generalize transpose to arrays of arrays [[]], arrays of objects [{}]…

…, objects of arrays {[]} and objects of objects {{}}.

closes #48

README.md

@@ -526,7 +526,7 @@ d3.range(49).map(function(d) { return d / 49; }); // GOOD: returns 49 elements.
 
 <a name="transpose" href="#transpose">#</a> d3.<b>transpose</b>(<i>matrix</i>) · [Source](https://github.com/d3/d3-array/blob/master/src/transpose.js), [Examples](https://observablehq.com/@d3/d3-transpose)
 
-Uses the [zip](#zip) operator as a two-dimensional [matrix transpose](http://en.wikipedia.org/wiki/Transpose).
+A two-dimensional [matrix transpose](http://en.wikipedia.org/wiki/Transpose). Accepts matrices as arrays of arrays [[]], arrays of objects [{}], objects of arrays {[]} and objects of objects {{}}.
 
 <a name="zip" href="#zip">#</a> d3.<b>zip</b>(<i>arrays…</i>) · [Source](https://github.com/d3/d3-array/blob/master/src/zip.js), [Examples](https://observablehq.com/@d3/d3-transpose)
 

src/transpose.js

@@ -1,15 +1,30 @@
-import min from "./min.js";
-
 export default function(matrix) {
-  if (!(n = matrix.length)) return [];
-  for (var i = -1, m = min(matrix, length), transpose = new Array(m); ++i < m;) {
-    for (var j = -1, n, row = transpose[i] = new Array(n); ++j < n;) {
-      row[j] = matrix[j][i];
+  // matrix is a key-value store of lines, themselves key-value stores of data.
+  // dimension y of the incoming matrix
+  const y = Object.keys(matrix);
+  if (!y.length) return [];
+
+  // dimension x of the incoming matrix
+  const line0 = matrix[y[0]],
+    x = new Set(Object.keys(line0)),
+    transpose = line0.length ? [] : {};
+
+  // prepare the transpose matrix with x as first dimension
+  for (const k of x) {
+    transpose[k] = matrix.length ? [] : {};
+  }
+  for (const [i, line] of Object.entries(matrix)) {
+    for (const k of x) {
+      // checks that each key is present in the line, otherwise:
+      // - remove that key from the transpose (for lines already read)
+      // - remove it from x (ignores it in future lines)
+      if (!(k in line)) {
+        delete transpose[k];
+        x.delete(k);
+      } else {
+        transpose[k][i] = line[k];
+      }
     }
   }
   return transpose;
 }
-
-function length(d) {
-  return d.length;
-}

test/transpose-test.js

@@ -35,3 +35,29 @@ tape("transpose(…) returns a copy", function(test) {
   test.deepEqual(tranpose, [[1, 3], [2, 4]]);
   test.end();
 });
+
+tape("transpose([objects]) transposes an array of objects", function(test) {
+  test.deepEqual(arrays.transpose([{a:1, b:2}, {a:3, b:4}, {a:5, b:6}]), {a: [1, 3, 5], b: [2, 4, 6]});
+  test.end();
+});
+
+tape("transpose([objects]) only uses properties present in all the objects", function(test) {
+  test.deepEqual(arrays.transpose([{a:1, b:2, c:-1}, {a:3, b:4}, {a:5, b:6, d:-1}]), {a: [1, 3, 5], b:[2, 4, 6]});
+  test.end();
+});
+
+tape("transpose(object) transposes an object of arrays", function(test) {
+  test.deepEqual(arrays.transpose({a: [1, 3, 5], b: [2, 4, 6]}), [{a:1, b:2}, {a:3, b:4}, {a:5, b:6}]);
+  test.end();
+});
+
+tape("transpose(object) ignores extra elements", function(test) {
+  test.deepEqual(arrays.transpose({a: [1, 3, 5], b: [2, 4, 6, 8]}), [{a:1, b:2}, {a:3, b:4}, {a:5, b:6}]);
+  test.end();
+});
+
+tape("transpose(object) transposes an object of objects", function(test) {
+  test.deepEqual(arrays.transpose({A: {a:1, b:2, c:-1}, B: {a:3, b:4}, C: {a:5, b:6, d:-1}}), { a: { A: 1, B: 3, C: 5 }, b: { A: 2, B: 4, C: 6 } });
+  test.end();
+});
+