 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Java Forum / General / December 2007How to code something in Java
I'm still relatively new to Java. I have a data set in which each pair has a number attached to it. Say the data set is {a,b,c,d,....z} Now form this data set, I have bunch of pairs and each pair has a number attached to it. e.g. {a,b} = 3 {a,y} = 5 I was wondering how will I code somethign like this in Java Ideally I would have an array where I would have an input like Array[x] [y].getValue() Any ideas. Thanks. On Nov 28, 3:36 pm, TheTravellingSalesman <saad.za...@gmail.com> wrote: > I'm still relatively new to Java. I have a data set in which each pair > has a number attached to it. [quoted text clipped - 14 lines] > > Thanks. Java does have multi-dimensional arrays, but you cannot index them by character literals (if that is what you were trying to do). someArray[x][y]=3 WILL work in Java (assuming that x and y are integer variables, but someArray['x']['y'] =3 won't. Have you looked into HashMaps? I suppose you could have a HashMap of HashMaps, just like you can have an array of arrays. Hard to say more without more details on the problem. > Java does have multi-dimensional arrays, but you cannot index them by > character literals (if that is what you were trying to do). Character literals work but String literals do not. A char -> int conversion is a widening primitive conversion, and as such does not need a cast to be converted for you. Using characters as their integral values is actually somewhat common in many circumstances.  SignatureBeware of bugs in the above code; I have only proved it correct, not tried it. -- Donald E. Knuth >I'm still relatively new to Java. I have a data set in which each pair >has a number attached to it. [quoted text clipped - 14 lines] > >Thanks. One idea would be to use a HashMap. Use a two character string, "ab", "ay" etc. as the key and use your number as the value. HashMap<String, int> rossum > I'm still relatively new to Java. I have a data set in which each pair > has a number attached to it. [quoted text clipped - 14 lines] > > Thanks. There are many options. Is your a to z the actual data you want to use, or just an example? Are there exactly 26 items? You could simply use a two dimensional int array which is 26x26 for one simple example: int[][] values = new int[26][26]; values[0][1] = 3; // a is 0, b is 1 values[0][24] = 5; // a is 0, y is 24 int v = values[0][1]; Using a 2D array means you can't tell if a given value is defined or not because it makes it work as if all possible values are defined. That may not be what you need. For an associative lookup, you will use one of the Map Collections. Java doesn't implement a 2D associative lookup so you have to do that yourself one way or another. A simple way would be to use a Map with a String as a key and the Integer as the value, and construct your keys as pairs of letters: Map<String,Integer> values = new HashMap<String,Integer>(); values.put("ab", 3); values.put("ay", 5); int v = values.get("ab"); SignatureCurt Welch http://CurtWelch.Com/ curt@kcwc.com http://NewsReader.Com/ > For an associative lookup, you will use one of the Map Collections. Java > doesn't implement a 2D associative lookup so you have to do that yourself [quoted text clipped - 12 lines] > Curt Welch http://CurtWelch.Com/ > c...@kcwc.com http://NewsReader.Com/ Map<String, Map> values... might work. You'd have to use it like this I think (I haven't actually tried this though): values.getValue('a').getValue('b') to find 3 at "ab". On Nov 28, 4:56 pm, lord.zol...@gmail.com wrote: > > For an associative lookup, you will use one of the Map Collections. Java > > doesn't implement a 2D associative lookup so you have to do that yourself [quoted text clipped - 18 lines] > though): > values.getValue('a').getValue('b') to find 3 at "ab". No... It would probably heave to be declared as: Map<String, Map<String, int>> values = ... > On Nov 28, 4:56 pm, lord.zol...@gmail.com wrote: >> > For an associative lookup, you will use one of the Map Collections. Java [quoted text clipped - 8 lines] > Map<String, Map<String, int>> values = ... >> values.getValue('a').getValue('b') to find 3 at "ab". If the OP does it that way, though, he must be aware that adding new elements to nested HashMaps is somewhat more complicated: if the first key isn't yet there, it will have to create a new nested HashMap bound to that first key before he can insert the second key and set the value Also, retrieving isn't just that simple: The way you suggested would throw NullPointerExceptions whenever the first key didn't exist in the outer HashMap. Unless the demands are too specific, I'd suggest either the "make a string of both keys"-approach, or if even that didn't work out, to create a Pair<Keytype1,Keytype2> class, and use that as Key to the HashMap: HashMap<Pair<Keytype1,Keytype2>,Integer> When creating the Pair-class, don't forget to override both its .equals() and .hashCode() methods! I'm not aware of any predefined general-purpose Pair class. >Ideally I would have an array where I would have an input like >Array[x] [y].getValue() class P { final java.lang.Object[] value; public P( final java.lang.Object ... value ){ this.value = value; } public int hashCode(){ return java.util.Arrays.hashCode( value ); } public boolean equals( final java.lang.Object value ) { return java.util.Arrays.equals ( this.value,(( P )value ).value ); }} public class Main { public static P p( final java.lang.Object ... value ) { return new P( value ); } public static void main( final java.lang.String[] args ) { final java.util.Map<P,java.lang.Integer> map = new java.util.HashMap<P,java.lang.Integer>(); map.put( p( 'a', 'b' ), 3 ); map.put( p( 'a', 'y' ), 5 ); java.lang.System.out.println( map.get( p( 'a', 'b' ))); java.lang.System.out.println( map.get( p( 'a', 'y' ))); }} 3 5 >map.put( p( 'a', 'b' ), 3 ); >map.put( p( 'a', 'y' ), 5 ); >java.lang.System.out.println( map.get( p( 'a', 'b' ))); >java.lang.System.out.println( map.get( p( 'a', 'y' ))); One might remove the need for the above call to »p« to simplify the interface: class P<E> { final E[] value; public P( final E ... value ){ this.value = value; } public int hashCode(){ return java.util.Arrays.hashCode( value ); } public boolean equals( final java.lang.Object value ) { return value instanceof P ? java.util.Arrays.equals ( this.value,(( P )value ).value ): false; }} interface MultiMap<K,V> { public void set( final V value, final K ... keys ); public V get( final K ... keys ); } class MultiHashMap<K,V> extends java.util.HashMap<P<K>,V> implements MultiMap<K,V> { public void set( final V value, final K ... keys ) { put( new P<K>( keys ), value ); } public V get( final K ... keys ){ return get( new P<K>( keys )); }} public class Main { static final MultiMap<java.lang.Character,java.lang.Integer> map = new MultiHashMap<java.lang.Character,java.lang.Integer>(); public static void main( final java.lang.String[] args ) { map.set( 3, 'a', 'b' ); map.set( 5, 'a', 'y' ); java.lang.System.out.println( map.get( 'a', 'b' )); java.lang.System.out.println( map.get( 'a', 'y' )); }} 3 5 So Overall what everyone is suggesting that I should use Hashmaps, i.e. my only way. Another Lame way I though of was to have a String array which will contain Strings like > >map.put( p( 'a', 'b' ), 3 ); > >map.put( p( 'a', 'y' ), 5 ); [quoted text clipped - 36 lines] > 3 > 5 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 ...
|
Reply to this Message
|
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.