Octal to Hexadecimal in C

What is an octal number system?

An octal number system is a number system which is having base-8 where base-8 represents the eight digits from 0 to 7.

What is a Hexadecimal number system?

A hexadecimal number system is a number system which is having base-16 where base-16 represents the sixteen digits, i.e.,0,1,2,3,4,5,6,7,8,9, A, B, C, D, E, F.

How to convert an octal number into a hexadecimal number

We cannot directly convert an octal number system into a hexadecimal number system. First, we need to convert the octal number into a binary number system, and then this binary number system will be converted into a hexadecimal number system.

Let's look at the approach.

Input the octal number.
An octal number system is converted into a binary number system.
Extract the 4 bits in a group from the right-side.
Provide the hexadecimal number to the extracted 4 bits.

For example, the octal number is 123.

Let's create a simple program which converts the octal into a hexadecimal number.

#include <stdio.h>
#include<string.h>
int main()
{
  int octaltobinary[]={0,1,10,11,100,101,110,111};
  char hexadecimal[10];
   char hex[10];
  long int binary=0;
  int octal;
  int rem=0;
  int position=1;
  int len=0;
   int k=0;
  printf("Enter a octal number");
  scanf("%d",&octal);
// Converting octal number into a binary number.  
while(octal!=0)
  {
      rem=octal%10;
      binary=octaltobinary[rem]*position+binary;
      octal=octal/10;
      position=position*1000;
  }
  printf("The binary number is : %ld",binary);
  
  // Converting binary number into a hexadecimal number.
  while(binary > 0)
    {
        rem = binary % 10000;
        switch(rem)
        {
            case 0:
                strcat(hexadecimal, "0");
                break;
            case 1:
                strcat(hexadecimal, "1");
                break;
            case 10:
                strcat(hexadecimal, "2");
                break;
            case 11:
                strcat(hexadecimal, "3");
                break;
            case 100:
                strcat(hexadecimal, "4");
                break;
            case 101:
                strcat(hexadecimal, "5");
                break;
            case 110:
                strcat(hexadecimal, "6");
                break;
            case 111:
                strcat(hexadecimal, "7");
                break;
            case 1000:
                strcat(hexadecimal, "8");
                break;
            case 1001:
                strcat(hexadecimal, "9");
                break;
            case 1010:
                strcat(hexadecimal, "A");
                break;
            case 1011:
                strcat(hexadecimal, "B");
                break;
            case 1100:
                strcat(hexadecimal, "C");
                break;
            case 1101:
                strcat(hexadecimal, "D");
                break;
            case 1110:
                strcat(hexadecimal, "E");
                break;
            case 1111:
                strcat(hexadecimal, "F");
            break;
        }
len=len+1;
        binary /= 10000;
    }
  for(int i=len-1;i>=0;i--)
{
    hex[k]=hexadecimal[i];
    k++;
}
hex[len]='\0';
printf("\nThe hexadecimal number is :");
for(int i=0; hex[i]!='\0';i++)
{
    printf("%c",hex[i]);
}

    return 0;
}

Output

Analysis of the above program

First, we take the user input, which would be an octal number, and it gets stored in an 'octal' variable.
After entering the user input, we convert the octal number into binary number. We iterate through the while(octal!=0) loop, which executes till the value of octal is not equal to zero. The final value of a binary number will be stored in a 'binary' variable.
After calculating the binary number, we will calculate the hexadecimal number. We will iterate through the elements of a binary number by using a while(binary>0) On each iteration, four digits from the right-side of a binary number are extracted and replaced with their corresponding hexadecimal number.
The hexadecimal[] variable contains the reverse value of the actual hexadecimal number. In order to get the actual hexadecimal number of an octal number, we define a new variable, i.e., hex[]. We will iterate the elements of a hexadecimal[] variable from the last, and stores each element in hex[].