Occurences of substring in a string

Question

Why is the following algorithm not halting for me? (str is the string I am searching in, findStr is the string I am trying to find)

    String str = "helloslkhellodjladfjhello";
    String findStr = "hello";
    int lastIndex = 0;
    int count =0;

    while(lastIndex != -1){

           lastIndex = str.indexOf(findStr,lastIndex);

           if( lastIndex != -1){
                 count ++;
          }
        lastIndex+=findStr.length();
    }
    System.out.println(count);

Thanks!

EDIT- updated, still not working

Answer 1

Increment lastIndex whenever you look for next occurence. Otherwise it's always finding the first substring (at position 0).

Answer 2

public int indexOf(int ch, int fromIndex)

Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.

So your lastindex value is always 0 and it always finds hello in the string.

Answer 3

try adding lastIndex+=findStr.length() to the end of your loop, otherwise you will end up in an endless loop because once you found the substring, you are trying to find it again and again from the same last position.

Answer 4

    String str = "helloslkhellodjladfjhello";
    String findStr = "hello";
    int lastIndex = 0;
    int count = 0;

    while ((lastIndex = str.indexOf(findStr, lastIndex)) != -1) {
        count++;
        lastIndex += findStr.length() - 1;
    }

    System.out.println(count);

at the end of the loop count is 3; hope it helps

Answer 5

do you really have to handle the matching yourself ? especially if all you need is the number of occurences, regular expressions are tidier :

    String str = "helloslkhellodjladfjhello";
    Pattern p = Pattern.compile("hello");
    Matcher m = p.matcher(str);
    int count = 0;
    while (m.find()){
     count +=1;
    }
    System.out.println(count);

Answer 6

You "lastIndex += findStr.length();" was placed outside the brackets, causing an infinite loop (when no occurence was found, lastIndex was always to findStr.length().

Here is the fixed version :

        String str = "helloslkhellodjladfjhello";
     String findStr = "hello";
     int lastIndex = 0;
     int count = 0;

     while (lastIndex != -1) {

      lastIndex = str.indexOf(findStr, lastIndex);

      if (lastIndex != -1) {
       count++;
       lastIndex += findStr.length();
      }
     }
     System.out.println(count);

Answer 7

The last line in was creating a problem... last endex would never be left at -1 so there would be an infinite loop... you can fix it by moving the code in the if block.

String str = "helloslkhellodjladfjhello";
String findStr = "hello";
int lastIndex = 0;
int count =0;

while(lastIndex != -1){

       lastIndex = str.indexOf(findStr,lastIndex);

       if( lastIndex != -1){
             count ++;
             lastIndex+=findStr.length();
      }
}
System.out.println(count);

Answer 8

A shorter version. ;)

String str = "helloslkhellodjladfjhello";
String findStr = "hello";
System.out.println(str.split(findStr).length-1);

Answer 9

How about using StringUtils.countMatches from Apache Commons Lang?

String str = "helloslkhellodjladfjhello";
String findStr = "hello";

System.out.println(StringUtils.countMatches(str, findStr));

That outputs:

3