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Java Forum / General / April 2006Comparing elements from one hashset to another
Hi, The following code was repeated three times to store 3 files into 3 individual hash sets. I now need to compare file A to file B, i want to see how many of the words in set A are also on set B. (Each set contains the same number of words). Im not sure how to implement this, can anyone help?? public FileIn() { try { // make sure there is a text file in the java directory where the java // code is saved in = new BufferedReader(new FileReader("C:\\Program Files\\Java\\example.txt")); set = new HashSet(); int Len = 1; while(Len>0) { String line = in.readLine(); try { Len = line.length(); System.out.println(line); set.add(line); } catch(NullPointerException npe){ Len = 0; //no more file to read } } > Hi, > [quoted text clipped - 3 lines] > contains the same number of words). Im not sure how to implement this, > can anyone help?? Take a look at the method retainAll that HashSet inherits from AbstractCollection. Patricia > Hi, > [quoted text clipped - 24 lines] > } > } Hello, what you want to do is make a union of the two collections (e.g. make a 3rd collection that has all elements that are contained by both of your source collections, a .size() on this 3rd collection gives you the answer to your problem). The fastest way to do this is (using A, B and C to match your question) : HashSet C = new HashSet(A); A.retainAll(B); C.size(); This obviously works just as well if you have more than 2 source collections Remon van Vliet Im not too sure i understand........how would this enable me to compare the values from two individual sets? would i be checking for duplicates?? farah > Im not too sure i understand........how would this enable me to compare > the values from two individual sets? would i be checking for > duplicates?? The retainAll() method essentially performs the intersection operation on two sets, which is (I think) what you want (i.e. A intersect B = {all x : (x is an element of A) and (x is an element of B)} ). - Oliver >> Im not too sure i understand........how would this enable me to compare >> the values from two individual sets? would i be checking for [quoted text clipped - 6 lines] > > - Oliver In particular, this does a non-destructive intersection using clone and retainAll: HashSet ab = (HashSet)a.clone(); ab.retainAll(b); The elements of ab are those that appear in both a and b. Of course, you can always do it manually by iterating over one of the sets and asking the other set whether it contains each element. But why write code yourself if it is already there and tested in java.util? Patricia -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 NotDashEscaped: You need GnuPG to verify this message Patricia Shanahan schreef: >>> Im not too sure i understand........how would this enable me to compare >>> the values from two individual sets? would i be checking for [quoted text clipped - 12 lines] > HashSet ab = (HashSet)a.clone(); > ab.retainAll(b); This also works if a is declared of type Set (don't code against implementations...): Set ab = new HashSet(a); ab.retainAll(b); This is what is recommended in Sun's Java tutorials, too. H.  SignatureHendrik Maryns ================== www.lieverleven.be http://aouw.org > This also works if a is declared of type Set (don't code against > implementations...): [quoted text clipped - 5 lines] > > H. :) Collection ab = new HashSet(a); Signaturewww.EdmundKirwan.com - Home of The Fractal Class Composition. Download Fractality, free Java code analyzer: www.EdmundKirwan.com/servlet/fractal/frac-page130.html Set ab = new HashSet(a); ab.retainAll(b); how can i check that this code works? it compiles but how can i check that it has done its task? >how can i check that this code works? it compiles but how can i check >that it has done its task? see the source code at http://mindprod.com/jgloss/hashset.html It dumps the result set so you can see it works. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Set ab = new HashSet(a); ab.retainAll(b); ok so the code compares the words from two individual sets.....how can i check the output of this so that it shows me which of the words within the two sets are identical??? Farah >ok so the code compares the words from two individual sets.....how can >i check the output of this so that it shows me which of the words >within the two sets are identical??? Again, please see the source code at http://mindprod.com/jgloss/hashset.html#COMPARING SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Set ab = new HashSet(a); ab.retainAll(b); ok so the code compares the words from two individual sets.....how can i check that it has identified any words that were contained in BOTH sets? (Id like the code to place any words that are contained in both sets int a seperate store and than return the number of words contained in that store (this being the total number of identical words that appear in both sets)) Farah >ok so the code compares the words from two individual sets.....how can >i check that it has identified any words that were contained in BOTH >sets? please see http://mindprod.com/jgloss/hashset.html#COMPARISON Run the code. Trace it. It solves your problem three different ways. All produce the expected result. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > Set ab = new HashSet(a); > ab.retainAll(b); [quoted text clipped - 9 lines] > > Farah ab.retainAll(b).retainAll(a).size() Roedy, For the retainAll method, a list of fruits that had been entered individually as strings were used. My code hoever uses info from files that are read into the program. Im having trouble modifying my code to work around this, can you help?? Farah -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 NotDashEscaped: You need GnuPG to verify this message farah uitte de volgende tekst op 04/02/2006 01:22 PM: > Roedy, > > For the retainAll method, a list of fruits that had been entered > individually as strings were used. My code hoever uses info from files > that are read into the program. Im having trouble modifying my code to > work around this, can you help?? Not if you give no information. I mean, do you really plan on doing any serious programming? It is very straightforward what happens in the code we gave you. If you don't understand it, I see no point in even trying to cope with file input. H. SignatureHendrik Maryns ================== www.lieverleven.be http://aouw.org >For the retainAll method, a list of fruits that had been entered >individually as strings were used. My code hoever uses info from files >that are read into the program. Im having trouble modifying my code to >work around this, can you help?? The only difference will be how you get the words from the files into the HashSets.. How you do that depends on the format of the files. Presuming they are simple text files with many words per line and punctuation, you might use a StreamTokenizer, http://mindprod.com/jgloss/streamtokenizer.html StringTokenizer http://mindprod.com/jgloss/stringtokenizern or Regex http://mindprod.com/jgloss/regex.html split that breaks the words apart at space or punctuation. You then add each word to HashSet A or B. If they are one per line, you can read them with a BufferedReader.readLine and trim. To read the file for regex, see http://mindprod.com/applets/fileio.html If there are duplicates WITHIN a file you will find out from the boolean returned by HashSet.add. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Just me or did you just ask the same question three times? Anyway, make a unit test and check if it produces the expected results (which it will). Remon van Vliet > Set ab = new HashSet(a); > ab.retainAll(b); [quoted text clipped - 9 lines] > > Farah On Thu, 30 Mar 2006 17:35:26 +0200, "Remon van Vliet" <remon@exmachina.nl> wrote, quoted or indirectly quoted someone who said : >HashSet C = new HashSet(A); >A.retainAll(B); >C.size(); the retainAll is easiest to code of the three algorithms, but I suspect that the sort method will be fastest for very large sets, which is the bulkiest to code. retainAll has the over head of creating a duplicate set and the overhead of looking up each element and removing nearly all the elements. sort has the overhead of loading the sort class, exporting two arrays, and sorting. lookup has the overhead of a hashed lookup on every element of one of the sets in the other. Code for all three algorithms is posted at http://mindprod.com/jgloss/hashset.html#COMPARING SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > On Thu, 30 Mar 2006 17:35:26 +0200, "Remon van Vliet" > <remon@exmachina.nl> wrote, quoted or indirectly quoted someone who [quoted text clipped - 20 lines] > Code for all three algorithms is posted at > http://mindprod.com/jgloss/hashset.html#COMPARING In terms of theoretical complexity, the sort method is O(n log(n)), but the retainAll method, and its manual equivalent, are O(n), assuming HashSet lookup is constant time. Of course, for any reasonably small problem size, the sort method might be faster. On the other hand, we were not asked for the fastest method, and the retainAll approach is really easy to code. Patricia i think in this instance i might try the easier of the methods.....im pretty new to java and not very competent with the language >The following code was repeated three times to store 3 files into 3 >individual hash sets. I now need to compare file A to file B, i want to >see how many of the words in set A are also on set B. (Each set >contains the same number of words). Im not sure how to implement this, >can anyone help?? there are two basic ways: extract the two sets as arrays, sort and compare linearly Enumerate Set A and look each element up in Set B to see if it exists. http://mindprod.com/jgloss/hashset.html#COMPARING SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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