C++ Program For Octal To Decimal Conversion

Given an octal number as input, we need to write a program to convert the given octal number into an equivalent decimal number.

Examples:

Input : 67
Output: 55

Input : 512
Output: 330

Input : 123
Output: 83

1. Simple Approach

• The idea is to extract the digits of a given octal number starting from the rightmost digit and keep a variable dec_value. 
• At the time of extracting digits from the octal number, multiply the digit with the proper base (Power of 8) and add it to the variable dec_value. 
• In the end, the variable dec_value will store the required decimal number.

Example:

If the octal number is 67. 

dec_value = 6*(8^1) + 7*(8^0) = 55

The below diagram explains how to convert an octal number (123) to an equivalent decimal value:  

Below is the implementation of the above idea. 

// C++ program to convert octal 
// to decimal
#include <iostream>
using namespace std;

// Function to convert octal 
// to decimal
int octalToDecimal(int n)
{
    int num = n;
    int dec_value = 0;

    // Initializing base value to 1, 
    // i.e 8^0
    int base = 1;

    int temp = num;
    while (temp) 
    {
        // Extracting last digit
        int last_digit = temp % 10;
        temp = temp / 10;

        // Multiplying last digit with 
        // appropriate base value and adding 
        // it to dec_value
        dec_value += last_digit * base;

        base = base * 8;
    }

    return dec_value;
}

// Driver code
int main()
{
    int num = 67;

    cout << octalToDecimal(num) << endl;
}

55

The complexity of the above method

Time complexity: O(logN) where N is the given number

Auxiliary space: O(1)

2. Using Predefined stoi() Function

Below is the C++ program for Octal to Decimal Conversion: 

// C++ program to convert octal 
// to decimal
#include <iostream>
using namespace std;
int OctToDec(string n) 
{ 
  return stoi(n, 0, 8); 
}

// Driver code
int main()
{

  string n = "67";
  cout << OctToDec(n);
  return 0;
}

// This code is contributed by phasing17

55

Please refer complete article on Program for Octal to Decimal Conversion for more details!