1package io.protobase.f7.formulas.statistical;
 2
 3import io.protobase.f7.errors.DivException;
 4import io.protobase.f7.formulas.AbstractFormula;
 5import io.protobase.f7.formulas.FormulaName;
 6import io.protobase.f7.models.GridColumnRowKey;
 7import io.protobase.f7.utils.Converters;
 8import io.protobase.f7.utils.Mappers;
 9
10import java.util.Arrays;
11import java.util.List;
12import java.util.function.BiFunction;
13import java.util.function.Function;
14import java.util.stream.Collectors;
15
16public class STDEV extends AbstractFormula {
17  public static FormulaName NAME = FormulaName.STDEV;
18  public static STDEV SELF = new STDEV();
19
20  public STDEV() {
21    super();
22  }
23
24  public STDEV(Function<Object, Object> lookup, BiFunction<GridColumnRowKey, Object, Object> collateralLookup) {
25    super(lookup, collateralLookup);
26  }
27
28  @Override
29  public Object internal(GridColumnRowKey origin, Object... values) {
30    checkAtLeastLength(values.length, 1, NAME);
31    List<Double> numbers = Arrays.stream(values)
32        .map(lookup)
33        .flatMap(Mappers::toFlatStreamGridFilterOnlyNumberLiterals)
34        .map(Converters::toDouble)
35        .collect(Collectors.toList());
36    if (numbers.size() == 1) {
37      return new DivException("Formula STDEV caused an error when attempting to divide by zero.");
38    }
39    double sum = 0.0;
40    double standardDev = 0.0;
41    int length = numbers.size();
42    for (double number : numbers) {
43      sum += number;
44    }
45    double mean = sum / length;
46    for (double number : numbers) {
47      standardDev += Math.pow(number - mean, 2);
48    }
49    return Math.sqrt(standardDev / (length - 1));
50  }
51}