Supriya,
Shabbir already provide a good article for number conversion.Go thru it for all type of number conversions.
For your reference i make a slight touch to existing (Shabbir) program.
To convert Hexadecimal no to octal no......(I just combined two functions into a single one)
Coding listed below:
//first convert Hex to Dec number
//Next convert Dec to Oct number
Code: CPP
#include<stdio.h>
#include<string.h>
#include<math.h>
void main()
{
char hex[100];
double dec=0.0;
int oct[100],temp[100];
int len,i,j,num=0;
printf("Enter a hexadecimal number");
scanf("%s",hex);
len=strlen(hex);
for(i=0;i<len;i++)
{
switch(hex[i])
{
case '0':
temp[i]=hex[i]-48; //Ascii code of 0 is 48 48-48=0//
break;
case '1':
temp[i]=hex[i]-48; //Ascii code of 1 is 49 49-48=1//
break;
case '2':
temp[i]=hex[i]-48; //Ascii code of 2 is 50 50-48=2//
break;
case '3':
temp[i]=hex[i]-48; //Ascii code of 3 is 51 51-48=3//
break;
case '4':
temp[i]=hex[i]-48; //Ascii code of 4 is 52 52-48=4//
break;
case '5':
temp[i]=hex[i]-48; //Ascii code of 5 is 53 53-48=5//
break;
case '6':
temp[i]=hex[i]-48; //Ascii code of 6 is 54 54-48=6//
break;
case '7':
temp[i]=hex[i]-48; //Ascii code of 7 is 55 55-48=7//
break;
case '8':
temp[i]=hex[i]-48; //Ascii code of 8 is 56 56-48=8//
break;
case '9':
temp[i]=hex[i]-48; //Ascii code of 9 is 57 57-48=9//
break;
case 'A':
temp[i]=hex[i]-55; //Ascii code of A is 65 65-55=10//
break;
case 'B':
temp[i]=hex[i]-55; //Ascii code of B is 65 66-55=11//
break;
case 'C':
temp[i]=hex[i]-55; //Ascii code of C is 65 67-55=12//
break;
case 'D':
temp[i]=hex[i]-55; //Ascii code of D is 65 68-55=13//
break;
case 'E':
temp[i]=hex[i]-55; //Ascii code of E is 65 68-55=14//
break;
case 'F':
temp[i]=hex[i]-55; //Ascii code of F is 65 69-55=15//
break;
}
}
for(i=0,j=len;i<len;i++,j--)
dec=dec+(temp[j-1]*pow(16,i));
printf("The decimal equivalent is %lg \n",dec);
num=(int)dec;
//OCTAL CONVERSION
for(i=0;num!=0;i++)
{
oct[i]=num%8;
num=num/8;
}
i--;
printf("\n Octal equivalent number:");
for(;i>=0;i--)
printf("%d",oct[i]);
printf("\n");
}