二进制到十进制程序没有给出正确的答案java

Question

So i have to write a program for homework where it takes a binary number and prints out its the decimal form. I've tried and and i just can't really get it. Like im printing out what it's doing every time in the loop and it looks like im getting the wrong value when i multiply the inputed data by the 2^i. If someone could help, that would be amazing

import java.util.Scanner;

public class Homework {

    public static void main(String[] args) {

        @SuppressWarnings("resource")
        Scanner userinput = new Scanner(System.in);

        System.out.println("Enter a binary number (only 1's or 0's): ");
        String binary_number = userinput.next();

        int value = 0;
        int square_value = 0; 

        for (int i = 0; i < binary_number.length(); i++) {


            char binary_place_holder = binary_number.charAt(i); 
            square_value = (int) Math.pow(2, i);

            if (binary_place_holder == '1') {
                value =  (square_value*binary_place_holder+value);
            }
            System.out.println(binary_place_holder+ "  " + square_value + "  " + value); 

        }
        System.out.print(binary_number + " == " + value);
    }
}

Answer 1

The way you determine the exponent is wrong: The exponent is not i .

You need to realize that you are looking at the string from left to right side so from the highest bit. This means the first character's exponent is 2^string_length-1 . Second character's exponent is 2^string_length-2 and so on. Therefore as the index i becomes larger, ie as we are scanning the input left to right, the exponent value becomes smaller.

So the exponent is:

int exponent = binary_number.length() - 1 - i;

Note : You can of course do things in reverse and start from the end of the input string and determine the exponent that way, I just wanted to do less changes to your code.

---

Also, you should add a check to make sure the input is a valid binary number. You can write your own function to do it but a simpler solution is to use regex matching:

binary_number.matches("[01]+") 

This will return true if the string binary_number contains only '0' and '1' characters, and false otherwise.

---

Applying all these changes to your code:

public static void main(String[] args) {
    @SuppressWarnings("resource")
    Scanner userinput = new Scanner(System.in);

    System.out.println("Enter a binary number (only 1's or 0's): ");
    String binary_number = userinput.next();

    if(!binary_number.matches("[01]+")) {
        System.err.println("\nPlease enter a valid binary number");
    } else {
        int value = 0;
        for (int i = 0; i < binary_number.length(); i++) {
            if (binary_number.charAt(i) == '1') {
                int exponent = binary_number.length() - 1 - i;
                value += (int) Math.pow(2, exponent);
            } 
        }
        System.out.print(binary_number + " == " + value);
    }
}

Answer 2

这可能被认为是作弊行为，但您可以像

System.out.println(Integer.parseInt(binary_number, 2));