Initial data commit

Task/Cumulative-standard-deviation/JavaScript/cumulative-standard-deviation-4.js

@@ -0,0 +1,55 @@
+(() => {
+    'use strict';
+
+    // ---------- CUMULATIVE STANDARD DEVIATION ----------
+
+    // cumulativeStdDevns :: [Float] -> [Float]
+    const cumulativeStdDevns = ns => {
+        const go = ([s, q]) =>
+            ([i, x]) => {
+                const
+                    _s = s + x,
+                    _q = q + (x * x),
+                    j = 1 + i;
+                return [
+                    [_s, _q],
+                    Math.sqrt(
+                        (_q / j) - Math.pow(_s / j, 2)
+                    )
+                ];
+            };
+        return mapAccumL(go)([0, 0])(ns)[1];
+    };
+
+    // ---------------------- TEST -----------------------
+    const main = () =>
+        showLog(
+            cumulativeStdDevns([
+                2, 4, 4, 4, 5, 5, 7, 9
+            ])
+        );
+
+    // --------------------- GENERIC ---------------------
+
+    // mapAccumL :: (acc -> x -> (acc, y)) -> acc -> [x] -> (acc, [y])
+    const mapAccumL = f =>
+        // A tuple of an accumulation and a list
+        // obtained by a combined map and fold,
+        // with accumulation from left to right.
+        acc => xs => [...xs].reduce((a, x, i) => {
+            const pair = f(a[0])([i, x]);
+            return [pair[0], a[1].concat(pair[1])];
+        }, [acc, []]);
+
+
+    // showLog :: a -> IO ()
+    const showLog = (...args) =>
+        console.log(
+            args
+            .map(x => JSON.stringify(x, null, 2))
+            .join(' -> ')
+        );
+
+    // MAIN ---
+    return main();
+})();