How can I use a map for calculations?

Question

I'm working on a program for class. Using a TreeMap to store IDs (String - Key) and earnings amounts (double - value). I'm importing the values from a text file using a Scanner. My problem at the moment is that I need the values to accumulate rather than overwriting with the last value read. So my question is how do you use a Map to do calculations like that? Any help would be appreciated.

score 3 · Answer 1 · answered Feb 29 '12 at 17:32

3

There is no implicit functionality in Map. Idea behind your homework assignment is for you to learn how to insert, find, get and replace to/from a Map. There are functions for each of these and ou should use all to get this done.

answered Feb 29 '12 at 17:32

Kashyap

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score 1 · Accepted Answer · answered Feb 29 '12 at 17:28

1

When adding a new value to your map, if the key already exists, you can get the associated value, add the new value to it, and put it back into the map. Example:

// Assuming that key and value were read from your file, and that
// myMap is declared as "Map<String, Double>"
if (myMap.containsKey (key)) {
    double oldValue = myMap.get (key);
    value += oldValue;
}

myMap.put (key, value);

answered Feb 29 '12 at 17:28

Laf

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@thekashyap if it makes you feel better this code doesn't work for me as it is. The second line gives an incompatible type error because the variable is a double but the key is a string... I really wish spec docs were easier to read. – Steve the Maker Feb 29 '12 at 18:02
@StevetheMaker How did you declare your `Map`? – Laf Feb 29 '12 at 18:12
@Laf I just used: TreeMap agentMap = new TreeMap(); When I tried to specify types in the constructor I was getting errors that I couldn't figure out so I thought I'd give this a try. – Steve the Maker Feb 29 '12 at 18:18
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You should use generics to declare your tree, so that you get type checking at compile time rather than errors at runtime. Your declaration should look like: `TreeMap agentMap = new TreeMap ();`. This is assuming that your key is a `String`, and your values are `Double`. – Laf Feb 29 '12 at 18:20
OMG... I just wasn't capitalizing the word Double. Why would that need to be caps sometimes and lowercase other times? – Steve the Maker Feb 29 '12 at 18:26
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Capitalized means that you are using the class, and the lowercase means you are using the primitive type. Maps can only store objects, so in this case you need a `Double`, and not a `double`. Java will automatically convert `Double` into `double` and vice-versa (you can read more information on auto-boxing [here](http://docs.oracle.com/javase/1.5.0/docs/guide/language/autoboxing.html)). – Laf Feb 29 '12 at 18:32
@StevetheMaker it does make me feel better that it didn't work.. you also learned that you didn't know enough of template specialization (generics), implicit type casting (auto-boxing) etc. Java's javadocs are sooo much better to browse than man pages and C++ comments.. – Kashyap Feb 29 '12 at 18:41
Well, I appreciate the hard-handed and the soft-handed approach. I'm trying to get a degree online and I don't want a pile of certs and no ability to code when I'm finished (I'm at least past fizzbuzz at this point). At the same time, I don't have a single human to bounce ideas off of, so SO and SE are essentially my classroom. – Steve the Maker Feb 29 '12 at 18:52
I wish to note some issue. For the first, why do you use TreeMap? It is a sorted implementation, its search takes O(log n) time. So, if you may consider using HashMap istead. – Alexandr Mar 01 '12 at 12:43
for the second, program to interfaces, so, never use concreate implementations when create a reference. Use Map agentMap = new TreeMap(); instead. – Alexandr Mar 01 '12 at 12:44
for the third, concerning performance issue, I would recommend to use primitive collections, you may read here for details http://stackoverflow.com/questions/2504959/storing-primitive-values-in-a-java-collection – Alexandr Mar 01 '12 at 12:47

score 1 · Answer 3 · answered Feb 29 '12 at 17:29

1

1) Check whether value with same key exists in the map 2) If it exists then read it and add the currently read value. Put it back into map

answered Feb 29 '12 at 17:29

JProgrammer

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How can I use a map for calculations?

3 Answers3