September 2017 Update

Task/Matrix-multiplication/JavaScript/matrix-multiplication-3.js

@@ -0,0 +1,2 @@
+[[51, -8, 26, -18], [-8, -38, -6, 34],
+      [33, 42, 38, -14], [17, 74, 72, 44]]

Task/Matrix-multiplication/JavaScript/matrix-multiplication-4.js

@@ -0,0 +1,52 @@
+((() => {
+    'use strict';
+
+    // matrixMultiply :: Num a => [[a]] -> [[a]] -> [[a]]
+    const matrixMultiply = (a, b) => {
+        const bCols = transpose(b);
+        return a.map(aRow => bCols.map(bCol => dotProduct(aRow, bCol)));
+    }
+
+    // dotProduct :: Num a => [[a]] -> [[a]] -> [[a]]
+    const dotProduct = (xs, ys) => sum(zipWith(product, xs, ys));
+
+
+    // GENERIC
+
+    // zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
+    const zipWith = (f, xs, ys) =>
+        xs.length === ys.length ? (
+            xs.map((x, i) => f(x, ys[i]))
+        ) : undefined;
+
+    // transpose :: [[a]] -> [[a]]
+    const transpose = xs =>
+        xs[0].map((_, iCol) => xs.map(row => row[iCol]));
+
+    // sum :: (Num a) => [a] -> a
+    const sum = xs =>
+        xs.reduce((a, x) => a + x, 0);
+
+    // product :: Num a => a -> a -> a
+    const product = (a, b) => a * b;
+
+
+    // TEST
+    return matrixMultiply(
+        [
+            [-1, 1, 4],
+            [6, -4, 2],
+            [-3, 5, 0],
+            [3, 7, -2]
+        ],
+
+        [
+            [-1, 1, 4, 8],
+            [6, 9, 10, 2],
+            [11, -4, 5, -3]
+        ]
+    );
+
+    // --> [[51, -8, 26, -18], [-8, -38, -6, 34],
+    //        [33, 42, 38, -14], [17, 74, 72, 44]]
+}))();

Task/Matrix-multiplication/JavaScript/matrix-multiplication-5.js

@@ -0,0 +1,2 @@
+[[51, -8, 26, -18], [-8, -38, -6, 34],
+[33, 42, 38, -14], [17, 74, 72, 44]]