Posted on Apr 24

Programming Session Day 8: Manual Octal and Decimal Conversion in Java

Day 8: Manual Octal ↔ Decimal Conversion with Integers Only

1. Octal → Decimal (Input as `int`)

public static int octalToDecimal(int octal) { int decimal = 0; int place = 1; // 8^0 initially while (octal > 0) { int digit = octal % 10; // rightmost digit (0–7) decimal += digit * place; // add digit * current power of 8 place *= 8; // next power of 8 octal /= 10; // shift right one digit } return decimal; } // Example: System.out.println(octalToDecimal(17)); // prints 15

How it works

% 10 extracts the least-significant digit of the octal number.
place tracks powers of 8 (1, 8, 64, ...).
Multiply digit by place, add to total, then update both place and octal.

2. Decimal → Octal (Output as `int`)

public static int decimalToOctal(int decimal) { int octal = 0; int factor = 1; // 10^0 for placing digits in decimal slots while (decimal > 0) { int remainder = decimal % 8; // next octal digit octal += remainder * factor; // place digit at correct decimal position decimal /= 8; // move to next digit in base 8 factor *= 10; // shift factor to next decimal place (10, 100, ...) } return octal; } // Example: System.out.println(decimalToOctal(15)); // prints 17

How it works

% 8 gets the least-significant octal digit.
Each digit is placed in the decimal representation by multiplying by factor (1, 10, 100, ...).
Update decimal, and increase factor for the next digit.

3. Complete Utility Class

public class ManualBaseConverter { public static int octalToDecimal(int octal) { int decimal = 0; int place = 1; while (octal > 0) { int digit = octal % 10; decimal += digit * place; place *= 8; octal /= 10; } return decimal; } public static int decimalToOctal(int decimal) { int octal = 0; int factor = 1; while (decimal > 0) { int remainder = decimal % 8; octal += remainder * factor; decimal /= 8; factor *= 10; } return octal; } public static void main(String[] args) { System.out.println("Octal 17 → Decimal " + octalToDecimal(17)); System.out.println("Decimal 15 → Octal " + decimalToOctal(15)); } }

Day 8 Key Takeaways

Pure integer arithmetic: no string operations or library calls.
Octal → Decimal: extract digits with %10, accumulate using an ever‑increasing place multiplier of 8.
Decimal → Octal: extract base‑8 digits with %8, build the octal number in decimal slots via an increasing factor multiplier of 10.