Loops in C Programming | for Loop | do-while Loop | while Loop | Nested Loop

Loops in C Programming
Computer Programming
PREPARED BY
PRIYOM MAJUMDER
BACHELOR OF COMMERCE (HONS.), 6TH SEM
ITM UNIVERSITY, GWALIOR
priyom99anms@gmail.com

1 | P a g e
LOOPS IN C PROGRAMMING
Loops
The versatility of computer lies in its ability to perform a set of instructions
repeatedly. This involves repeating some portion of the programs either a
specified number of times or until a particular condition is satisfied. This
repetitive operation is done through a loop control instructions. There are three
methods by way of which we can repeat a part of program. They are:
a. Using for statement
b. Using a while statement
c. Using a do-while statement
The - for loop:
The most popular looping is instructions. The for allows us to specify three
things about loop in a single line:
a. Setting a loop counter to an initial value.
b. Testing the loop counter to determine whether its value has reached the
number of repetitions desired.
c. Increasing the value of loop counter each time the program segment
within the loop has been executed.
The general form of for statement is as under:
for (initialize counter; test counter; increment counter)
{
do this;
and this;
}
Let us write down the simple interest program using for.

2 | P a g e
Let us now examine how the for statement gets executed:
 When the for statement is executed for first time, the value of count is
set to an initial value of 1.
 Now the condition count<=3 is tested. Since count is 1 the condition is
satisfied and the body of the loop is executed for the first time.
 Upon reaching the closing brace of for, control is sent back to the for
statement, where the value of count gets incremented by 1.
 Again the test is performed to check whether the new of count exceeds 3.
 If the value of count is still within the range 1 to 3, the statements
within the brace of for are executed again.
 The body of the for loop continuous to get executed till count doesn’t
exceed final value of 3.
 When count reaches the value 4 the control exits from loop and is
transferred to the statement (if any) immediately after the body of for.
The following figure would help in further clarifying the concept of execution of
the for loop.
It is important to note that the initialization, testing and incrimination part of a
for loop can be replaced by any valid expression.
Let us now write down the program to print numbers from 1 to 10 in different
ways.

3 | P a g e
main()
{
int i;
for(i=1; i<=10; i=i+1)
printf(“%dn”, i);
}
Note that the initialization, testing and incrimination of loop counter is done in
the for statement itself. Instead of i+1, the statements of i++ and 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.
main()
{
int i;
for(i=1; i<=10;)
{
i = i+1;
}
}
Here the incrimination is done within the body of the loop and not in the for
statement. Note that in spite of this the semicolon after the condition is
necessary.
main()
{
int i=1;
for(; i<=10;)
{
i=i+1;
}
}
Here, neither the initialization, nor the incrimination is done in the statement,
but still the two semicolons are necessary.
main()
{
int i;
for(i=0; i++<10;)
}

4 | P a g e
Here the comparison as well as the incrimination is done through the same
statement, i++<10. Since the ++ operator comes after i firstly comparison is
done, followed by incrimination. Note that it is necessary to initialize i to 0.
main()
{
int i;
for(i=0; ++i<=10;)
}
Here both, the comparison and the incrimination is done through the same
statement, ++i<=10. Since ++ precedes i firstly incrimination is done, followed
by comparison. Note that it is necessary to initialize i to 0.
The -while loop:
It is often the case in programming that you want to do something a fixed
numbers of times. Perhaps you want to calculate gross salaries of ten different
persons, or you want to convert temperatures from centigrade to Fahrenheit for
15 different cities.
The while loop is ideally suited for such cases. The general syntax of while loop
is:
initialize loop counter;
while (test loop counter using condition)
{
do this;
and this;
increment loop counter;
}
Let us look at a simple example, which uses while loop.

5 | P a g e
The above program executes all statements after the while 3 times. The logic
for calculating the simple interest is written within a pair of braces immediately
after the while keyword. These statements form what is called the ‘body’ of the
loop. The parenthesis after the while contain a condition. So long as this
condition remains true all statements within the body of while loop keep
getting executed repeatedly. To begin with the variable count is initialized to 1
and every time the simple interest logic is executed to the value of count is
incremented by one. The variable count is many a time called either a ‘loop
counter’ or an ‘index variable’.
Note the following points about while…
 The statements within while loop would keep on getting executed till the
condition being tested remains true. When the condition becomes false,
the control passes to the first statements that follows the body of while
loop. In place of the condition there can be any other valid expression. So
long as the expression evaluates to a non-zero values the statements
within the loop would get executed.
 The condition being tested may use relational or logical operators as
shown in the following examples:
o while(i<=10)
o while(i>=10 && j<=15)
o while(j>10 && (b<15||c<20)
 The statements within the loop may be single line or a block of
statements. In the first case the parenthesis are optional. For example:
while(i<10)
{
i=i+1;
}
Is same as
while (i<10)
i=i+1;
 As a result the while must test a condition that will eventually become
false, otherwise the loop would be executed forever, indefinitely,
main()
{
int i=1;
while(i<=10)
}

6 | P a g e
This is an indefinite loop, since i remain equal to 1 forever. The correct
would be as under:
main()
{
int i=1;
while(i<=10)
{
i++;
}
}
 Instead of incrementing a loop counter, we can even decrement it and
still manage to get the body of the loop executed repeatedly. This is
shown below:
main()
{
int i=5;
while(i>=1)
{
i--;
}
}
 It is not necessary that a loop counter must only be an int. It can be a
float.
main()
{
float a=10.0;
while(a<=10.5)
{
printf(“nRaindrops on roses”);
printf(“…and whiskers on kittens”);
a=a+0.1;
}
}
The do-while loop:
The do-while loop looks like this:
do
{
this;
and this;
}while(this condition is true);

7 | P a g e
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.
The 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. The difference is brought
about clearly by the following program.
main()
{
while(4<1)
printf(“n Hello There”);
}
Here, since the condition fails the first time itself, printf() will not get executed
at all. Let’s now write the same program using a do-while loop.
main()
{
do
{
printf(“Hello Theren”);
}while(4<1);
}
In this program the printf() would be executed once, since first the body of the
loop is executed and then the condition is tested.
There are some occasions when we want to execute a loop at least once no
matter what.
Nesting of loops:
The way if statements can be nested, similarly whiles and for’s can also be
nested. To understand how nested loops work; look at the program given in the
next page.

8 | P a g e
When you run the program you get the following output:
Here, for each of i the inner loop is cycled through 4 times, with the variable j
taking values from 1 to 4. The inner loop terminates when the value of j
exceeds 4, and the outer loop terminates when the value of i 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 indentions make the program
easier to understand.
Instead of using two statements, one to calculate sum and another to print it
out, we can compact this into one single statement saying:
printf(“i=%d j=%d sum=%dn”, i, j, i+j);
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.

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