Loops in C++ Programming
Loops in C++
A
loop means “executing some set of code repeatedly till a certain no of times”.
For Example if we want to print the table of 3, we can make the program using
loop. Why? Because in printing the table of 3, we have to start with 3 and then
repeatedly add 3 till we get 30. So we can say this is a program of loop.
There
are two categories of loop in C++ Programming:
·
Entry Controlled
o While Loop
o For Loop
·
Exit Controlled
o Do…While Loop
A
Loop in C++ has essentially 3 parts:
·
Initialization of Loop Variable
·
Test Condition
·
Updating /Re-Initialization
of Loop Variable
Let us understand the concept of
these 3 parts by following examples:
// Program to print the table of 3
using While Loop
Line 1.
#include<iostream.h>
Line 2.
void main()
Line 3.
{
Line 4.
int a=3; //Initialization
of loop variable
Line 5.
while(a<=30) //Test
Condition
Line 6.
{
Line 7.
cout<<a<<endl;
Line 8.
a=a+3; ////Re-initialization
of loop variable
Line 9.
}
Line 10. }
Code
Explanation:
At
Line 4 we start with the value of Loop Variable as 3
At
Line 5 condition is checked for “a” to be either less than or equal to 30. If
the condition is true, the program enters the loop body which starts at Line 6
and ends at Line 8. Here the condition is true so Line 7 and Line 8 executes
and again the program reaches at Line 5 to check the condition with updated
value of loop variable “a”. This process continues till we get value of “a” as
33 which ends the loop because condition becomes false.
.png) |
| For Loop Program |
Code
Explanation:
Line
4: In for loop all three parts of while loop come into one place as we can see
at line 4. We start with the value of
Loop Variable as 3 as Step 1 (Marked in Red). Then condition is checked for “a”
to be either less than or equal to 30 Step 2(Marked in Red). If the condition is true, the program enters
the loop body which starts at Line 5. Here the condition is true so Line 6
executes and again the program reaches at Line 4 Step 4 (Marked in Red). Now
again the program reaches Step 2 to check the condition with updated value of
loop variable “a”. This process of Step 2-3 -4 continues till we get value of “a” as
33 which ends the loop because condition becomes false.
Line 1.
#include<iostream.h>
Line 2.
void main()
Line 3.
{
Line 4.
int a=3; //Initialization
of loop variable
Line 5.
do
Line 6.
{
Line 7.
cout<<a<<endl;
Line 8.
a=a+3; //Re-initialization
of loop variable
Line 9.
} while(a<=30); //Test
Condition
Line 10. }
Code
Explanation:
At
Line 4 we start with the value of Loop Variable as 3
At
Line 5 do statement causes the program to enter the loop body (from Line 6 to
Line 9) without any condition check. Statement 7 and 8 execute and then at Line
9 the updated value of “a” is checked to be either less than or equal to 30. If
the condition is true, the program enters the loop body again which starts at
Line 6 and ends at Line 9. This process from Line 9 to Line 6, Line 7, Line 8 continues till we get value of
“a” as 33 which ends the loop because condition becomes false.
We
get the output as following from each program explained above:
3
6
9
12
15
18
21
24
27
30
Now
we understand the difference among these 3 loops
There
will be no output in while loop and for loop because the condition becomes
false at the entry of the loop and loop does not execute even a single time.
That’s why while and for are called “Entry Controlled” Loop.
But
in case of do….while loop, the condition
is checked at the end of the loop at Line 9 and so the loop executes 1 time
even though the condition is false. SO we can say that “do..while loop executes
at least 1 time compulsorily”.
Do..while
LOOP is most suited for Menu Driven Programs when we definitely want to display
a Menu to the user.
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