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Fix loss of precision in standard deviation #1087

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Fix loss of precision in standard deviation #1087

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14 changes: 8 additions & 6 deletions json-path/src/main/java/com/jayway/jsonpath/internal/function/numeric/StandardDeviation.java
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Original file line number Diff line number Diff line change
Expand Up @@ -6,19 +6,21 @@
* Created by mattg on 6/27/15.
*/
public class StandardDeviation extends AbstractAggregation {
private Double sumSq = 0d;
private Double sum = 0d;
private Double count = 0d;
private double mean = 0d;
private double sumOfSquaredDifferences = 0d;
private long count = 0;

@Override
protected void next(Number value) {
sum += value.doubleValue();
sumSq += value.doubleValue() * value.doubleValue();
double number = value.doubleValue();
count++;
double delta = number - mean;
mean += delta / count;
sumOfSquaredDifferences += delta * (number - mean);
}

@Override
protected Number getValue() {
return Math.sqrt(sumSq/count - sum*sum/count/count);
return Math.sqrt(sumOfSquaredDifferences / count);
}
}
19 changes: 19 additions & 0 deletions json-path/src/test/java/com/jayway/jsonpath/internal/function/NumericPathFunctionTest.java
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Expand Up @@ -120,4 +120,23 @@ public void testStddevOfEmptyListNegative(Configuration conf) {
// assertThat(using(conf).parse(NUMBER_SERIES).read("$.numbers.foo()")).isEqualTo(numberSeries);
// }


@ParameterizedTest
@MethodSource("configurations")
public void testStdDevWithLargeOffset(Configuration conf) {
Object result = com.jayway.jsonpath.JsonPath.using(conf)
.parse("[10000000000,10000000001,10000000002]").read("$.stddev()");
Number value = (Number) conf.jsonProvider().unwrap(result);
assertEquals(Math.sqrt(2.0 / 3.0), value.doubleValue(), 1e-12);
}

@ParameterizedTest
@MethodSource("configurations")
public void testStdDevOfIdenticalLargeValues(Configuration conf) {
Object result = com.jayway.jsonpath.JsonPath.using(conf)
.parse("[1e200,1e200]").read("$.stddev()");
Number value = (Number) conf.jsonProvider().unwrap(result);
assertEquals(0.0, value.doubleValue(), 0.0);
}

}