Plz Help me fix this runtime error :

Question

Q. The question is we have to replace the occurrence of the "ab" with "12" and return the new string and recursion is to be used, so if, the input string is "xabx" the output should be "x12x".

public class Replaceab {

public static String replaceH(String input,int index,String help){
if(index == input.length()){
return help;
}

if(input.substring(index,index+2).equals("ab")){
help = help + "12";
return replaceH(input,index+2,help);

}else{
help = help + input.substring(index,index+1);
return replaceH(input,index+1,help);
}

}
// Return the changed string
public static String replace(String input){

if(input.length()==1){
return input;
}
String help = "";
return replaceH(input,0,help);

}
public static void main(String[] args) {
// TODO Auto-generated method stub
String s = "xabx";
s = replace(s);
System.out.println(s); /* the ideal output should be x12x but I am getting
runtime error */

}
}

Answer 1

I haven't gone through all your code, but this may be the source of the problem;
change:
if(input.substring(index,index+2)=="ab")
to:
if(input.substring(index,index+2).equals("ab"))
String comparisons using == compare the object not the value.

Answer 2

ok, also you need to change:
if(index==input.length())
to:
if(index==input.length()-1)

Answer 3

Is this a hacker challenge, like the ones from hacker earth? If so can you post a link or at least the requirements and test cases?

Adding this worked for me on this case, but it may not work with other cases:

if(index + 1 == input.length()){
return help + input.substring(index);
}

Answer 4

My other answer should fix the out of bounds exception.

but you'll still need to append the last character to the help String.

Answer 5

Now, this change worked for 4 test cases ( earlier one was working for 2 cases) out of 6. Btw what logic did you hit upon for that edit ?

Sorry friend, It's not a publically visible question. So link sharing won't work else I would have shared the same in the beginning :).

Answer 6

The output was missing the last character prior to that. The substring when given an index will go from that index to the end of the String. If help is missing the last char then the index is at the end of the input string and we need to append it.

Do you have to use recursion? can you copy the challenge and post it here? etc etc lol

Answer 7

Did a little testing and it looks like it wouldn't pass if the input was "ab" or a repeating pattern of "ababababab" etc. So I altered the code a little bit. Try this and see if it passes.

public class Replaceab {

public static String replaceH(String input,int index,String help){
if(index + 1 == input.length()){
return help + input.substring(index);
}

if(index + 1 >= input.length()-1
&& input.substring(index,index+2).equals("ab")) {
return help + "12";
}

int endIndex = (index + 2) <= (input.length()-1) ? index + 2 : input.length() -1;
if(input.substring(index,endIndex).equals("ab")){
help = help + "12";
return replaceH(input,endIndex,help);
}else{
help = help + input.substring(index,index+1);
return replaceH(input,index+1,help);
}

}

public static String replace(String input){

if(input.length()==1){
return input;
}

String help = "";
return replaceH(input,0,help);
}

public static void main(String[] args) {
String s = "abababababab";
s = replace(s);
System.out.println(s);
}
}

Answer 8

ya sorry, I copied my old snippet. The correct one is with .equals() and I am unable to figure out why it is not clearing all the test cases. Only two of the test cases are going correct and remaining ones are showing Runtime error ( String Index Out of Bounds Exception).

Answer 9

ya logically, I also hit upon the same thought of changing (index== input.length()-1) . But after implying the same, none of the test cases pass. :(

Answer 10

Give a manual dry run to the code, I think it should run fine. When I tried to figure out what is actually happening, I used the debugger mode of eclipse and found that the if condition " if(input.substring(index,index+2).equals("ab"))" is being continuously being omitted. Plz help me out.