The programs that we have developed so far used either a sequential or a decision control instruction. In the first one, the calculations

were carried out in a fixed order; while in the second, an appropriate set of instructions were executed depending upon the outcome of the condition(s) being tested.

The for Loop

Perhaps one reason why few programmers use while is that they are too busy using the for, which is probably the most popular looping instruction. The for allows us to specify three things about a loop in a single line:

The general form of for statement is as under:

for ( initialize counter ; test counter ; increment counter )
 { 
    do this ; and this ; and this ;
 }

Let us now write down the simple interest program using for. Compare this program with the one that we wrote using while. The flowchart is also given in Figure 6.1 for a better understanding.

START

STOP

Yes

No

si = p * n * r / 100

count = 1

count = count + 1

is count <= 3

INPUT p, n, r

PRINT si

Figure 6.1

/* Calculation of simple interest for 3 sets of p, n and r */

#include <stdio.h> 
int main( ) 
{ 
    int p, n, count ; 
    float r, si ; 
    for ( count = 1 ; count <= 3 ; 
    count = count + 1 ) 
    { 
        printf ( "Enter values of p, n, and r " ) ; 
        scanf ( "%d %d %f", &p, &n, &r ) ; 
        si = p * n * r / 100 ; 
        printf ( "Simple Interest = Rs.%f\\n", si ) ; 
    } 
        return 0 ; 
}

If you compare this program with the one written using while, you can observe that the three steps—initialization, testing and incrementation—required for the loop construct have now been incorporated in the for statement.

Let us now examine how the for statement gets executed:

Figure 6.2 /would help in further clarifying the concept of execution of the for loop./

START

STOP

test

True

False

body of loop

initialize

increment

Figure 6.2

It is important to note that the initialization, testing and incrementation part of a for loop can be replaced by any valid expression. Thus the following for loops are perfectly ok.

for ( i = 10 ; i ; i -- ) printf ( "%d ", i ) ; 
for ( i < 4 ; j = 5 ; j = 0 ) printf ( "%d ", i ) ; 
for ( i = 1; i <=10 ; printf ( "%d ", i++ ) ) ; 
for ( scanf ( "%d", &i ) ; i <= 10 ; i++ ) printf ( "%d", i ) ;

Let us now write down the program to print numbers from 1 to 10 in different ways. This time we would use a for loop instead of a while loop.

1.

#include <stdio.h> 
int main( ) 
{ 
    int i ; 
    for ( i = 1 ; i <= 10 ; i = i + 1 ) 
    printf ( "%d\\n", i ) ; 
    return 0 ; 
}

Note that the initialization, testing and incrementation of loop counter is done in the for statement itself. Instead of i = i + 1, the statements i++ or i += 1 can also be used.

Since there is only one statement in the body of the for loop, the pair of braces have been dropped. As with the while, the default scope of for is the immediately next statement after for.

2.

#include <stdio.h> 
int main( ) 
{ 
    int i ; 
    for ( i = 1 ; i <= 10 ; ) 
    { 
        printf ( "%d\\n", i ) ; i = i + 1 ;
    }
    return 0 ; 
}

Here, the incrementation is done within the body of the for loop and not in the for statement. Note that, in spite of this, the semicolon ( ; ) after the condition is necessary.

3.

#include <stdio.h> 
int main( ) 
{ 
    int i = 1 ; 
    for ( ; i <= 10 ; i = i + 1 ) 
    printf ( "%d\\n", i ) ;
    return 0 ; 
}

Here the initialization is done in the declaration statement itself, but still the semicolon before the condition is necessary.

4.

#include <stdio.h> 
int main( ) 
{ 
    int i = 1 ; 
    for ( ; i <= 10 ; ) 
    { 
        printf ( "%d\\n", i ) ; 
        i = i + 1 ; 
    }
        return 0 ; 
}

Here, neither the initialization, nor the incrementation is done in the for statement, but still the two semicolons are necessary.

5.

#include <stdio.h> 
int main( ) 
{ 
    int i ; 
    for ( i = 0 ; i++ < 10 ; ) 
    printf ( "%d\\n", i ) ; 
    return 0 ;
}

Here, the comparison as well as the incrementation is done through the same expression, i++ < 10. Since the ++ operator comes after i, first comparison is done, followed by incrementation. Note that it is necessary to initialize i to 0.

6.

#include <stdio.h> 
int main( ) 
{ 
    int i ; 
    for ( i = 0 ; ++i <= 10 ; ) 
    printf ( "%d\\n", i ) ; 
    return 0 ; 
}

Here again, both, the comparison and the incrementation are done through the same expression, ++i <= 10. Since ++ precedes i firstly incrementation is done, followed by comparison. Note that it is necessary to initialize i to 0.

Nesting of Loops

The way if statements can be nested, similarly whiles and fors can also be nested. To understand how nested loops work, look at the program given below.

/* Demonstration of nested loops */

#include <stdio.h> 
int main( ) 
{ 
    int r, c, sum ; 
    for ( r = 1 ; r <= 3 ; r++ ) /* outer loop */ 
    { 
        for ( c = 1 ; c <= 2 ; c++ ) /* inner loop */ 
        { 
            sum = r + c ; 
            printf ( "r = %d c = %d sum = %d\\n", r, c, sum ) ; 
        } 
    } return 0 ; 
}

When you run this program, you will get the following output:

r = 1 c = 1 sum = 2 
r = 1 c = 2 sum = 3 
r = 2 c = 1 sum = 3 
r = 2 c = 2 sum = 4 
r = 3 c = 1 sum = 4 
r = 3 c = 2 sum = 5

Here, for each value of r, the inner loop is cycled through twice, with the variable c taking values from 1 to 2. The inner loop terminates when the value of c exceeds 2, and the outer loop terminates when the value of r exceeds 3.

As you can see, the body of the outer for loop is indented, and the body of the inner for loop is further indented. These multiple indentations make the program easier to understand.

Instead of using two statements, one to calculate sum and another to print it out, we can compact them into one single statement by saying:

printf ( "r = %d c = %d sum = %d\\n", r, c, r + c ) ;

The way for loops have been nested here, similarly, two while loops can also be nested. Not only this, a for loop can occur within a while loop, or a while within a for.

Multiple Initializations in the for Loop

The initialization expression in the for loop can contain more than one statement separated by a comma. For example,

for ( i = 1, j = 2 ; j <= 10 ; j++ )

Multiple statements can also be used in the incrementation expression of for loop; i.e., you can increment (or decrement) two or more variables at the same time. Similarly multiple conditions are allowed in the test expression. These conditions must be linked together using logical operators && and/or ||.

The break Statement

We often come across situations where we want to jump out of a loop instantly, without waiting to get back to the condition. The keyword break allows us to do this. When break is encountered inside any loop, control automatically passes to the first statement after the loop. A

break is usually associated with an if. Let’s consider the following example:

Example 6.1: Write a program to determine whether a number is prime or not. A prime number is said to be prime if it is divisible only by 1 or itself.

All we have to do to test whether a number is prime or not, is to divide it successively by all numbers from 2 to one less than itself. If remainder of any of these divisions is zero, the number is not a prime. If no division yields a zero then the number is a prime number. Following program implements this logic:

#include <stdio.h> 
int main( ) 
{ 
    int num, i ; 
    printf ( "Enter a number " ) ; 
    scanf ( "%d", &num ) ; i = 2 ; 
    while ( i <= num - 1 ) 
    { 
        if ( num % i == 0 ) 
        { 
            printf ( "Not a prime number\\n" ) ;
            break ; 
        } 
        i++ ; 
    } 
    if ( i == num ) 
    printf ( "Prime number\\n" ) ; 
}

In this program, the moment num % i turns out to be zero, (i.e., num is exactly divisible by i), the message “Not a prime number” is printed and the control breaks out of the while loop. Why does the program require the if statement after the while loop at all? Well, there are two possibilities the control could have reached outside the while loop:

When the loop terminates in the second case, it means that there was no number between 2 to num - 1 that could exactly divide num. That is, num is indeed a prime. If this is true, the program should print out the message “Prime number”.

The keyword break, breaks the control only from the while in which it is placed. Consider the following program, which illustrates this fact:

#include <stdio.h> 
int main( ) 
{
     int i = 1 , j = 1 ; 
     while ( i++ <= 100 ) 
     { 
        while ( j++ <= 200 ) 
        { 
            if ( j == 150 ) 
            break ; 
            else 
            printf ( "%d %d\\n", i, j ) ; 
        } 
    } 
    return 0 ; 
}

In this program when j equals 150, break takes the control outside the inner while only, since it is placed inside the inner while.

The continue Statement

In some programming situations, we want to take the control to the beginning of the loop, bypassing the statements inside the loop, which have not yet been executed. The keyword continue allows us to do this. When continue is encountered inside any loop, control automatically passes to the beginning of the loop.

A continue is usually associated with an if. As an example, let’s consider the following program:

#include <stdio.h>
int main( ) 
{ 
    int i, j ; 
    for ( i = 1 ; i <= 2 ; i++ ) 
    { 
        for ( j = 1 ; j <= 2 ; j++ ) 
        { 
            if ( i == j ) 
            continue ; 
            printf ( "%d %d\\n", i, j ) ; 
        } 
    } return 0 ; 
}

The output of the above program would be…

1 2 2 1

Note that when the value of i equals that of j, the continue statement takes the control to the for loop (inner) bypassing the rest of the statements pending execution in the for loop (inner).

The do-while Loop The do-while loop looks like this:

do 
{ 
    this ; 
    and this ; 
    and this ; 
    and this ; 
} while ( this condition is true ) ;

There is a minor difference between the working of while and do-while loops. This difference is the place where the condition is tested. The while tests the condition before executing any of the statements within the while loop. As against this, the do-while tests the condition after

having executed the statements within the loop. Figure 6.3 would clarify the execution of do-while loop still further.

START

STOP

test True

False

body of loop

initialize

increment

Figure 6.3

This means that do-while would execute its statements at least once, even if the condition fails for the first time. The while, on the other hand will not execute its statements if the condition fails for the first time. This difference is brought about more clearly by the following program:

#include <stdio.h> 
int main( ) 
{ 
    while ( 4 < 1 ) 
    printf ( "Hello there \\n" ) ; 
    return 0 ; 
}

Here, since the condition fails the first time itself, the printf( ) will not get executed at all. Let’s now write the same program using a do-while loop.

#include <stdio.h> 
int main( ) 
{
    do 
    { 
        printf ( "Hello there \\n" ) ; 
    }
    while ( 4 < 1 ) ; 
    return 0 ; 
}

In this program, the printf( ) would be executed once, since first the body of the loop is executed and then the condition is tested.

The Odd Loop

The loops that we have used so far executed the statements within them a finite number of times. However, in real life programming, one comes across a situation when it is not known beforehand how many times the statements in the loop are to be executed. This situation can be programmed as shown below.

/* Execution of a loop an unknown number of times */

#include <stdio.h> 
int main( ) 
{ 
    char another ; 
    int num ; 
    do 
    { 
        printf ( "Enter a number " ) ; 
        scanf ( "%d", &num ) ; 
        printf ( "square of %d is %d\\n", num, num * num ) ; 
        printf ( "Want to enter another number y/n " ) ; 
        fflush ( stdin ) ; 
        scanf ( "%c", &another ) ; 
    } 
    while ( another == 'y' ) ; 
    return 0 ; 
}

And here is the sample output…

Enter a number 5 square of 5 is 25 Want to enter another number y/n y

Enter a number 7 square of 7 is 49 Want to enter another number y/n n

In this program, the do-while loop would keep getting executed till the user continues to answer y. The moment user answers n, the loop terminates, since the condition ( another == ‘y’ ) fails. Note that this loop ensures that statements within it are executed at least once even if n is supplied first time itself.

Perhaps you are wondering what for have we used the function fflush( ). The reason is to get rid of a peculiarity of scanf( ). After supplying a number when we hit the Enter key, scanf( ) assigns the number to variable num and keeps the Enter key unread in the keyboard buffer. So when it’s time to supply Y or N for the question ‘Want to enter another number (y/n)’, scanf( ) will read the Enter key from the buffer thinking that user has entered the Enter key. To avoid this problem, we use the function fflush( ). It is designed to remove or ‘flush out’ any data remaining in the buffer. The argument to fflush( ) must be the buffer which we want to flush out. Here we have used ‘stdin’, which means buffer related with standard input device, i.e., keyboard.

Though it is simpler to program such a requirement using a do-while loop, the same functionality if required, can also be accomplished using for and while loops as shown below.

/* odd loop using a for loop */

#include <stdio.h> 
int main( ) 
{ 
    char another = 'y' ; 
    int num ; 
    for ( ; another == 'y' ; ) 
    { 
        printf ( "Enter a number " ) ; 
        scanf ( "%d", &num ) ; 
        printf ( "square of %d is %d\\n", num, num * num ) ;
        printf ( "Want to enter another number y/n " ) ; 
        fflush ( stdin ) ; 
        scanf ( "%c", &another ) ; 
    } 
    return 0 ; 
}

/* odd loop using a while loop */

#include <stdio.h> 
int main( ) 
{ 
    char another = 'y' ; 
    int num ; 
    while ( another == 'y' ) 
    { 
        printf ( "Enter a number " ) ; 
        scanf ( "%d", &num ) ; 
        printf ( "square of %d is %d\\n", num, num * num ) ; 
        printf ( "Want to enter another number y/n " ) ; 
        fflush ( stdin ) ; 
        scanf ( " %c", &another ) ; 
    } 
    return 0 ; 
}

break and continue are used with do-while just as they would be in a while or a for loop. A break takes you out of the do-while bypassing the conditional test. A continue sends you straight to the test at the end of the loop.

Summary

Exercise

[A] What will be the output of the following programs:

1.

#include <stdio.h> 
int main( ) 
{ 
    int i = 0 ; 
    for ( ; i ; ) 
    printf ( "Here is some mail for you\\n" ) ; 
    return 0 ; 
}

2.

#include <stdio.h> 
int main( ) 
{ 
    int i ; 
    for ( i = 1 ; i <= 5 ; printf ( "%d\\n", i ) ) ; 
    i++ ; 
    return 0 ; 
}

3.

#include <stdio.h> 
int main( ) 
{ 
    int i = 1, j = 1 ; 
    for ( ; ; ) 
    { 
        if ( i > 5 ) 
        break ; 
        else j += i ; 
        printf ( "%d\\n", j ) ; 
        i += j ; 
        return 0 ; 
    } 
}

[B] Answer the following:

the i____________ expression the t____________ expression

the i____________ expression

1. An if statement 2. A for loop 3. A program 4. The main( ) function

1. Only once 2. At least once 3. More than once 4. None of the above

1. Initialization, execution of body, testing 2. Execution of body, initialization, testing 3. Initialization, testing, execution of body 4. None of the above

1. int i = 1 ; while ( 1 ) { i++ ; }

2. for ( ; ; ) ;

3. int t = 0, f ; while ( t ) { f = 1 ; }

4. int y, x = 0 ; do { y = x ; } while ( x == 0 ) ;

#include <stdio.h> 
int main( ) 
{ 
    int j = 1 ; 
    while ( j <= 255 ) ; 
    { 
        printf ( "%c %d ", j, j ) ; 
        j++; 
    } 
    return 0 ; 
}

#include <stdio.h> 
int main( ) 
{ 
    int x=10, y = 100%90 ; 
    for ( i = 1 ; i <= 10 ; i++ ) ; 
    if ( x != y ) ; 
    printf ( "x = %d y = %d\\n", x, y ) ; 
    return 0 ; 
}
  1. The printf( ) function is called 10 times.
  2. The program will produce the output x=10 y=10.
  3. The ; after the if (x!=y) would not produce an error.
  4. The program will not produce any output.
  5. The printf( ) function is called infinite times.
  1. for loop works faster than a while loop.
  2. All things that can be done using a for loop can also be done

using a while loop. 3. for ( ; ; ) implements an infinite loop. 4. for loop can be used if we want statements in a loop to get

executed at least once. 5. for loop works faster than a do-while loop.

[C] Attempt the following:

using a for loop:

29 * 1 = 29 29 * 2 = 58

…

i = 2 + ( y + 0.5 x )

Write a program that will produce a table of values of i, y and x, where y varies from 1 to 6, and, for each value of y, x varies from 5.5 to 12.5 in steps of 0.5.

a = p ( 1 + r / q ) nq

Write a program to read 10 sets of p, r, n & q and calculate the corresponding as.

1

1!

2

2!

3

3! ……

.

If x is input through the keyboard, write a program to calculate the sum of first seven terms of this series.

(m) Write a program to produce the following output:

1

2 3

4 5 6

7 8 9 10

(n) Write a program to produce the following output:

A B C D E F G F E D C B A

A B C D E F F E D C B A

A B C D E E D C B A

A B C D D C B A

A B C C B A

A B B A

A A


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