how to find the index of an item in an array java

Question

I want to find the index of the start up letter and then show the index of that item in array or array list in java. Example: I have:

String[] arr={"apple","at","book","bad","car","cat"};

when I input a: then i will get index =0, b: i will get index=2, c: i will get index=4

Because my array item is over 20,000 , so using linear Search is too slow. and my list is unsorted, so i can't use Binary Search also. I want to get index of the item like the example above, what can i solve with this?

If you have an unsorted list, the index of one arbitrary item starting with letter x is pretty useless, I think. — Smutje, Feb 28 '14 at 07:31
possible duplicate of [how-to-find-the-index-of-an-element-in-an-array-in-java] [1]: http://stackoverflow.com/questions/1522108/how-to-find-the-index-of-an-element-in-an-array-in-java — Gundamaiah, Feb 28 '14 at 07:35
because i can't sort my array to the sorted list, because it is not in English, it is in Vietnamese. I can't sort word in Vietnamese — Ruby Man, Feb 28 '14 at 07:36
If the array is constant, maybe you can find the indexes for each letter in advance and then use the stored indexes when needed. — meir shapiro, Feb 28 '14 at 07:37
If i use this linear Search, it seem to work slow with the size of big array. — Ruby Man, Feb 28 '14 at 07:44
Compile a map of index to first character when building the array. — Smutje, Feb 28 '14 at 07:45
Are you able to interrupt the construction of this array? If yes, store the starting character and the actual array index in a map. — Smutje, Feb 28 '14 at 07:53

Henrik · Answer 1 · 2014-02-28T08:04:31.310

0

If you want to get all the indexes of words starting with a certain letter then try this one:

While adding the Words to your Array/list (that will hold all your words) you could also add it to a map that will hold all indexes for every first letters.

 Map<String, ArrayList<Integer>> myMap = new HashMap<String, ArrayList<Integer>>();

 public void yourmethod() {
    //adding all your words to an Array/arraylist goes here. (arr[] in this case)

    string firstLetter = yourword.substring(0,1);

    if(myMap.constainsKey(firstLetter)) {
       myMap.get(letter).add(yourword);
    } else {
         myMap.put(firstLetter, yourword);
    }
 }

edited Feb 28 '14 at 08:04

answered Feb 28 '14 at 07:50

Henrik

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So, if the user continue type the next letter, example: ap ? so can it help, sir? – Ruby Man Feb 28 '14 at 08:09
ap? didnt you say the startup letter (one letter)? – Henrik Feb 28 '14 at 08:12
not only one, the search is like the search word in Dictionary. one letter , the one more letter, ... – Ruby Man Feb 28 '14 at 08:14
So you will be able to search for i.e.: a, ap, app, appl, apple to in this case get index 0 and if you only type the a letter you will get index 0 and 1 ? – Henrik Feb 28 '14 at 08:15

score 0 · Answer 2 · answered Feb 28 '14 at 07:53

You can run some initialization code (before the user starts to type letters in).

// initialize an array that has a cell for each letter with value -1
int[] firstIndexes = new int[26];
for(int i=0;i<firstIndexes.length;i++) {
    firstIndexes[i] = -1;
}

// loop over original array and look for each letter's first occurence
for(int i=0;i<wordsArray.length;i++) {
    char c=wordsArray[i][0];
    if(firstIndexes[c-'a'] < 0) {
        firstIndexes[c-'a'] = i;
    }
}

Then when the user types a letter you just need to find its index in the 'firstIndexes' array.

how to find the index of an item in an array java

2 Answers2