From 3d5927acbf88ba3955cecab361712fd3be70f29a Mon Sep 17 00:00:00 2001
From: Pete Looney <petelooney81@gmail.com>
Date: Mon, 12 Oct 2020 16:35:28 -0500
Subject: [PATCH] added matrixMult.js

---
 Maths/matrixMult.js | 87 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 Maths/matrixMult.js

diff --git a/Maths/matrixMult.js b/Maths/matrixMult.js
new file mode 100644
index 000000000..d3a4c6b78
--- /dev/null
+++ b/Maths/matrixMult.js
@@ -0,0 +1,87 @@
+
+// MatrixCheck tests to see if all of the rows of the matrix inputted have similar size columns
+const matrixCheck = (matrix)=>{
+  let columnNumb;
+  for (let index = 0; index < matrix.length; index++){
+    if (index == 0){
+      columnNumb = matrix[index].length;
+    } else if (matrix[index].length != columnNumb){
+      console.log('The columns in this array are not equal')
+    } else {
+      return columnNumb;
+    }
+  }
+}
+
+// tests to see if the matrices have a like side, i.e. the row length on the first matrix matches the column length on the second matrix, or vise versa.
+const twoMatricesCheck = (first, second)=>{
+  const [firstRowLength, secondRowLength, firstColLength, secondColLength] = [first.length, second.length, matrixCheck(first), matrixCheck(second)];
+    if (firstRowLength != secondColLength || secondRowLength != firstColLength){
+      console.log('These matrices do not have a common side');
+      return false;
+    } else {
+      return true;
+    }
+}
+
+// returns an empty array that has the same number of rows as the left matrix being multiplied.
+  //Uses Array.prototype.map() to loop over the first (or left) matrix and returns an empty array on each iteration.
+const initiateEmptyArray = (first, second)=>{
+  if (twoMatricesCheck(first, second)){
+    const emptyArray = first.map(()=>{
+      return [''];
+    })
+    return emptyArray;
+  }else{
+    return false;
+  }
+}
+
+// Finally, `matrixMult` uses `Array.prototype.push()`, multiple layers of nested `for` loops, the addition assignment `+=` operator and multiplication operator `*` to perform the dot product between two matrices of differing sizes.
+    // Dot product, takes the row of the first matrix and multiplies it by the column of the second matrix, the `twoMatricesCheck` tested to see if they were the same size already.
+    // The dot product for each iteration is then saved to its respective index into `multMatrix`.
+const matrixMult = (firstArray, secondArray)=>{
+  let multMatrix = initiateEmptyArray(firstArray, secondArray);
+  for (let rm = 0; rm < firstArray.length; rm++){
+    rowMult = [];
+    for (let col = 0; col < firstArray[0].length; col++){
+      rowMult.push(firstArray[rm][col]);
+    }
+    for (let cm = 0; cm < firstArray.length; cm++){
+      colMult = [];
+      for (let row = 0; row < secondArray.length; row++){
+        colMult.push(secondArray[row][cm]);
+      }
+      let newNumb = 0;
+      for (let index = 0; index < rowMult.length; index++){
+        newNumb += rowMult[index] * colMult[index];
+      }
+      multMatrix[rm][cm] = newNumb;
+    }
+  }
+  return multMatrix;
+}
+
+const firstMatrix = [
+  [1, 2],
+  [3, 4]
+];
+
+const secondMatrix = [
+  [5, 6],
+  [7, 8]
+];
+
+console.log(matrixMult(firstMatrix, secondMatrix)); // [ [ 19, 22 ], [ 43, 50 ] ]
+
+const thirdMatrix = [
+  [-1, 4, 1],
+  [7, -6, 2],
+];
+const fourthMatrix = [
+  [2, -2],
+  [5, 3],
+  [3, 2],
+];
+
+console.log(matrixMult(thirdMatrix, fourthMatrix)); // [ [ 21, 16 ], [ -10, -28 ] ]
\ No newline at end of file