Updated quantile to use sorted values for min and max

Fixes #192
mathigon · Nov 22, 2024 · 9abfc28 · 9abfc28
1 parent f7aeeb4
commit 9abfc28
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Showing 2 changed files with 33 additions and 22 deletions.
diff --git a/src/statistics.ts b/src/statistics.ts
@@ -3,30 +3,36 @@
 // (c) Mathigon
 // ============================================================================
 
-
-import {total} from '@mathigon/core';
-import {lerp} from './arithmetic';
-
+import { total } from "@mathigon/core";
+import { lerp } from "./arithmetic";
 
 /** Calculates the mean of an array of numbers. */
 export function mean(values: number[]) {
   return values.length ? total(values) / values.length : 0;
 }
 
 /** Finds the quantile of an array of numbers for the cumulative probability p. */
-export function quantile(values: number[], p: number, method: number = 1): number {
+export function quantile(
+  values: number[],
+  p: number,
+  method: number = 1
+): number {
   const n = values.length;
   if (!n) return 0;
 
-  const sorted = values.slice(0).sort((a, b) => (a - b));
-  if (p === 0) return values[0];
-  if (p === 1) return values[n - 1];
+  const sorted = values.slice(0).sort((a, b) => a - b);
+  if (p === 0) return sorted[0];
+  if (p === 1) return sorted[n - 1];
 
   // See https://en.wikipedia.org/wiki/Quantile#Estimating_quantiles_from_a_sample
-  if (![1, 2, 3].includes(method)) throw new RangeError('Invalid quantile method.');
-  const index = (method === 1) ? n * p - 0.5 : // Matlab, Mathematica
-    (method === 2) ? (n - 1) * p :             // Excel, NumPy, Google Docs, R, Python (option)
-      (n + 1) * p - 1;                         // Python, Excel (option)
+  if (![1, 2, 3].includes(method))
+    throw new RangeError("Invalid quantile method.");
+  const index =
+    method === 1
+      ? n * p - 0.5 // Matlab, Mathematica
+      : method === 2
+      ? (n - 1) * p // Excel, NumPy, Google Docs, R, Python (option)
+      : (n + 1) * p - 1; // Python, Excel (option)
 
   if (Number.isInteger(index)) return sorted[index];
   const floor = Math.floor(index);
@@ -46,7 +52,7 @@ export function mode(values: number[]) {
   const counts = new Map<number, number>();
 
   let maxCount = -1;
-  let result: number|undefined = undefined;
+  let result: number | undefined = undefined;
 
   for (const v of values) {
     if (!counts.has(v)) counts.set(v, 0);
@@ -80,14 +86,14 @@ export function stdDev(values: number[]) {
 
 /** Calculates the covariance of the numbers in two arrays aX and aY. */
 export function covariance(aX: number[], aY: number[]) {
-  if (aX.length !== aY.length) throw new Error('Array length mismatch.');
+  if (aX.length !== aY.length) throw new Error("Array length mismatch.");
   const sum = aX.reduce((a, v, i) => a + v * aY[i], 0);
-  return (sum - total(aX) * total(aY) / aX.length) / aX.length;
+  return (sum - (total(aX) * total(aY)) / aX.length) / aX.length;
 }
 
 /** Calculates the correlation between the numbers in two arrays aX and aY. */
 export function correlation(aX: number[], aY: number[]) {
-  if (aX.length !== aY.length) throw new Error('Array length mismatch.');
+  if (aX.length !== aY.length) throw new Error("Array length mismatch.");
   const covarXY = covariance(aX, aY);
   const stdDevX = stdDev(aX);
   const stdDevY = stdDev(aY);

diff --git a/test/statistics-test.ts b/test/statistics-test.ts
@@ -3,12 +3,10 @@
 // (c) Mathigon
 // =============================================================================
 
+import tape from "tape";
+import { mode, quantile } from "../src";
 
-import tape from 'tape';
-import {mode, quantile} from '../src';
-
-
-tape('mode', (test) => {
+tape("mode", (test) => {
   test.equal(mode([]), undefined);
   test.equal(mode([2]), 2);
   test.equal(mode([2, 3]), undefined);
@@ -19,7 +17,7 @@ tape('mode', (test) => {
   test.end();
 });
 
-tape('quantile', (test) => {
+tape("quantile", (test) => {
   test.equal(quantile([], 0.5), 0);
 
   const evenPopulation = [1, 2, 3, 4, 5];
@@ -35,5 +33,12 @@ tape('quantile', (test) => {
   test.equal(quantile(oddPopulation, 0.5), 3.5);
   test.equal(quantile(oddPopulation, 0.75), 5);
   test.equal(quantile(oddPopulation, 1), 6);
+
+  const randomPopulation = [4, 1, 2, 6, 3, 5];
+  test.equal(quantile(randomPopulation, 0), 1);
+  test.equal(quantile(randomPopulation, 0.25), 2);
+  test.equal(quantile(randomPopulation, 0.5), 3.5);
+  test.equal(quantile(randomPopulation, 0.75), 5);
+  test.equal(quantile(randomPopulation, 1), 6);
   test.end();
 });