C Program to Convert Decimal to Octal and vice-versa

#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