Re: How to compute the number of occurance of each element in an array list?

From:
Michael Preminger <michaelp@hio.no>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 03 May 2007 23:35:09 +0200
Message-ID:
<f1dkie$19r4$1@news01.tp.hist.no>
John wrote:

why do I have the following errors?

n in thread "main" java.lang.NullPointerException
 at multipleChoiceQuestion.setUserResponses(multipleChoiceQuestion.java:199)
 at survey.takeSurveyOrQuiz(survey.java:120)
 at surveyQuizBank.takeSurveyOrQuiz(surveyQuizBank.java:83)
 at connection.finishQuestion(connection.java:262)
 at terminal.processKeyboardInput(terminal.java:39)
 at surveyQuizTester.main(surveyQuizTester.java:19)

I guess I didnot intialize the Map properly.

userResponses=new HashMap();


The initialization is O.K, so your problem probably lies somewhere else,
beyond the scope of this question.

You will have to provide us with some more information to get a useful
answer.

Michael

Is this correct?

"michaelp" <michaelp@hio.no> wrote in message
news:f1cos9$128k$1@news01.tp.hist.no...

Den 03.05.2007 13:59, Ruan skrev:

Are you suggesting hashtable?


This is a good suggestion. A HashTable is a map, the same is a HashMap and
other classes.

What is its method to add the value of a
specifica key when a new user takes the question?

And what is the method when there is no previous key and have to add


a new

one?


You need to represent a response by a pattern, for example as a String
pattern, "(a,b)", "(a,c)", and so on. But you do not need to do it in
advance, as long as you have a method that generates the pattern
automatically as the response comes from the user. These patterns are your
keys.

The number of occurrence is the value, and unless you use Auto-Boxing
(dont if you do not know what it means), has to be treated as an object.
Integer is a good choice.

Now: A Map has the methods.
      put(key, value) with which you associate a key with a value
      and get(key), with which you extract a value based on a key.

if a key (that is automatically generated by the application) has not been
used yet, get() returns null, and you assign it with put(Stringpattern,
new Integer(1))
otherwise you increment the already existing value.

Like the following:

Integer value = (Integer)mymap.get(stringpattern);
if (value == null){
mymap.put(stringpattern, new Integer(1));
}else{
int nr_occ=value.intValue() + 1;
mymap.put(Integer.valueOf(int_nr_occ));
}

Hope it helps.

Michael

"Patricia Shanahan" <pats@acm.org> wrote in message
news:f1blc7$8rc$1@ihnp4.ucsd.edu...

John wrote:
...

Or in another word, how do I know the number of occurence of each


element in

the ArrayList?


...

Replace the ArrayList with a Map, with responses as keys, and the count
of the number of appearances so far of the response as value.

Patricia

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