Create a String from chars at indexes 0, 1, 4, 5, 8, 9

Question

I have came across this problem on codingbat.com:

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

Here is my code:

public String altPairs(String str) {

  String result = "";

  for (int i = 0; i < str.length(); i++) {
    result += str.substring(i, i+1);

    if (i > 0 && str.indexOf(str.substring(i, i+1)) % 2 != 0) i += 2;
  }

  return result;
}

For "CodingHorror", it should return "Congrr".

---

• Why does my code return "Congrro" for the parameter "CodingHorror"?

Answer 1

First the second index of substring is exclusive. If you want 2 characters directly, you should use i+2 instead of i+1:

str.substring(i, i+2)

Then, you just have to increment i by 4 at each loop turn. Use i+=4 instead of i++, and you won't need the conditional increment.

for (int i = 0; i < str.length() - 1; i+=4) {
    result += str.substring(i, i+2);
}

Or, if you allow to break a pair of characters at the end of the String:

for (int i = 0; i < str.length(); i+=4) {
    if (i < str.length() - 1)
        result += str.substring(i, i+2);
    else
        result += str.substring(i, i+1);
}

Answer 2

public String altPairs(String str) {
  String newStr = "";
    for(int i=0;i<str.length();i++)
    {
      newStr = newStr +str.charAt(i);
      if(i%2!=0)
        i=i+2;
    }
  return newStr;
}

I did it this way Inside loop add a condition if i is odd then i=i+2; which is increasing the value of i to +3 index

Answer 3

check this out..

public String altPairs(String str) {
String result = "";
for (int i = 0; i < str.length(); ) {
result += str.substring(i, i+1);
i+=4;
}
return result;}