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

## Example 1: Program to Convert Decimal to Octal

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

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;
}

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>

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;
}

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```

```