");
scanf("%d", &n);
do
{
printf("Enter value for x: ");
scanf("%f", &x);
sum=sum+x;
i=i+1;
} while (i<=n);
mean = sum/ n;
printf("\nMean =%5.2f", mean);
}
Test Output:
Enter value for n: 5
Enter value for x: 10
Enter value for x: 10.5
Enter value for x: 12.12
Enter value for x: 13.45
Enter value for x: 23.90
Mean=13.99
11.4 for
Loop
The for loop is useful when the number of iterations of the
loop is known before the loop is executed. The head of the ‘for’
loop comprises of three constituents separated by semicolons. The general
syntax is:
for (initial statement; conditional expression; loop statement)
{
body of the loop ;
}
§
The initial statement
is used to initialise the loop variable
§ The
conditional expression is used to check whether or not the loop is to be
continued again
§ The
loop statement is used to change the loop-variable value for further
iteration.
Note that all the three constituents are optional;
hence, the following typical for statement is valid that forms an infinite loop to
display the message “Hello, this
is an infinite loop!”
repeatedly, as the conditional expression is always considered as TRUE, if the
same is absent).
for (;;)
{
printf("Hello, this is an infinite
loop!\n");
}
This will keep on executing the underlying printf
statement forever until interrupted by some other means. This might be done so
by incorporating either a return or a break statement inside the loop after printf
statement.
Example 7: Programme corresponding to the algorithm given in
Exercise-7 under Lesson-1?
/* c programme for Exercise-7,
Lesson-1 */
#include <stdio.h>
void main()
{
int n;
for (n = 1; n <= 4; n++)
printf("\n\nLoop%d", n);
}
Output:
Loop1
Loop2
Loop3
Loop4
Example 8: Programme to write a table of 2 using for
loop.
#include <stdio.h>
void main()
{
int a=0;
int i;
for ( i=1; i<=10; i++)
{
a = 2 * i;
printf(“ %d”,a);
}
}
Here, ‘a’ and ‘i’ are int type variables. Within for loop first step
is to initialise ‘i’ with value ‘1’and then
condition is checked that its value is less than or equal to ‘10’,
if TRUE, statements within the body of the loop are executed and the value of ‘i’
is incremented by ‘1’. Once again, the condition is checked; if it is
found valid, the statements within the body of the loop are executed and so on
until the condition becomes FALSE. As a result, the following table is
displayed on the output device:
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12 |
14 |
16 |
18 |
20 |
Example 9: Programme equivalent to the algorithm given in
Example-1(d) under Lesson-1 to compute average of any ten numbers using for
loop.
/* c programme to sum 10 numbers using for loop*/
#include <stdio.h>
void main()
{
int i;
float x, mean, sum = 0.0;
for (i=1; i<=10; i++)
{
printf("\nEnter value for x: ");
scanf("%f", &x);
sum = sum + x;
}
mean = sum/10.0;
printf("\nMean =%5.2f", mean);
}
Test Output:
Enter value for n: 5
Enter value for x: 10
Enter value for x: 10.5
Enter value for x:
12.12
Enter value for x:
13.45
Enter value for x:
23.90
Mean=13.99