How to return the index array of a character in a string in Java

Question

I need to write a method to return the index array of a character in a string in Java. Is the following good (correctness, efficiency, as short code as possible) enough?

int[] charIndexArray(String s, char c) {
    int start = 0;
    List<Integer> list = new ArrayList<Integer>();
    while ((start = s.indexOf(c, start)) != -1) {
        list.add(start);
        start++;
    }
    int arr[] = new int[list.size()];
    for (int i = 0; i < ret.length; i++)
        arr[i] = list.get(i);
    return arr;
}

Answer 1

You can replace the code at the end that copies it to an array with a call to the toArray() method . Other than that, looks pretty good.

Answer 2

Instead of:

while ((start = s.indexOf(c, start)) != -1) {
    list.add(start);
    start++;
}

consider:

for (int i = 0; i < s.length(); i++) {
    if (s.charAt(i) == c) {
      list.add(i);
    }
 }

because the indexOf causes the creation of a whole other loop to search for the next instance of your character.

You code is quietly doing:

while (start != -1) {
    start = -1;
    for ( int i=start;i<s.length();i++){
      if ( charAt(i) == c ) {
        start = i;
        break;
      }
    }
    if ( start != -1 ) { 
    list.add(start);
    start++;
  }
}

Which does not seem more efficient. But it turns out that after spending way too much time on this:

static int[] charIndexArrayByBits(String s, char c) {
    int start = 0;
    int[] list = new int[s.length()];
    int count = -1;
    while ((start = s.indexOf(c, start)) != -1) {
      list[++count] = start;
      start++;
    }
    return Arrays.copyOf(list, count);
  }

is faster. But I would not consider it more efficient in the general case because you are allocating an int array which would be larger space wise.

Answer 3

The code do not look good.

You use two loop instead of one.

Try to use methods.

charAt(int pos) for string and Arrays.copy

The OP shold not read more ;p

The first is the location this kind of method should be placed in some util class and be static IMHO.

public class CharSequenceUtil {

    private static int[] EMPTY_INT_ARRAY = new int[0];

    /**
    * Method search the position of given character in char sequence.
    *
    * @param CharSequence seq - Sequence of char that will be investigate 
    * @param char c - Character that is analysed.
    *
    * @return int array with positions of char c in CharSequence instanace
    * @throws NullPointerException if seq is null.
    */
    public static int[] charIndexArray(CharSequence seq, char c) {

      if(seq == null) {
        throw new NullPointerExcetion("The seq must not be null");
      }

      if(seq.length() == 0) {
        return EMPTY_INT_ARRAY;
      }

      int[] positions = new int[seq.lenth()];
      int stor = -1; 

      for(int pos = 0; pos < seq.length(); seq++) {
         if(c == seq.charAt(pos)) {
          positions[++stor] = pos;
         }
      }

      if(stor == -1) {
        return EMPTY_INT_ARRAY;
      }

      return Arrays.copyOf(positions, stor);
    }
}