poi/src/test/java/org/apache/poi/ss/formula/functions/TestFrequency.java

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   this work for additional information regarding copyright ownership.
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package org.apache.poi.ss.formula.functions;

import static org.apache.poi.ss.formula.functions.Frequency.histogram;
import static org.junit.jupiter.api.Assertions.assertArrayEquals;
import static org.junit.jupiter.api.Assertions.assertEquals;

import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.ss.usermodel.CellRange;
import org.apache.poi.ss.usermodel.FormulaEvaluator;
import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.ss.util.CellRangeAddress;
import org.junit.jupiter.api.Test;

/**
 * Testcase for the function FREQUENCY(data, bins)
 */
class TestFrequency {

    @Test
    void testHistogram() {
        assertArrayEquals(new int[]{3, 2, 2, 0, 1, 1},
                histogram(
                        new double[]{11, 12, 13, 21, 29, 36, 40, 58, 69},
                        new double[]{20, 30, 40, 50, 60})
        );

        assertArrayEquals(new int[]{1, 1, 1, 1, 1, 0},
                histogram(
                        new double[]{20, 30, 40, 50, 60},
                        new double[]{20, 30, 40, 50, 60})

        );

        assertArrayEquals(new int[]{2, 3},
                histogram(
                        new double[]{20, 30, 40, 50, 60},
                        new double[]{30})

        );
    }

    @Test
    void testEvaluate() {
        Workbook wb = new HSSFWorkbook();
        FormulaEvaluator evaluator = wb.getCreationHelper().createFormulaEvaluator();

        int[] data = {1, 1, 2, 3, 4, 4, 5, 7, 8, 9, 9, 11, 3, 5, 8};
        int[] bins = {3, 6, 9};
        Sheet sheet = wb.createSheet();
        Row dataRow = sheet.createRow(0); // A1:O1
        for (int i = 0; i < data.length; i++) {
            dataRow.createCell(i).setCellValue(data[i]);
        }
        Row binsRow = sheet.createRow(1);
        for (int i = 0; i < bins.length; i++) { // A2:C2
            binsRow.createCell(i).setCellValue(bins[i]);
        }
        Row fmlaRow = sheet.createRow(2);
        CellRange<? extends Cell> arrayFmla = sheet.setArrayFormula("FREQUENCY(A1:O1,A2:C2)", CellRangeAddress.valueOf("A3:A6"));
        Cell b3 = fmlaRow.createCell(1); // B3
        b3.setCellFormula("COUNT(FREQUENCY(A1:O1,A2:C2))"); // frequency returns a vertical array of bins+1

        Cell c3 = fmlaRow.createCell(2);
        c3.setCellFormula("SUM(FREQUENCY(A1:O1,A2:C2))"); // sum of the frequency bins should add up to the number of data values

        assertEquals(5, (int) evaluator.evaluate(arrayFmla.getFlattenedCells()[0]).getNumberValue());
        assertEquals(4, (int) evaluator.evaluate(arrayFmla.getFlattenedCells()[1]).getNumberValue());
        assertEquals(5, (int) evaluator.evaluate(arrayFmla.getFlattenedCells()[2]).getNumberValue());
        assertEquals(1, (int) evaluator.evaluate(arrayFmla.getFlattenedCells()[3]).getNumberValue());

        assertEquals(4, (int) evaluator.evaluate(b3).getNumberValue());
        assertEquals(15, (int) evaluator.evaluate(c3).getNumberValue());

    }
}