charAt（）和Math.pow（）

Question

Background:

I am making a simple base converter for a class assignment. I am fairly close to completion but need to tidy up the conversion algorithm to convert the user inputted value (in a given base) to a base 10 value.

Attempt:

import java.util.Scanner;

public class BaseConverter {
public static void main(String[] args) {
    String numIn, intro1, prompt1, prompt2, output1;
    int baseIn;
    double numOut;
    boolean flag = true;
    Scanner kb = new Scanner(System.in);

    intro1 = "This program converts values in different number bases to a decimal value.";
    prompt1 = "Type in a number base (2 - 9): ";

    while (flag == true){
        System.out.println(intro1);
        System.out.println(prompt1);
        baseIn = kb.nextInt();
        //Checking base value for outliers outside given range
        if (baseIn < 2 || baseIn > 9) {
            System.out.println("Base must be between 2 and 9");
            System.exit(1);
        }
        prompt2 = "Type in a base "+baseIn+" number: ";
        System.out.println(prompt2);
        numIn = kb.next();

        // System.out.println(numStore);
        int counter = 0;
        // Let's pretend baseIn is 3 and i starts at 3 
        for (int i = numIn.length(); i >= 1; i--){
            numOut = (numIn.charAt(i-1) * Math.pow(baseIn, counter++));
            System.out.println(numOut);
        }
    }//while
}// method
}//class

The problem:

This line does not return the expected value

numOut = (numIn.charAt(i-1) * Math.pow(baseIn, counter++));

For example, in string "10", numOut should be (0*(2*0)) or zero on the first iteration of the for loop. Instead, it returns 48.0.

My Thoughts:

I have a sneaking suspicion it has to do with the charAt() method as debugging the Math.pow() method showed it returning expected values. Suppose it's something to do with all the different variable types? I'm unsure.

Answer 1

Yes you're right charAt is the problem.

When you type let's say "10", the integer value of the character '0' is 48 and for '1' it's 49 according to the encoding table Java uses to encode characters.

If you take a look at it, you see that 0 is encoded as 0x0030 = 3*16^1 = 48 , 1 as 0x0031 = 3*16^1 + 1*16^0 = 49 and so on.

If you want to get the numeric value of the character itself you can use

numOut = Character.getNumericValue(numIn.charAt(i-1)) * Math.pow(baseIn, counter++);

Answer 2

The charAt method returns the char of your input, in this case '0' , not 0 . The Unicode value for the char '0' is not 0 , it's 48 .

Fortunately, the values '0' through '9' are consecutive Unicode values, 48 through 57 respectively, so you can "subtract" out the 48 by subtracting '0' before multiplying.

numOut = ( (numIn.charAt(i-1) - '0') * Math.pow(baseIn, counter++));

You'll still need to validate that what the user typed is in fact a valid "digit" in the chosen base.

You'll also want to add the values of numOut together to get the decimal result at the end.