Question

Currently going through an exercise in my book, but i'm stuck ( I havent learned arrays yet, this chapter is on loops nested loops and for loops)

The first part of the problem is to take a credit card number and then sum every other number backwards

Consider 4358 9795, which should output the sum 5+7+8 + 3 = 23.

Heres my solution which isn't working

class Checkit{
    private String creditNum;
    private int sum;
    public Checkit(String creditNum)
    {
        this.creditNum = creditNum;
        sum = 0;
    }

    public int getSum()
    {
        for (int i = creditNum.length() ; i > 0 ; i--)
        {
            char ch = creditNum.charAt(i-1);
            if(i%2 == 1 )
            {
                sum+=ch;
            }
        }
        return sum;
    }


    }


public class test{
    public static void main(String [] args)
    {
        Checkit sampleNumber = new Checkit("4358 9795");
        System.out.println(sampleNumber.getSum());
    }
}

I'm not exactly sure whats wrong with my logic. ch is taking all of the values of my credit card number 5,7,8,3. But for some reason the sum is messing up.

BONUS PART

Take Each number that wasn't added and double it, so 9+9 + 5 + 4, double each of those terms ( that becomes 18 + 18 + 10 + 8), and then get the sum of 1 + 8 + 1 + 8 + 1 + 0 + 8.

I tried the bonus part, but for some reason every time I get 9, 9, 5,4 and times that char value by 2, I get letters. I don't think I can multiply chars by integers, so should I do conversions? Note I didn't learn arrays yet

NEW CODE

class Checkit {
    private String creditCardNum;
    private int sum;

    public Checkit(String creditCardNum) {
        sum = 0;
        this.creditCardNum = creditCardNum;
    }

    public int getSum() {
        creditCardNum = creditCardNum.replaceAll("\\s+", "");
        for (int i = creditCardNum.length(); i > 0; i--) {
            char ch = creditCardNum.charAt(i - 1);
            if (i % 2 == 0) {
                sum += Character.getNumericValue(ch);

            }
        }
        return sum;
    }

    public int doubleDigitSum() {
        sum = 0;
        creditCardNum = creditCardNum.replaceAll("\\s", "");
        for (int i = creditCardNum.length(); i > 0; i--) {

            char ch = creditCardNum.charAt(i - 1);
            if (i % 2 == 1) {
                int newChar = Character.getNumericValue(ch) * 2;
                String newCharString = Integer.toString(newChar);
                for (int j = 0; j < newCharString.length(); j++) {
                    char sumThis = newCharString.charAt(j);
                    sum += Character.getNumericValue(sumThis);
                }


            }

        }
        return sum;

    }
}

public class DataSet{
    public static void main(String [] args) {
        Checkit data = new Checkit("4358 9795");
        System.out.println(data.getSum());
        System.out.println(data.doubleDigitSum());
    }
}

Answer 1

You're adding char values rather than int values. Replace sum+=ch; with

sum += ch - '0';

The reason why this works as opposed to the original solution is that when you're adding char values to an int value, the char is converted to a decimal via its ASCII value. For example, '5' has a ASCII dec value of 53. However, '5' - '0' equals 5. More on these ASCII values can be found here, http://www.asciitable.com/

Answer 2

Your issue is that you are not converting the char to an integer:

public int getSum()
    {
        for (int i = creditNum.length() ; i > 0 ; i--)
        {
            char ch = creditNum.charAt(i-1);
            if(i%2 == 1 )
            {
                sum+=ch;
            }
        }
        return sum;
    }

Fun fact that's causing your error - chars are numbers! They're simply a number that represents the ascii character code of the letter. So when you add them to a sum, Java is OK with that and just adds the corresponding number for the char '8' for example, which is 56.

To make your code work, you need to properly convert to an integer:

public int getSum()
    {
        for (int i = creditNum.length() ; i > 0 ; i--)
        {
            char ch = creditNum.charAt(i-1);
            if(i%2 == 1)
            {
                sum+=Character.getNumericValue(ch);
            }
        }
        return sum;
    }

Answer 3

for (int i = creditNum.length() ; i > 0 ; i--)     {
    char ch = creditNum.charAt(i-1);

第一次迭代通过循环得到IndexOutOfBounds异常。然后你永远不会检查信用卡号码的初始字符，因为你的终止标准是（i> 0），而不是（i> = 0）

为什么不使用i - = 2作为增量而不是if-check？

Error with multiplying char with a number and add with char ( Big Java Ex 6.2)

3 个答案: