# C Program to Convert Octal Number to Decimal and vice-versa

To understand this example, you should have the knowledge of the following C programming topics:

## Example 1: Program to Convert Decimal to Octal

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

// function prototype
int convertDecimalToOctal(int decimalNumber);

int main() {

int decimalNumber;

printf("Enter a decimal number: ");
scanf("%d", &decimalNumber);

printf("%d in decimal = %d in octal", decimalNumber, convertDecimalToOctal(decimalNumber));

return 0;
}

// function to convert decimalNumber to octal
int convertDecimalToOctal(int decimalNumber) {
int octalNumber = 0, i = 1;

while (decimalNumber != 0) {
octalNumber += (decimalNumber % 8) * i;
decimalNumber /= 8;
i *= 10;
}

return octalNumber;
}
``````

Output

```Enter a decimal number: 78
78 in decimal = 116 in octal```

## Example 2: Program to Convert Octal to Decimal

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

// function prototype
long long convertOctalToDecimal(int octalNumber);

int main() {

int octalNumber;

printf("Enter an octal number: ");
scanf("%d", &octalNumber);

printf("%d in octal = %lld in decimal", octalNumber, convertOctalToDecimal(octalNumber));

return 0;
}

// function to convert octalNumber to decimal
long long convertOctalToDecimal(int octalNumber) {
int decimalNumber = 0, i = 0;

while(octalNumber != 0) {
decimalNumber += (octalNumber%10) * pow(8,i);
++i;
octalNumber/=10;
}

i = 1;

return decimalNumber;
}

``````

Output

```Enter an octal number: 116
116 in octal = 78 in decimal
```

Note: This program doesn't work if we input non-octal numbers i.e., numbers that contain 8 or 9. For example, 187, 96, 985, etc., are not octal numbers.

## Example 3: Check for Octal Number and Convert it to Decimal

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

// function prototypes
int checkOctal(int);
long long convertOctalToDecimal(int);

int main() {

int octalNumber;
int condition;

// repeat loop as long as user
// gives a non-octal number
do {
printf("Enter an octal number: ");
scanf("%d", &octalNumber);

// check if number is octal
condition = checkOctal(octalNumber);

if (!condition) {
printf("%d is not an octal number!\n", octalNumber);
}
} while (condition == 0);

printf("%d in octal = %lld in decimal", octalNumber, convertOctalToDecimal(octalNumber));

return 0;
}

// function to check octal number
int checkOctal(int octalNumber) {
int remainder;

// check each digit of input number
while(octalNumber != 0) {
remainder = octalNumber % 10;

// return 0 if a digit is 8 or 9
if (remainder >= 8) {
return 0;
}

octalNumber/= 10;
}

// return 1 if number is octal
return 1;
}

// function to convert octalNumber to decimal
long long convertOctalToDecimal(int octalNumber) {
int decimalNumber = 0, i = 0;

while(octalNumber != 0) {
decimalNumber += (octalNumber%10) * pow(8,i);
++i;
octalNumber/=10;
}

i = 1;

return decimalNumber;
}```
```
```Enter an octal number: 96
96 is not an octal number!
Enter an octal number: 25
25 in octal = 21 in decimal
```