C Program to Convert Octal to Decimal

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In this tutorial, we will learn about how to create a program in C that can convert any given octal number into its equivalent decimal number. At last we will also take a look at function-driven program that does the same job but using user-defined function named OctToDec().

But before going through the program. If you are not aware of

Octal Number
Decimal Number
Octal to Decimal Conversion Process

then refer to Octal to Decimal conversion step by step process. Now let's move on to the program.

Octal to Decimal in C

To convert octal number to decimal number in C programming, you have to ask from user to enter the octal number first. Then convert it into decimal number, and then display the equivalent value in decimal form as output.

#include<stdio.h>
#include<conio.h>
#include<math.h>
int main()
{
    int octnum, decnum=0, i=0, rem;
    printf("Enter any Octal Number: ");
    scanf("%d", &octnum);
    while(octnum!=0)
    {
        rem = octnum%10;
        decnum = decnum + (rem*pow(8,i));
        i++;
        octnum = octnum/10;
    }
    printf("\nEquivalent Decimal Value = %d", decnum);
    getch();
    return 0;
}

As the above program was written under Code::Blocks IDE, therefore after successful build and run, you will get the following output. This is the first snapshot of the sample run:

Now supply any octal number say 345 as input and then press ENTER key to see the output that shows you its equivalent decimal value as shown in the second snapshot given below:

Program Explained

Receive any octal number say 345 in octnum variable
Create a while loop, that runs until the value of octnum becomes zero
As at first run, the condition octnum!=0 or 345!=0 evaluates to be true, therefore program flow goes inside the loop
And octnum%10 or 345%10 or 5 gets initialized to rem, and then decnum + (rem*pow(8,i)) (here decnum and i was initialized with 0 at start of the program) or 0 + (5*pow(8,0)) or 0 + (5*8⁰) or 0 + 5*1 or 5 gets initialized to decnum
The value of i gets incremented and becomes 1, and octnum/10 or 345/10 or 34 gets initialized to octnum
Now the program flow goes back to the condition of the while loop and this time again octnum!=0 or 34!=0 evaluates to be true, and the program flow again goes back inside the loop
And octnum%10 or 34%10 or 4 gets initialized to rem, and then decnum + (rem*pow(8, i)) or 5 + (4*pow(8, 1)) or 5 + 4*8¹ or 5 + 32 or 37 gets initialized to decnum
The value of i gets incremented and becomes 2, and octnum/10 or 34/10 or 3 gets initialized to octnum
Now the program flow goes back to the condition of the while loop and the condition octnum!=0 or 3!=0 evaluates to be true, and the program flow again goes back inside the loop
And octnum%10 or 3%10 or 3 gets initialized to rem, and then decnum + (rem*pow(8, i)) or 37 + (3*pow(8, 2)) or 37 + 3*8² or 37 + 192 or 229 gets initialized to decnum
The value of i gets incremented and becomes 3, and octnum/10 or 3/10 or 0 gets initialized to octnum
Now the program flow goes back again to the condition of the loop, and this time octnum!=0 or 0!=0 evaluates to be false, therefore program flow exits from the while loop, and the value of decnum which is 229 gets printed as equivalent decimal value of given octal number 345 as output
Here we have used the function pow() of math.h library, that takes two argument, the first one is responsible for base and the second is responsible for exponent. For example 8 is base and 1 is exponent in 8¹

Octal to Decimal in C without pow() Function

You can also create the program without using any pow() function of math.h library. Here is the program without using any pre-defined function:

#include<stdio.h>
#include<conio.h>
int main()
{
    int octnum, decnum=0, rem, mul=1;
    printf("Enter any Octal Number: ");
    scanf("%d", &octnum);
    while(octnum!=0)
    {
        rem = octnum%10;
        decnum = decnum + (rem*mul);
        mul = mul*8;
        octnum = octnum/10;
    }
    printf("\nEquivalent Decimal Value = %d", decnum);
    getch();
    return 0;
}

Here is the final snapshot of the above program's sample run:

Octal to Decimal in C using User-defined Function

Let's create function-driven program in which the function OctToDec() created in such way, it takes one argument as octal number, and converts it into its equivalent decimal value and then return it to the main() function as shown in the program given below:

#include<stdio.h>
#include<conio.h>
int OctToDec(int oct);
int main()
{
    int octnum, decnum=0, rem, mul=1;
    printf("Enter any Octal Number: ");
    scanf("%d", &octnum);
    decnum = OctToDec(octnum);
    printf("\nEquivalent Decimal Value = %d", decnum);
    getch();
    return 0;
}
int OctToDec(int oct)
{
    int rem, dec=0, mul=1;
    while(oct!=0)
    {
        rem = oct%10;
        dec = dec + (rem*mul);
        mul = mul*8;
        oct = oct/10;
    }
    return dec;
}

The final snapshot of the above program's sample run is given below:

Same Program in Other Languages

C Online Test

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