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Java

Posted by Bryan Beasman
Bryan Beasman
3,435 Points

Why is my method returning numbers?

Please help!

I am attempting to solve this code challenge:

Given a string, return a string made of the chars at indexes 0,1, 4,5, 8,9 ... so "kittens" yields "kien".

Here is the method I built:

public static String altPairs(String str) {
    char[] strArr = str.toCharArray();
    String newStr = "";

    for (int i = 0; i < strArr.length; i += 4) {
      for (int j = i + 1; j == i + 1 && j < strArr.length; j++) {
        newStr += strArr[i] + strArr[j];
      }
    }

    return newStr;
  }

For some reason, the method is returning a string of numbers. For example: altPairs("firetruck") returned 207230

Why is this???

Bryan Beasman
Bryan Beasman
3,435 Points

My code pasted a little weird. My apologies for the mess.

Moderator edit: Added markdown to code posted for readability.

1 Answer

Alexander Nikiforov
.a{fill-rule:evenodd;}techdegree seal-36
Alexander Nikiforov
Java Web Development Techdegree Graduate 22,175 Points

Change the code from:

newStr += strArr[i] + strArr[j];

To

                newStr += strArr[i];
                newStr += strArr[j];

should work. For a detailed explanation see here:

http://stackoverflow.com/questions/21387948/the-concatenation-of-chars-to-form-a-string-gives-different-results

Haven't finished reading yet. But in general, be careful adding two char-s...

Bryan Beasman
Bryan Beasman
3,435 Points

Welp, that solved my issue. But that is so weird. So it thought I was trying to add the two indices together and then cast the result to a String?

Alexander Nikiforov
.a{fill-rule:evenodd;}techdegree seal-36
Alexander Nikiforov
Java Web Development Techdegree Graduate 22,175 Points

The problem is in 'Arithmetic promotion' and in the ways += operator works as you can read from Stack Overflow post, or here:

http://www.codemiles.com/java/arithmetic-promotion-t3487.html

First of all, when you try to add two char-s you will get a number.

Why?

Because quote:

Under the hood a char is represented by a numeric value. The characters for the numbers 0-9 are stored in ascending numeric values. If we use 0 as a base value then simple subtraction can convert a 0-9 char into the equivalent int value

Why under the hood char is represented by number, I don't know for sure yet, but it is a fact...

All I can say that there is no straightforward way to convert two char to the third one... Because char represents one character. And you cannot add 'a' to 'b' and get 'ab', because 'ab' is a String...

So writing 

        System.out.println('a' + 'b');

Will give "195".

So coming back to your code:

When you write

newStr += strArr[i] + strArr[j];

First of all expresion inside is evaluated, and that is strArr[i] + strArr[j] and as you saw that gives a number.

And because that expression is evaluated first you got the number...

My solution avoids this problem...

There is also other solution:

newStr = newStr + strArr[i] + strArr[j];

It will also work.

But the operation like here:

                newStr = newStr + (strArr[i] + strArr[j]);

Will NOT work.

As you can see: it is just simple Math, and you have to make sure do not add two chars first.