Question

I am trying to create a method that will count the number of occurrences of digits in a string and record them into an array.

For example if the string entered into the method is “1223000”, then counter[1] =1, counter[2] =2, counter[3] = 1, counter[0] = 3.

I keep getting a arrayindexoutofbounds error, here is the code I have soo far:

//method: count number of occurences for digits
    public static int[] count(String s){

        int[] counter = new int[10];

        for(int j= 0; j < s.length(); j++){
            if (Character.isDigit(s.charAt(j)))
                counter[s.charAt(j)] += 1;
        }

        return counter;
    }

Answer 1

See my comment for how to correct the issue.

You should also consider looping the characters of the string directly, rather than tracking the position in the string and using charAt.

For example,

public static int[] countDigits(final String str) {
  final int[] freq = new int[10];
  for (final char c : str.toCharArray()) {
    if (Character.isDigit(c)) {
      ++freq[c - '0'];
    }
  }
  return freq;
}

Testing the above using the below code results in no error (java -ea DigitFreqTest).

final String input = "1223000";
final int[] freq = countDigits(input);
assert freq[0] == 3 && freq[1] == 1 && freq[2] == 2 && freq[3] == 1;

---

Note the above does not support Unicode… in that case, you may wish to instead use Character.getNumericValue.

public static Map<Integer, Integer> countNumerals(final String str) {
  final Map<Integer, Integer> freq = new HashMap<Integer, Integer>(10);
  for (final char c : str.toCharArray()) {
    if (Character.isDigit(c)) {
      final int num = Character.getNumericValue(c);
      Integer occ = freq.get(num);
      if (occ == null) {
        occ = 0;
      }
      freq.put(num, occ + 1);
    }
  }
  return freq;      
}

Note I had to improvise using a Map<Integer, Integer> because Java does not inherently provide a multiset collection 🙁