Loops

WHAT IS LOOP CONSTRUCT IN C ?
• A LOOP IS A REPETITION CONTROL
STRUCTURE THAT ALLOWS YOU TO EFFICIENTLY
WRITE A LOOP THAT NEEDS TO EXECUTE A SPECIFIC
NUMBER OF TIMES.

C programming has three types of
loops:
1.for loop
2.while loop
3.do...while loop

1.FOR LOOP
• BASIC SYNTAX OF FOR LOOP

2. WHILE LOOP
• BASIC SYNTAX OF WHILE LOOP

3. DO… WHILE LOOP
•BASIC SYNTAX OF DO… WHILE LOOP

WHILE LOOP CONSTRUCT
IN C
A WHILE LOOP IN C PROGRAMMING REPEATEDLY EXECUTES A
TARGET STATEMENT AS LONG AS A GIVEN CONDITION IS TRUE

• THE SYNTAX OF THE WHILE LOOP IS:
WHILE (TESTEXPRESSION)
{
// THE BODY OF THE LOOP
}
SYNTAX OF THE WHILE LOOP

HOW WHILE LOOP WORKS
• THE WHILE LOOP EVALUATES THE TEST EXPRESSION INSIDE THE
PARENTHESES ().
• IF TEST EXPRESSION IS TRUE, STATEMENTS INSIDE THE BODY OF WHILE LOOP
ARE EXECUTED. THEN, TEST EXPRESSION IS EVALUATED AGAIN.
• THE PROCESS GOES ON UNTIL TEST EXPRESSION IS EVALUATED TO FALSE.
• IF TEST EXPRESSION IS FALSE, THE LOOP TERMINATES (ENDS).
• TO LEARN MORE ABOUT TEST EXPRESSIONS (WHEN TEST EXPRESSION IS
EVALUATED TO TRUE AND FALSE), CHECK OUT RELATIONAL AND LOGICAL
OPERATORS.

EXAMPLE 1- WHILE LOOP
• // PRINT NUMBERS FROM 1 TO 5
#include<stdio.h>
int main()
{
int i = 1;
while (i <= 5) {
printf("%dn", i);
++i; }
return 0;
}
OUTPUT
1 2 3 4 5

• HERE, WE HAVE INITIALIZED I TO 1.
• WHEN I = 1, THE TEST EXPRESSION I <= 5 IS TRUE. HENCE, THE BODY OF
THE WHILE LOOP IS EXECUTED. THIS PRINTS 1 ON THE SCREEN AND THE
VALUE OF I IS INCREASED TO 2.
• NOW, I = 2, THE TEST EXPRESSION I <= 5 IS AGAIN TRUE. THE BODY OF
THE WHILE LOOP IS EXECUTED AGAIN. THIS PRINTS 2 ON THE SCREEN AND THE
VALUE OF I IS INCREASED TO 3.
• THIS PROCESS GOES ON UNTIL I BECOMES 6. THEN, THE TEST EXPRESSION I
<= 5 WILL BE FALSE AND THE LOOP TERMINATES.

Imagine teacher asked you to print numbers from 1
to 100, it might be time consuming but you may do
it. But what if she asked you to print 1 lakh or more
numbers?
Will you use the printf statement 1 lakh times to print
it?
OF COURSE NOT !
We humans always try to find a way to make our task
simple and quickly.
And to solve this problem we have created the for
loop.
So what actually is a for loop?
A for loop is just an advanced form of while
loop where we have to initialize, check if the
condition is true or false, execute the code
and update counter in the beginning itself.
Let’s understanding
FOR LOOP using an
Analogy.

We All have played
this game but did we
realise that the birds
here try to follow a
loop?
Every bird here waits for
its turn and when called it
strikes and the same
process is repeated for
other birds
But the number of birds are
limited and hence we need to
know this since the beginning
other wise we will run out of
birds and pigs remains
undestroyed.
Similarly
Similarly in FOR LOOP we
have to write when do we
need to stop at the very
beginning to prevent an
infinite loop formation.

for (birds =1; birds <= 3;
birds++)
{
launch the bird;
}
A for loop consist of three elementary steps :
• The initialization (start)
• The Test Expression(tells when to continue or
stop the loop)
• The Update counter. (updates after every
iteration)

for (int i = 0 ; i <= limit ; i++/i--
etc.)
{
execution of code ;
}
data type
variabl
e conditional
operator
updating data
using
increment or
decrement
operator
curly
braces

1) Start
2) write the starting digit and the
condition along with how you
want to update the counter.
3) Use curly braces to enclose the
code of for loop.
4) Inside the curly braces type the
code you want to execute.
5) On running the program ,
compiler will check the condition
and if its true, it will execute the
code and update the counter.
6) The step 5 is repeated until the
condition remains true.
7) If the condition becomes false,
the compiler stops printing the
code and comes out of the loop.
Understanding the working.

DO-WHILE LOOP CONSTRUCT IN C
LANGUAGE
• THE ‘C’ DO-WHILE STATEMENT CREATES A
STRUCTURED LOOP THAT EXECUTES AS LONG AS A
SPECIFIED CONDITION IS TRUE AT THE END OF EACH
PASS THROUGH THE LOOP, AND IF THE VALUE OF
EXPRESSION IS FALSE THEN THE LOOP IS EXITED.

SYNTAX OF A DO-WHILE LOOP
• THE SYNTAX OF A D0-WHILE LOOP IN THE C
PROGRAMMING LANGUAGE IS:
Initialize;
do
{
statement(s);
increment/decrement;
}
while (condition);

FLOWCHART OF DO WHILE LOOP IN C.

WORKING OF A DO-WHILE LOOP:
• The do-while loop is similar to the while loop with one
important difference. The body of do-while loop is executed
at least once. Only then, the test expression is evaluated.
• If test Expression is true, the body of the loop is executed
again and test Expression is evaluated once more.
• This process goes on until test Expression becomes false.
• If test Expression is false, the loop ends.

AN EXAMPLE OF DO-WHILE
LOOP WITH A SIMPLE CODE
The following programme prints the numbers
between 1 to 100 which are the multiples of ‘n’
(n is the number given by user) using do while
loop:

#include<stdio.h>
int main()
{
int i = 1,n; // declare and initialize i to 1
printf("Enter the number of which you want
the multiples.n=");
scanf("%d",&n);
do //the following is the body of loop
{
// check whether i is multiple of n or not
if(i % n == 0)
printf("%d ", i);
i++; // increment i by 1
}while(i < 100); // stop the loop when i
becomes greater than 100
return 0;
}

A "While" Loop is used
to repeat a specific
block of code an
unknown number of
times, until a condition
Advantages

Disadvantages
While loop can cause the problem if
the index length is incorrect.
It is also slow as the compiler adds
the runtime code to perform the
conditional check on every iteration
through this loop.

A “do while” loop is a control flow
statement that executes a block of
code at least once, and then
repeatedly executes the block, or
not, depending on a given
Boolean condition at the end of
the block
Advantages

Disdvantage
1.Condition must be updated
within body of loop.
2. Caution must be used to
avoid an infinite loop.

Advantages
How many times the loop will
execute before the loop starts is
for loop.

Dissadvantages
It cannot be used for accessing or
viewing any specific element of an
array or collection like array and string
etc.
It’s used when you want to traverse
through whole of the array or
collection framework you are using in
your program

Loops

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