Java, replacing characters in a for-loop

Question

Okay so, obviously I'm new with Java and what I currently wish to do is a very simple program to encrypt a string by breaking it into an array of characters and replacing the characters with new ones.

So what I did so far was to create a key-array containing the alphabet, which I'm comparing the split-up string with, and I'm trying to replace the characters with a value-array which is basically just the alphabet backwards.

My code so far works when I'm just printing out the value, but it wont properly replace the characters.

public class Main {

    public static void main(String[] args) {

        char[] keyArray =  {'a', 'b', 'c', 'd',  'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
        char[] valueArray = {'z', 'y', 'x', 'w', 'v', 'u', 't', 's', 'r', 'p', 'q', 'o', 'n', 'm', 'l', 'k', 'j', 'i', 'h', 'g', 'f', 'e', 'd',  'c', 'b', 'a'};

        String myString = "abcxyz";
        char[] myStringArray = myString.toCharArray();

        for(int x = 0; x<myString.length(); x++)
        {
            for(int i = 0; i<keyArray.length; i++)
            {
                if(myStringArray[x] == keyArray[i])
                {
                    //System.out.println(valueArray[i]); would give the output "zyxcba" as expected 
                    myStringArray[x] = valueArray[i]; // this will only change the characters in the first half of keyArray 
                }
            }
        }

        System.out.println(myStringArray); //Outputs "abccba" instead of "zyxcba"
    }
}

This might help: http://stackoverflow.com/q/11588916/535275 – Scott Hunter Oct 20 '14 at 01:38 — Scott Hunter, Oct 20 '14 at 01:38

score 3 · Accepted Answer · answered Oct 20 '14 at 01:39

The problem you have is that you continue to loop through the key array even after you've already made the replacement - allowing it to replace it a second time!

You'll need to 'break' out of the for loop once you've done your replacement.

public class Main {

    public static void main(String[] args) {

        char[] keyArray =  {'a', 'b', 'c', 'd',  'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
        char[] valueArray = {'z', 'y', 'x', 'w', 'v', 'u', 't', 's', 'r', 'p', 'q', 'o', 'n', 'm', 'l', 'k', 'j', 'i', 'h', 'g', 'f', 'e', 'd',  'c', 'b', 'a'};

        String myString = "abcxyz";
        char[] myStringArray = myString.toCharArray();

        for(int x = 0; x<myString.length(); x++)
        {
            for(int i = 0; i<keyArray.length; i++)
            {
                if(myStringArray[x] == keyArray[i])
                {
                    //System.out.println(valueArray[i]); would give the output "zyxcba" as expected 
                    myStringArray[x] = valueArray[i]; // this will only change the characters in the first half of keyArray 
                    break; //Exit the loop checking against the keyArray
                }
            }
        }

        System.out.println(myStringArray); //Outputs "abccba" instead of "zyxcba"
    }
}

Aaah, I understand! Thank you so much, everything works as expected now :) — s1nclair, Oct 20 '14 at 01:41

score 0 · Answer 2 · answered Oct 20 '14 at 02:11

public class Main {

public static void main(String[] args) {

    char[] keyArray =  {'a', 'b', 'c', 'd',  'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
    char[] valueArray = {'z', 'y', 'x', 'w', 'v', 'u', 't', 's', 'r', 'p', 'q', 'o', 'n', 'm', 'l', 'k', 'j', 'i', 'h', 'g', 'f', 'e', 'd',  'c', 'b', 'a'};

    String myString = "abcxyz";
    char[] myStringArray = myString.toCharArray();

    for(int x = 0; x<myString.length(); x++)
    {
        for(int i = 0; i<keyArray.length; i++)
        {
            if(myStringArray[x] == keyArray[i])
            {
                //System.out.println(valueArray[i]); would give the output "zyxcba" as expected 
                myStringArray[x] = valueArray[i]; // this will only change the characters in the first half of keyArray
                break; //Introduced break so that when the answer is set, break and move to the next iteration of myStringArray
            }
        }
    }


    System.out.println(myStringArray); //Outputs "abccba" instead of "zyxcba"
}

}

I introduced break in between to control when the item is matched.

There are many other ways of making this optimal though. But the main thing is you should not make x*i comparison when it is not required.

I will also add one more thing that instead of converting your string into array you can do the operation using string methods charAt or substring. — scorpionmance, Oct 20 '14 at 02:14
Thank you for your answer! Yeah I've yet to explore the various options of doing it, this is my first program in Java. I will take a look at the string operations for sure! But what do you mean with the x*i comparison? Are you saying I should be using something else than for-loops? — s1nclair, Oct 20 '14 at 09:23
By x * i, I mean that the total number of cases for this will be myString.length() * keyArray.length() which is not a efficient way to solve the problem, in this following case you are obtaining your result well before end of the inner loop. — scorpionmance, Oct 21 '14 at 03:22

Java, replacing characters in a for-loop

2 Answers2