CODEWITHSUNDEEP
javacollectionshashmaphashtable
user1079065
user1079065

Reputation: 2205

counting occurance of characters using hashtable in Java

I am trying to count occurrence of each character in a string. So if I input aaabbbccc I am expecting o/p as {a=3, b=3, c=3} but instead I keep on getting it as {a=1, b=1, c=1} as my hashtable contains method fails and returns false. What is wrong in the code?

I also know that there is a HashMap collection quite similar to hashtable. but as I am learing java I want to try the same code with all datastructures just to get an idea of methods. The code with map is not much different than what I am doing here still this code fails. and I am not able to figure out where is the bug in the code.

I have following code:

Hashtable<Character, Integer> stringHash = new Hashtable<Character, Integer>();

This stringHash is a class level variable.

for(int i=0; i<s.length(); i++){
        if(stringHash ==null || stringHash.size()==0){
            stringHash.put(s.charAt(i), 1);
        }
        else{
            if(! stringHash.contains(s.charAt(i)) ){
                System.out.println(s.charAt(i));
                stringHash.put(s.charAt(i), 1);
            }
            else{
                int count = stringHash.get(s.charAt(i));
                stringHash.put(s.charAt(i), count++);
            }   
        }
        System.out.println(stringHash + " " + s.charAt(i) + "  "+ stringHash.contains(s.charAt(i)));
}

Upvotes: 0

Views: 11402

Answers (6)

ring bearer
ring bearer

Reputation: 20783

Your code

if(stringHash ==null || stringHash.size()==0){
    stringHash.put(s.charAt(i), 1);
}

would throw NPE if somehow the hashmap is null. Luckily it seems that you have initialized it properly. The block rather should have been

if(stringHash ==null){
    stringHash = new HashMap()
    stringHash.put(s.charAt(i), 1);
}

Again, that would not have fixed your bug. You should use containsKey instead of contains that checks for value in HashTable. What you are looking to implement can be summarized in following pseudocode.

initialize hashmap
for each character c in inputString
  count = 0
  if hashmap has a key for c
     count = get value for c from hashmap
  end if
  put in to hashmap c, count + 1
end for

In Java this would look like : 

Map<Character, Integer> charCountMap = new HashMap<>();
for(char c : inputString.toCharArray()){
  int count = 0;
  if(charCountMap.containsKey(c)){
    count = charCountMap.get(c);
  }
  charCountMap.put(c,count+1);
}

Or for the adventurous, here is Java8 version

Map<Character,Long> map = s.chars().mapToObj(i->(char)i)
                                     .collect(Collectors
                                                .groupingBy(e -> e,
                                                   Collectors.counting()));
System.out.println(map);

Finally, do not use HashTable its a legacy class, no one uses it now a days. Stick with HashMap or other flavors of Map implementations.

Upvotes: 2

Rigre Garciand&#237;a
Rigre Garciand&#237;a

Reputation: 159

Don't use Hashtable, you can simplify that code a lot, something like this should work:

import java.text.MessageFormat;
import java.util.HashMap;
import java.util.Map;

import org.apache.commons.lang.StringUtils;

public class Main {

    public static void main(String[] args) {
        Map<Character, Long> countMap = count("aabbbcccc");
        for (Map.Entry<Character, Long> entry : countMap.entrySet()) {
            System.out
                    .println(MessageFormat.format("Char ''{0}'' with count ''{1}''", entry.getKey(), entry.getValue()));
        }
    }

    private static Map<Character, Long> count(String value) {
        Map<Character, Long> countMap = new HashMap<Character, Long>();

        if (StringUtils.isNotBlank(value)) {
            for (int i = 0; i < value.length(); i++) {
                Long count = countMap.get(value.charAt(i));

                count = count == null ? 1 : count + 1;
                countMap.put(value.charAt(i), count);
            }
        }

        return countMap;
    }
}

Upvotes: 0

Tawcharowsky
Tawcharowsky

Reputation: 625

Why do you use hashMap for counting character occurance? I would use integer array of size 255 like so:

    int[] counter = new int[256];
    String s = "aaabbbcccc";
    for(int i = 0; i < s.length(); i++){
        counter[s.charAt(i)]++;
    }

    for(int i = 0; i < counter.length; i++)
        if(counter[i] > 0)
            System.out.println(((char)i) + " occurs " + counter[i] + " times");

that coude would give an output:

a occurs 3 times
b occurs 3 times
c occurs 4 times

Upvotes: 0

digidude
digidude

Reputation: 289

Try something like this....The reason your code is failing is that you are checking contains() on HashTable instead of its keySet. Hope that helps

 public static void main(String[] args) {
    // TODO Auto-generated method stub
    String s = "aaaaabbcccc";
    Hashtable<Character, Integer> counter = new Hashtable<Character, Integer>();
    int count = 0;
    for(int i=0;i<s.length();i++){
        if(!counter.keySet().contains(s.charAt(i))){
            counter.put(s.charAt(i), 1);
        } else {
            count = counter.get(s.charAt(i));
            counter.put(s.charAt(i), ++count);
        }
    }

    for(char c:counter.keySet()) {
        System.out.println("Character : "+c+" - Occurences : "+counter.get(c));
    }
}

o/p

Character : b - Occurences : 2
Character : c - Occurences : 4
Character : a - Occurences : 5

Upvotes: 2

Tyler Durden
Tyler Durden

Reputation: 11532

Debug my code questions are discouraged, but in the way of solving the general problem of counting characters in a string I can suggest a much simpler method:

public static int[] countCharacters( String s ){
    int[] count = new int[ 256 ];
    for( int xChar = 0; xChar < s.length(); xChar++ ) count[ s.charAt( xChar ) ]++;
    return count;
}

This assumes you have single byte characters.

Upvotes: 0

Jashaszun
Jashaszun

Reputation: 9270

This code works for me:

String s = "aaabbbccc";

Map<Character, Integer> stringHash = new HashMap<Character, Integer>();
for (char ch : s.toCharArray())
    stringHash.put(ch, stringHash.containsKey(ch) ? (stringHash.get(ch) + 1) : 1);

System.out.println(stringHash);
// output: "{a=3, b=3, c=3}"

I am using a Map<K, V> instead of HashTable<K, V>, but this is more common.

Upvotes: 4

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