Double sort in numpy.median (Trac #295)

[numpy-gitbot]

Original ticket http://projects.scipy.org/numpy/ticket/295 on 2006-09-26 by @FrancescAlted, assigned to unknown.

numpy.median is computing a sort twice for arrays with an even number of elements. The next patch is a cure for this:

--- ./numpy/lib/function_base.py        2006-09-26 10:05:27.000000000 +0200
+++ /usr/local/lib/python2.5/site-packages/numpy/lib/function_base.py   2006-09-26 18:13:14.000000000 +0200
@@ -1046,11 +1046,10 @@
     """median(m) returns a median of m along the first dimension of m.
     """
     sorted = msort(m)
+    index = sorted.shape[0]/2
     if sorted.shape[0] % 2 == 1:
-        return sorted[int(sorted.shape[0]/2)]
+        return sorted[index]
     else:
-        sorted = msort(m)
-        index = sorted.shape[0]/2
         return (sorted[index-1]+sorted[index])/2.0

Also, and although median shouldn't have to be well defined in other values different from numbers, it can be forced to work decently for string, object and boolean arrays. Behind is a patch for this.

--- ./numpy/lib/function_base.py        2006-09-26 10:05:27.000000000 +0200
+++ /usr/local/lib/python2.5/site-packages/numpy/lib/function_base.py   2006-09-26 18:13:27.000000000 +0200
@@ -1046,11 +1046,10 @@
     """median(m) returns a median of m along the first dimension of m.
     """
     sorted = msort(m)
-    if sorted.shape[0] % 2 == 1:
-        return sorted[int(sorted.shape[0]/2)]
+    index = sorted.shape[0]/2
+    if sorted.shape[0] % 2 == 1 or sorted.dtype.kind in ["S", "O", "b"]:
+        return sorted[index]
     else:
-        sorted = msort(m)
-        index = sorted.shape[0]/2
         return (sorted[index-1]+sorted[index])/2.0

Feel free to apply whatever patch you consider more appropriate.