C program to convert Octal to Decimal number system

Write a C program to input Octal number from user and convert to Decimal number system. How to convert from Octal number system to Decimal number system in C program. Logic to convert octal to decimal number system in C programming.

Input octal number: 172

Decimal number: 122

Required knowledge

Basic C programming, While loop

Must know – Program to find last digit of a number.

Octal number system

Octal number system is a base 8 number system. Octal number system uses 8 symbols to represent all its numbers i.e. 01234567

Decimal number system

Decimal number system is a base 10 number system. Decimal number system uses 10 symbols to represent all its numbers i.e. 0123456789

Logic to convert from octal to decimal

Algorithm Conversion Octal to Binary
begin:
    read(octal);
    decimal ← 0; rem ← 0; place ← 0;
    While(octal !=0)
    begin:
        rem ← octal % 10;
        decimal ← decimal + (8place * rem);
        octal ← octal / 10;
        place ← place + 1;
    end;
    write('Decimal =' decimal);
end;

Program to convert octal to decimal number system

/**
 * C program to convert Octal number system to Decimal number system
 */

#include <stdio.h>
#include <math.h>

int main()
{
    long long octal, tempOctal, decimal;
    int rem, place;
    
    /* Input octal number from user */
    printf("Enter any octal number: ");
    scanf("%lld", &octal);
    tempOctal = octal;

    decimal = 0;
    place = 0;
    
    /*
     * Convert octal to decimal
     */
    while(tempOctal > 0)
    {
         /* Extract the last digit of octal */
        rem = tempOctal % 10;

        /* Convert last octal digit to decimal */
        decimal += pow(8, place) * rem;

        /* Remove the last octal digit */
        tempOctal /= 10;

        place++;
    }

    printf("Octal number = %lld\n", octal);
    printf("Decimal number = %lld", decimal);

    return 0;
}

Enter any octal number: 172
Octal number = 172
Decimal number = 122

Happy coding 😉