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Cisco, CCNA Exam and Answer
Lecture 6
The for Loop
The for loop is the most popular looping
control among all. The for loop allows us to specify three things 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/decreasing the value of loop counter each time the
program segment within the loop is executed.
The general form of for statement is as
follows:
for
(initialize counter; test counter; increment/decrement counter){
do this;
and this;
and this;
}
For example, the program below prints 1 to
10.
main(){
int counter;
for (counter=1;counter<=10;counter++){
printf (“%d\n”, counter);
}
}
Let’s examine the structure of for more
detail:
a)
Initially
counter is set to 1. Check if it is less than or equal to 10. Yes it is.
b)
Move into the
body of for. Print the variable counter.
c)
Increment
counter by 1.
d)
counter is
now 2. Check if it is less than or equal to 10. Yes it is.
e)
Move into the
body of for. Print the variable counter.
f)
Increment
counter by 1.
……………………………………………………………………………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………………………………………………………………………
counter
is now 11. Check if it is less than or equal to 10. No, it is not.
End
of the loop.
Now, let’s play with for loop for a while!
main(){
int counter=1;
for (;counter<=10;counter++){
printf (“%d\n”, counter);
}
}
This is valid. We have initialized counter
to 1 before we came to for loop. But inside the for loop, the semi-colon is
still required.
main(){
int counter;
for (counter=1;counter<=10;){
printf (“%d\n”, counter);
counter++;
}
}
This is valid as well. We have incremented
counter by 1 just before the end of the ending brace. It makes counter to 2 and
again the control goes back to the for loop. But inside the for loop, the
semi-colon is still required.
main(){
int counter=1;
for (;counter<=10;){
printf (“%d\n”, counter);
counter++;
}
}
And valid so as
this! We have initialized counter to 1. Then we have incremented the variable
just as before. The for loop only contains the checking of condition. Both the
semi-colons are required.
main(){
int counter=1;
for (;;){
printf (“%d\n”, counter);
counter++;
}
}
This is an
infinite for loop. As we have omitted the condition checking from the loop, it
prints counter for infinite times.
for Loops: Notes and Observations
1.
The initialization,
loop-continuation condition and increment/decrement can contain arithmetic
expressions. Assume that x=2 and y=10. The statement-
for(j=x;
j<=4*x*y; j=j+y/x)
is equivalent
to-
for(j=2;
j<=80; j=j+5)
2.
There can be a decrement as
well (or you can say a negative increment). For example-
main(){
int counter;
for (counter=10;counter>=1;counter--)
printf(“%d”, counter);
}
prints 10 9 8 7
6 5 4 3 2 1.
3.
If the loop continuation
condition is initially false, the body portion of the loop is not performed.
main(){
int i,loopcontrol=20,mark=0;
for (i=1;i>=loopcontrol;i++){
mark=1;
}
if(mark==0)
printf (“The loop continuation
condition is initially false”);
else
printf(“We have entered into the
loop”);
}
the above
program will all the time print The loop
continuation condition is initially false as it never
went into the body of for because the loop continuation condition is false (1
is not greater than or equal to 20).
Qs
We
want to go 1 to 100 incremented by 1 (1 2 3 4 …
100)
We
want to go 1 to 100 incremented by 3 (1 4 7 10 …
100)
We
want to go to 1 from 100 decremented by 1 (100 99 98 … 1)
We
want to go to 0 from 100 decremented by 5 (100 95 90 … 0)
Nested for Loops
while and for
loops can be nested. A nested loop is loop inside a loop. Let us demonstrate
the procedure.
main(){
int i, j;
for (i=1;i<=3;i++){
for (j=1; j<=2;j++){
printf(“i=%d j=%d sum=%d\n”,
i, j, i+j);
}
}
}
The output of
the program is as follows-
i=1
j=1 sum=2
i=1
j=2 sum=3
i=2
j=1 sum=3
i=2
j=2 sum=4
i=3
j=1 sum=4
i=3
j=2 sum=5
Let’s Play
What will be the output?
#include<stdio.h>
#include<conio.h>
void
main(){
int i;
clrscr();
for(i=1;i<=10;printf("%d\n",i)){
i++;
}
getch();
}
What will be the
output?
#include<stdio.h>
#include<conio.h>
void
main(){
int i;
clrscr();
for(;i=0;){
printf("Hello there\n");
}
getch();
}
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 conditional test. 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. As an example, let’s consider the
following example.
#include<stdio.h>
#include<conio.h>
void main( )
{
int i;
clrscr();
for(i=0;i<10;i++){
if(i==5)
break;
printf("%d",i);
}
getch();
}
The output of
the program is 0 1 2 3 4. When the value of i is 5, the program continues
inside the if where it finds break statement. The program execution then goes to the end of
for loop and then it continues executing the rest of the statements (getch()
and the final curly brace).
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>
#include<conio.h>
void main( )
{
int i;
clrscr();
for(i=0;i<10;i++){
if(i==5)
continue;
printf("%d",i);
}
getch();
}
The output of the above program would be 0 1 2 3 4 6 7 8 9.
The program prints 0 1 2 3 4 as up to this point, the if condition
is false. Then when the value of i is 5, it goes into the if block and executes
the continue statement. The program then omits the printf and increases i by
one and then again does the rest- prints 6 7 8 9 as this time again the
condition of if is false.
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