Java：简单递归问题

Question

I'm extremely new to the whole idea of recursion, and it's blowing my mind a bit to be honest. I'm trying to turn this function I wrote into a recursive one...

public static int to_number(String s)
{
    int total = 0;
    int n=s.length();
    for(int i = 0 ; i < n ; i++) { 
        char c = s.charAt(i);
        if (Character.isDigit(c)){
            int value=Character.getNumericValue(c);
            total +=value;
        }
    }
    System.out.println(total);
    return total;
}

It reads in a string, such as "3aaa6a3", iterates through the string, if the char is a digit, it adds it to total, and so on. What I have so far...

    public static int to_number(String s)
{
    int total = 0;
    int n=s.length();
    int i=0;
    if (i == n){
        return 0; //if gone through the whole string
    }
    char c = s.charAt(i);
        if (Character.isDigit(c)){
            int value=Character.getNumericValue(c);
            total +=value;
        }

    System.out.println(total);
    i++;
    to_number(); //trying to call the same function
    return total;
}

I feel like I'm close, but just not getting it. Thanks for your time and effort!

Answer 1

Not gonna give you the code, but as a recursive function, you want to process the first character of the input string, then call yourself with the remaining string, ie to_number(s.substring(1)) , and combine the result. Recursion ends when input string is empty.

Answer 2

try this

public void intToNum(String s, int total[])
{

     if(s.isEmpty())
        return;

     char c = s.charAt(0);

if (Character.isDigit(c)){
            int value=Character.getNumericValue(c);
            total[0] +=value;
        }

intToNum(s.substring(1),total);

}

and in your main, call the function as

int [] total = new int[1];

intToNum(input,total);

System.out.println(total[0]);

or another approach is

public int intToNum(String s) {

     if(s.isEmpty())
        return 0;

     char c = s.charAt(0);

if (Character.isDigit(c)){

            int value=Character.getNumericValue(c);

            return value + intToNum(s.substring(1));
        }

return intToNum(s.substring(1));

}

Answer 3

This answer contains the solution. Just look at it when you are stuck. I also encourage you to read and understand the code instead of just copying it. This is one of the most important concepts in programming, so make sure you understand it and are familiar with it. :)

How it works: The method receives a string. If the string is longer than 1 character, the method splits it in half, calls itself on the two substrings and adds the two results. Those calls will do the same thing, until the string fragments are only 1 (or 0) characters long. In that case, it just returns their value (or 0, if there is no value).

public class RecursionVsIteration {
    public static void main(String[] args) {
        String str = "2.938fyfh0293urhp2398rpod8723uoihr98y";
        System.out.println("Iterative: " + toNumberIterative(str));
        System.out.println("Recursive: " + toNumberRecursive(str));
    }

    public static int toNumberIterative(String s) {
        int total = 0;
        int n = s.length();
        for (int i = 0; i < n; i++) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                int value = Character.getNumericValue(c);
                total += value;
            }
        }
        return total;
    }

    public static int toNumberRecursive(String s) {
        int n = s.length();
        // termination criteria
        if (n == 0) { // emtpy string
            return 0;
        }
        if (n == 1) { // on character string
            char c = s.charAt(0);
            return Character.isDigit(c) ? Character.getNumericValue(c) : 0;
        }

        // recursive call (split the string in half and call the method on both substrings)
        return toNumberRecursive(s.substring(0, n / 2)) + toNumberRecursive(s.substring(n / 2, n));
    }
}