Java: Using a HashMap as a sparse vector

Question

I am using HashMap with integer keys,

HashMap<Integer, Double> feature_vector = new HashMap<Integer, Double>();

and while reading data, add a new member when the value is not 0,

if (value != 0)
                feature_vector.add(index, value);

For anything that deals with this feature_vector, how can I just iterate over the "non-empty" positions of this feature_vector? I don't want to increment the key from 0 to the size of the feature_vector and check the value of every single key, which slows the program down.

Meaning there will be cases when the feature vector looks like: 1->7, 3->0, 4->5, 5->9, ... (the second key has no value, so there would not be a second key and its value)

Thanks in advance for your time and help!

If you're already not adding indexes that have zero values to the map, then the only entries inside will be the nonzero ones. You can use the HashMap.entrySet() method (which is iterable) to explore the contents (both keys and values together), or use HashMap.keySet() for just the keys (indices). — William Price
– William Price, Commented Sep 12, 2014 at 4:10
Damn, totally forgot about the entrySet() method... Thank you William! — Meteorite
– Meteorite, Commented Sep 12, 2014 at 4:13
Not sure if it's required but if you use a TreeMap instead of a 'HashMap', the keys / entries will be sorted by Integer value. — AngerClown
– AngerClown, Commented Sep 12, 2014 at 11:03

Sean Owen · Accepted Answer · 2014-09-12 07:27:31Z

2

More efficient than iterating over keys is to iterate directly over key-value pairs:

for (Map.Entry<Integer,Double> entry : feature_vector.entrySet()) {
  Integer key = entry.getKey();
  Double value = entry.getValue();
  ...
}

This avoids looking up the value.

answered Sep 12, 2014 at 7:27

Sean Owen

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Multithreader · Accepted Answer · 2014-09-12 04:13:49Z

1

You can use this:

    Set<Integer> keys = feature_vector.keySet();

    for(Integer key : keys) {
        Double value = feature_vector.get(key);
        //do what ever you want with the key and value in here...
    }

answered Sep 12, 2014 at 4:13

Multithreader

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