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  <title>Matrix addition — shape-safe</title>
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  <h1>Dimension-safe matrix add</h1>
  <p>Edit <code>goodA</code>, <code>goodB</code>, <code>badA</code>, <code>badB</code>, then <strong>Run</strong>.</p>
  <pre id="out"></pre>
  <script>
    (function () {
      function sameShape(a, b) {
        if (!a.length || !b.length) {
          return false;
        }
        if (a.length !== b.length) {
          return false;
        }
        var cols = a[0].length;
        if (cols === 0) {
          return false;
        }
        var all = a.concat(b);
        var i;
        for (i = 0; i < all.length; i++) {
          if (all[i].length !== cols) {
            return false;
          }
        }
        return true;
      }
      function addMatricesSafe(a, b) {
        if (!sameShape(a, b)) {
          return null;
        }
        var rows = a.length;
        var cols = a[0].length;
        var result = [];
        var i, j;
        for (i = 0; i < rows; i++) {
          result[i] = [];
          for (j = 0; j < cols; j++) {
            result[i][j] = a[i][j] + b[i][j];
          }
        }
        return result;
      }
      var goodA = [[1, 2], [3, 4]];
      var goodB = [[4, 3], [2, 1]];
      var badA = [[1, 2]];
      var badB = [[1, 2], [3, 4]];
      var lines = [];
      lines.push(JSON.stringify(addMatricesSafe(goodA, goodB)));
      lines.push(String(addMatricesSafe(badA, badB)));
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
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JavaScript Matrix Addition — Shape-safe (Try It)