Recursive count of a char in Str -- Java

Question

guys I'm having some problems with something really basic. I have a suggested solution to a problem that looks like (below) but I don't understand how the static argument of countX == str.substring(1) is supposed to search the entire String, the argument is a static 1 (?!?!) :

   public int countX(String str) {
    if (str.length() == 0) return 0;
    if (str.charAt(0) == 'x') return 1 + countX(str.substring(1));
    return countX(str.substring(1));
}

Instead I thought of this solution before looking up for the solution but can't figure out how to identify the right most char of the substring for comparison with the searched char 'x' (line : 3)

 public int countX(String str) {
  if (str.length()>0) {
    if (str.charAt(str.substring(str.length()-1) == 'x'))
      return countX (str.substring(str.length()-1)) + 1;
    else
      return countX (str.substring(str.length() -1)
  }
  else
    return 0;
}

Any suggestion for my ignorance about the first solution and my mistake on the second ?Thanks in advance

yeah and couldn't find a way to define what I call String.substring.length () — Luca Conin
– Luca Conin, Commented Feb 2, 2019 at 19:43
substring(1) will return the String minus the first char. So if you do "abcd".substring(1); it will return "bcd" — GBlodgett
– GBlodgett, Commented Feb 2, 2019 at 19:54
got that but what I can't understand is why the subsequent recursive call let's say of abcd.substring(1); call "cd" this time. Should I just take it as it is (it's the way substring works) ? — Luca Conin
– Luca Conin, Commented Feb 2, 2019 at 19:59
owwwww just realized that the recursive call is done on already a substring after the first time so that's why it keeps chopping off the string. Thanks your analogy just started the right thought chain :)) — Luca Conin
– Luca Conin, Commented Feb 2, 2019 at 20:07

Stefan Freitag · Accepted Answer · 2019-02-02 19:49:14Z

1

Let me explain the first solution. The idea is to split the string into its first character and the rest.

If the first character is an x we have to add 1 to the count and continue recursively.
If it is not an x we have to add nothing and continue recursively.
For the rest: let us split this up again into its first character and its rest (aka recursion)

To catch also the situation of an empty string, the length of the input is checked first. If length is 0, then the count of x is zero.

answered Feb 2, 2019 at 19:49

Stefan Freitag

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2 Comments

Luca Conin Over a year ago

thanks for the answer but what I couldn't get is how can the recursion continue if the argument of substring in the recursive call countX(str.substring(1)) [line 3] is always 1, shouldn't the whole thing go into stackOverflow and explode leaving corpses everywhere ?

Stefan Freitag Over a year ago

No, this one takes the 2nd character until end of string to the next recursion level. With each level the string to examine is one character shorter.

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