Add d3.median.

Makefile

@@ -45,8 +45,10 @@ d3.core.js: \
 	src/core/ascending.js \
 	src/core/descending.js \
 	src/core/mean.js \
+	src/core/median.js \
 	src/core/min.js \
 	src/core/max.js \
+	src/core/number.js \
 	src/core/sum.js \
 	src/core/quantile.js \
 	src/core/zip.js \

d3.js

@@ -66,12 +66,17 @@ d3.mean = function(array, f) {
       i = -1,
       j = 0;
   if (arguments.length === 1) {
-    while (++i < n) if (!isNaN(a = array[i]) && a != null) m += (a - m) / ++j;
+    while (++i < n) if (d3_number(a = array[i])) m += (a - m) / ++j;
   } else {
-    while (++i < n) if (!isNaN(a = f.call(array, array[i], i)) && a != null) m += (a - m) / ++j;
+    while (++i < n) if (d3_number(a = f.call(array, array[i], i))) m += (a - m) / ++j;
   }
   return j ? m : undefined;
 };
+d3.median = function(array, f) {
+  if (arguments.length > 1) array = array.map(f);
+  array = array.filter(d3_number);
+  return array.length ? d3.quantile(array.sort(d3.ascending), .5) : undefined;
+};
 d3.min = function(array, f) {
   var i = -1,
       n = array.length,
@@ -100,6 +105,9 @@ d3.max = function(array, f) {
   }
   return a;
 };
+function d3_number(x) {
+  return x != null && !isNaN(x);
+}
 d3.sum = function(array, f) {
   var s = 0,
       n = array.length,

src/core/mean.js

@@ -5,9 +5,9 @@ d3.mean = function(array, f) {
       i = -1,
       j = 0;
   if (arguments.length === 1) {
-    while (++i < n) if (!isNaN(a = array[i]) && a != null) m += (a - m) / ++j;
+    while (++i < n) if (d3_number(a = array[i])) m += (a - m) / ++j;
   } else {
-    while (++i < n) if (!isNaN(a = f.call(array, array[i], i)) && a != null) m += (a - m) / ++j;
+    while (++i < n) if (d3_number(a = f.call(array, array[i], i))) m += (a - m) / ++j;
   }
   return j ? m : undefined;
 };

src/core/median.js

@@ -0,0 +1,5 @@
+d3.median = function(array, f) {
+  if (arguments.length > 1) array = array.map(f);
+  array = array.filter(d3_number);
+  return array.length ? d3.quantile(array.sort(d3.ascending), .5) : undefined;
+};

src/core/number.js

@@ -0,0 +1,3 @@
+function d3_number(x) {
+  return x != null && !isNaN(x);
+}

test/core/median-test.js

@@ -0,0 +1,43 @@
+require("../env");
+require("../../d3");
+
+var vows = require("vows"),
+    assert = require("assert");
+
+var suite = vows.describe("d3.median");
+
+suite.addBatch({
+  "median": {
+    topic: function() {
+      return d3.median;
+    },
+    "returns the median value for numbers": function(median) {
+      assert.equal(median([1]), 1);
+      assert.equal(median([5, 1, 2, 3, 4]), 3);
+      assert.equal(median([20, 3]), 11.5);
+      assert.equal(median([3, 20]), 11.5);
+    },
+    "ignores null, undefined and NaN": function(median) {
+      assert.equal(median([NaN, 1, 2, 3, 4, 5]), 3);
+      assert.equal(median([1, 2, 3, 4, 5, NaN]), 3);
+      assert.equal(median([10, null, 3, undefined, 5, NaN]), 5);
+    },
+    "can handle large numbers without overflowing": function(median) {
+      assert.equal(median([Number.MAX_VALUE, Number.MAX_VALUE]), Number.MAX_VALUE);
+      assert.equal(median([-Number.MAX_VALUE, -Number.MAX_VALUE]), -Number.MAX_VALUE);
+    },
+    "returns undefined for empty array": function(median) {
+      assert.isUndefined(median([]));
+      assert.isUndefined(median([null]));
+      assert.isUndefined(median([undefined]));
+      assert.isUndefined(median([NaN]));
+      assert.isUndefined(median([NaN, NaN]));
+    },
+    "applies the optional accessor function": function(median) {
+      assert.equal(d3.median([[1, 2, 3, 4, 5], [2, 4, 6, 8, 10]], function(d) { return d3.median(d); }), 4.5);
+      assert.equal(d3.median([1, 2, 3, 4, 5], function(d, i) { return i; }), 2);
+    }
+  }
+});
+
+suite.export(module);