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Adding NGram range support for feature extraction #2

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madhawa-gunasekara wants to merge 3 commits into from
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Adding NGram range support for feature extraction #2

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d8c16a2
Adding NGram range support for feature extraction
Apr 3, 2016
6376eae
Adding validation for N-Gram range
Apr 5, 2016
81597ad
Fixing non-negative validation
Apr 5, 2016
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42 changes: 40 additions & 2 deletions opennlp-tools/src/main/java/opennlp/tools/doccat/NGramFeatureGenerator.java
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,8 @@

package opennlp.tools.doccat;

import opennlp.tools.util.InvalidFormatException;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
Expand All @@ -27,12 +29,48 @@
*/
public class NGramFeatureGenerator implements FeatureGenerator {

//default values for bigrams
private int minGram = 2;
private int maxGram = 2;

public NGramFeatureGenerator() {
}

public NGramFeatureGenerator(int minGram, int maxGram) throws InvalidFormatException {
if (minGram > 0 && maxGram > 0) {
if (minGram <= maxGram) {
this.minGram = minGram;
this.maxGram = maxGram;
} else {
throw new InvalidFormatException("Minimum range value (minGram) should be less than or equal to maximum range value (maxGram)!");
}
} else {
throw new InvalidFormatException("Both minimum range value (minGram) & maximum range value (maxGram) should be greater than or equal to 1!");
}

@kelkarn kelkarn Apr 5, 2016

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This is good, but what if minGram/maxGram are negative?

}

/**
* Extract N-Grams from the given text fragments according to the N-Gram range,
* i.e. a,b,c,d with minGram = 2 & maxGram = 3 then features would be a:b, a:b:c, b:c, b:c:d, c:d
* i.e. a,b,c,d with minGram = 2 & maxGram = 2 then features would be a:b, b:c, c:d
*
* @param text the text fragments to extract features from
* @param extraInfo optional extra information to be used by the feature generator
* @return a collection of features
*/
public Collection<String> extractFeatures(String[] text, Map<String, Object> extraInfo) {

List<String> features = new ArrayList<String>();

for (int i = 0; i < text.length - 1; i++) {
features.add("ng=" + text[i] + ":" + text[i + 1]);
for (int i = 0; i <= text.length - minGram; i++) {
String feature = "ng=";
for (int y = 0; y < maxGram && i + y < text.length; y++) {
feature = feature + ":" + text[i + y];
int gramCount = y + 1;
if (maxGram >= gramCount && gramCount >= minGram) {
features.add(feature);
}
}
}

return features;
Expand Down