java String - String index out of range , charAt

Question

I try to make a program which it can find palindromic number (it has to be pruduct of two 3-digits number and I hope that it contain 6 digit but it is not important). Here is my code:

public class palindromicNumber {
    public static void getPalindromicNumber() {
        boolean podminka = false;
        int test;
        String s;
        for (int a = 999; podminka == false && a > 100; a--) {
            for (int b = 999; podminka == false && b > 100; b--) {
                test = a * b;
                s = Integer.toString(test);
                int c = 0;
                int d = s.length();
                while (c != d && podminka == false) {

                    if (s.charAt(c) == s.charAt(d)) { // I think that problem is here but I can't see what
                        System.out.println(s);
                        podminka = true;
                    }
                    c++;
                    d--;
                }
            }
        }
    }
}

and if I want to compile it :

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 6
at java.lang.String.charAt(String.java:695)
at faktorizace.palindromicNumber.getPalindromicNumber(palindromicNumber.java:24)
at faktorizace.Faktorizace.main(Faktorizace.java:19)

Java Result: 1

Jon Skeet · Accepted Answer · 2012-07-25 16:37:27Z

2

There are two problems here:

You're starting off with the wrong upper bound, as other answers have mentioned
If c starts off odd and d starts off even, then c will never equal d. You need to use
```
while (c < d && !podminka) // Prefer !x to x == false
```

Additionally, judicious use of break and return would avoid you having to have podminka at all.

As another aside, you've got a separation of concerns issue. Your method currently does three things:

Iterates over numbers in a particular way
Checks whether or not they're palandromic
Prints the first it finds

You should separate those out. For example:

public void printFirstPalindrome() {
    long palindrome = findFirstPalindrome();
    System.out.println(palindrome);
}

public long findFirstPalindrome() {
    // Looping here, calling isPalindrome
}

public boolean isPalindrome(long value) {
    // Just checking here
}

I suspect findFirstPalindrome would normally take some parameters, too. At this point, you'd have methods which would be somewhat easier to both write and test.

answered Jul 25, 2012 at 16:37

Jon Skeet

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2 Comments

United121 Over a year ago

How can I find the largest palindrome ? I tried every palindromic number get to the array and the buble sort and then I called for array[0] but the compilator didn't give me resul ( it give me nothing :D ) Do you have any solution or idea ?

Jon Skeet Over a year ago

@user1551954: One question at a time... this sounds like a separate one, so ask a new question, after reading tinyurl.com/so-hints

Chris Dargis · Accepted Answer · 2012-07-25 18:03:33Z

1

String indices go from [0..length - 1]

Change int d = s.length(); to int d = s.length() - 1;

Update: As a quick aside, you are setting podminka to true when

s.charAt(c) == s.charAt(d)

If s = 100101 for example, you will terminate all of the loops on the first iteration of the while loop because the first and last characters are the same.

edited Jul 25, 2012 at 18:03

answered Jul 25, 2012 at 16:35

Chris Dargis

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4 Comments

user784540 Over a year ago

When you've changed 'd' update code that is related to this variable too. For instance comparison and conditional operators involved this variable.

United121 Over a year ago

Ouuu , that is the mistake what i'm looking for .It is easy to I figure it out when I know where I must look. :D

Chris Dargis Over a year ago

@user1551954: A quick fix could be setting podminka = true if the two characters are NOT equal. However, this would still terminate all of the loops. You should really take a look at @Jon Skeet 's answer.

United121 Over a year ago

So I figured out promblem ( I can find palindromic number ) but next problem appears now , I found pal. number but it isn't the largest one . I do what Jon Skeet show me so the problem is in : public long findFirstPalindrome() { // Looping here, calling isPalindrome }

Rob Wagner · Accepted Answer · 2012-07-25 16:36:21Z

0

int d = s.length();

An array of the strings chars will only go from 0 - length-1.

s.charAt(d) will always be out of bounds on the first iteration.

answered Jul 25, 2012 at 16:36

Rob Wagner

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Comments

AlexR · Accepted Answer · 2012-07-25 16:37:42Z

0

Take a look on JDK source code:

public char charAt(int index) {
    if ((index < 0) || (index >= count)) {
        throw new StringIndexOutOfBoundsException(index);
    }
    return value[index + offset];
}

You can see that this exception is thrown when index is less then zero or exceeds the string length. Now use debugger, debug your code and see why do you pass this wrong parameter value to charAt().

answered Jul 25, 2012 at 16:37

AlexR

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1 Comment

corsiKa Over a year ago

You say exceeds, don't you mean meets or exceeds? After all, "hello".charAt(5) doesn't exceed the length, but throws an exception because it meets the length.

robi terebesi · Accepted Answer · 2012-07-25 16:37:46Z

0

       public class palindromicNumber {
           public static void getPalindromicNumber(){
              boolean podminka = false;
               int test;
               String s;
            for(int a = 999;podminka == false && a>100; a-- ){
              for(int b = 999;podminka == false && b>100; b-- ){
                test = a*b;
                s = Integer.toString(test); 
                int c = 0;
                int d = s.length();
                while(c!=d && podminka == false){                          

                  if(s.charAt(c)==s.charAt(d - 1)){  
                    System.out.println(s);
                      podminka = true;                                
                }
                      c++;
                       d--;
                 }
                  }

}

try this! string count starts from 0!

answered Jul 25, 2012 at 16:37

robi terebesi

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4 Comments

Jon Skeet Over a year ago

What does your inner loop do with a number such as 123?

United121 Over a year ago

I tried this ,error is gone but the result is unsatisfactory ( wrong :D ) . When I started it I got this : 990009 but it isn't palindromic number . Do you have any solution ?

Chris Dargis Over a year ago

@user1551954: Consider what happens with the algorithm when s = 990009. Follow the logic.

United121 Over a year ago

I think that it is correct . I take first character and compare to the last one . Then I take second character and compare to the penultimate one and over and over again . When I'm in the middle the algorithm stop and return final number .

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