C Program to Convert Decimal to Octal

In this tutorial, we will learn about how to create a program in C that converts any given decimal number (by user at run-time) to its equivalent octal number. At last we have also created a user-defined function program that does the same job.

Before going through the program, if you are not aware of

Decimal Number
Octal Number
Decimal to Octal Conversion

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

Decimal to Octal in C

To convert decimal number to octal number in C programming, you have to ask from user to enter the decimal number' and convert it into octal number and then display the equivalent value in octal as output like here in the program given below:

#include<stdio.h>
#include<conio.h>
int main()
{
    int decnum, octnum[50], i=0;
    printf("Enter any Decimal number: ");
    scanf("%d", &decnum);
    while(decnum != 0)
    {
        octnum[i] = decnum%8;
        i++;
        decnum = decnum/8;
    }
    printf("\nEquivalent Octal Value = ");
    for(i=(i-1); i>=0; i--)
        printf("%d", octnum[i]);
    getch();
    return 0;
}

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

Now supply any decimal number say 159 and press ENTER key to see its equivalent octal value as shown in the second snapshot of the sample run given here:

Program Explained

Receive any decimal number from user at run-time as input
Create a while loop that runs until the value of entered decimal number is not equal to zero (0). In other words, while loop runs until the value of decnum (given decimal number by user) becomes 0
Let's suppose that user has entered 159 as input. Therefore at first run of the while loop, the condition decnum!=0 or 159!=0 evaluates to be true, therefore program flow goes inside the loop
And decnum%8 or 159%8 (while dividing the number 159 by 8, we will get a remainder as 7) or 7 gets initialized to octnum[i] or octnum[0] (as i holds 0 as its initial value)
Now the value of i gets incremented and becomes 1. And then decnum/8 or 159/8 or 19 gets initialized to decnum
Now the program flow goes back to the while loop and checks the condition, that is decnum!=0 or 19!=0 evaluates to be true, therefore program flow again goes inside the loop, and decnum%8 or 19%8 or 3 gets initialized to octnum[i] or octnum[1].
The value of i gets incremented and becomes 2. And then decnum/8 or 19/8 or 2 gets initialized to decnum. Program flow goes back again to the while loop and checks the condition
That is decnum!=0 or 2!=0 evaluates to be true, therefore program flow goes inside the loop again
And decnum%8 or 2%8 (talking about only whole number, 2 can not be divided by 8, therefore 2 will be the remainder in this case) or 2 gets initialized to octnum[i] or octnum[2]
The value of i gets incremented and becomes 3. And decnum/8 or 2/8 or 0 gets initialized to decnum.
Program flow again goes back to the while loop, and checks the condition, that is decnum!=0 or 0!=0 evaluates to be false, therefore program flow does not goes inside the loop (that is, now the program flow exits from the loop)
Now we have stored all the equivalent octal digit one by one of given decimal number in an array named octnum[] here
We have to create a for loop, that will be initialized with one less then the current value of i. As at last run of the while loop, we have incremented the value of i. But as decnum becomes 0, therefore program flow goes out from the loop, but i holds its value as one more than the required value, therefore we have to delete or skip last index or number.
We have to print the octal digit one by one in reverse order. That is, loop runs from i-1 to greater than or equal to to 0
Inside the loop, print the octal digit one by one
In this way, we have printed the equivalent octal value of given decimal number by user at run-time

Decimal to Octal in C without Modulous Operator

Now let's create same purpose program that is to convert any given decimal number into its equivalent octal value but without using any modulous operator as shown here in the program given below.

#include<stdio.h>
#include<conio.h>
int main()
{
    int decnum, octnum[100], i=0, temp, chck, rem;
    printf("Enter any Decimal number: ");
    scanf("%d", &decnum);
    while(decnum!=0)
    {
        temp = decnum/8;
        chck = temp*8;
        rem = decnum - chck;
        octnum[i] = rem;
        i++;
        decnum = temp;
    }
    printf("\nEquivalent Octal Value = ");
    for(i=i-1; i>=0; i--)
        printf("%d", octnum[i]);
    getch();
    return 0;
}

Here is the final snapshot of the sample run:

Program Explained

As we all knows that modulous operator (%) only finds the remainder while dividing any number with any other number
So here we have to find out the remainder while dividing the number decnum by 8
Therefore, we have simply divided the number by 8 and initialized its value to a variable say temp
And then we have again multiply the same number temp with 8 and initialized to another variable say chck
Now subtract and initialize the value of decnum - chck (here decnum is the original number) to a variable say rem
For example, let's suppose user has supplied 159 as input
Therefore at first run of the while loop, decnum/8 or 159/8 or 19 gets initialized to temp
And temp*8 or 19*8 or 152 gets initialized to chck
Then decnum-chck or 159-152 or 7 gets initialized to rem
Now if you will replace the three line code with rem = decnum%8, you will get the same value that is 7 will be initialized to rem here

Therefore, we have successfully replaced modulous operator and created the program for same purpose but without using any modulous operator as shown in the above program.

Decimal to Octal in C using User-defined Function

Now let's create a function-driven program for same purpose. The question is, Write a program in C that converts Decimal number to Octal number using user-defined function. The answer to this question is:

#include<stdio.h>
#include<conio.h>
void DecToOct(int dec);
int octnum[50];
static int i;
int main()
{
    int decnum;
    printf("Enter any Decimal number: ");
    scanf("%d", &decnum);
    DecToOct(decnum);
    printf("\nEquivalent Octal Value = ");
    for(i=(i-1); i>=0; i--)
        printf("%d", octnum[i]);
    getch();
}
void DecToOct(int dec)
{
    while(dec != 0)
    {
        octnum[i] = dec%8;
        i++;
        dec = dec/8;
    }
}

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

Here we have declared the variable i as static variable. Because static variable remembers its previous value. And we have not initialized the value of i with 0, because static variable holds 0 as its initial value. That is if you will not initialize any value to any static variable, therefore 0 gets automatically initialized to it. We have created the array octnum[] and the variable i outside the main() function so that both the array and variable stays known throughout the program, that is inside both the function namely, main() and DecToOct().

Same Program in Other Languages

C Online Test

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