feat: add interquartile range algorithm

Author: sarthakgupta1907-cell

Maths/InterquartileRange.js

@@ -0,0 +1,31 @@
+/**
+ * @function interquartileRange
+ * @description Calculates the interquartile range (IQR) of an array.
+ *   IQR is the difference between the 75th and 25th percentiles,
+ *   representing the middle 50% spread of the data.
+ * @param {number[]} arr - An array of numbers
+ * @returns {number} - The interquartile range
+ * @see https://en.wikipedia.org/wiki/Interquartile_range
+ * @example interquartileRange([1, 2, 3, 4, 5]) => 2
+ * @example interquartileRange([1, 2, 3, 4, 5, 6, 7]) => 3
+ */
+const interquartileRange = (arr) => {
+  if (!Array.isArray(arr) || arr.length === 0) {
+    throw new TypeError('Input must be a non-empty array')
+  }
+  if (!arr.every((n) => typeof n === 'number' && !isNaN(n))) {
+    throw new TypeError('All elements must be numbers')
+  }
+  const sorted = [...arr].sort((a, b) => a - b)
+  const getPercentile = (data, p) => {
+    if (p === 0) return data[0]
+    if (p === 100) return data[data.length - 1]
+    const index = (p / 100) * (data.length - 1)
+    const lower = Math.floor(index)
+    const upper = Math.ceil(index)
+    return data[lower] + (data[upper] - data[lower]) * (index - lower)
+  }
+  return getPercentile(sorted, 75) - getPercentile(sorted, 25)
+}
+
+export { interquartileRange }

Maths/test/InterquartileRange.test.js

@@ -0,0 +1,41 @@
+import { interquartileRange } from '../InterquartileRange.js'
+
+describe('interquartileRange', () => {
+  test('interquartileRange([1,2,3,4,5]) returns 2', () => {
+    expect(interquartileRange([1, 2, 3, 4, 5])).toBeCloseTo(2)
+  })
+
+  test('interquartileRange([1,2,3,4,5,6,7]) returns 3', () => {
+    expect(interquartileRange([1, 2, 3, 4, 5, 6, 7])).toBeCloseTo(3)
+  })
+
+  test('interquartileRange identical elements returns 0', () => {
+    expect(interquartileRange([5, 5, 5, 5])).toBe(0)
+  })
+
+  test('interquartileRange works with unsorted array', () => {
+    expect(interquartileRange([5, 3, 1, 4, 2])).toBeCloseTo(2)
+  })
+
+  test('interquartileRange works with negative numbers', () => {
+    expect(interquartileRange([-3, -2, -1, 0, 1, 2, 3])).toBeCloseTo(3)
+  })
+
+  test('interquartileRange does not mutate original array', () => {
+    const arr = [5, 3, 1, 4, 2]
+    interquartileRange(arr)
+    expect(arr).toEqual([5, 3, 1, 4, 2])
+  })
+
+  test('throws TypeError for empty array', () => {
+    expect(() => interquartileRange([])).toThrow(TypeError)
+  })
+
+  test('throws TypeError for non-array input', () => {
+    expect(() => interquartileRange('123')).toThrow(TypeError)
+  })
+
+  test('throws TypeError for array with non-numbers', () => {
+    expect(() => interquartileRange([1, 'two', 3])).toThrow(TypeError)
+  })
+})