From 0d31b919ace5b9a1d9e169f77793616bbb749586 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bart=20van=20Merri=C3=ABnboer?=
 <bart.vanmerrienboer@gmail.com>
Date: Fri, 27 May 2016 16:26:55 -0400
Subject: [PATCH] Fix zero gradient for subtensor assignment.

The variable that is being assigned has its gradient correctly
calculated (g[k]) but later on when the gradient of the variable being
assigned to is calculated g[k] is set to 0. This gives the correct
gradient for the variable being assigned to, but because it shares the
same storage it actually overrides the earlier gradient incorrectly to
zero. This fixes that.
---
 src/gradfuns.lua | 7 ++++++-
 test/test.lua    | 5 +++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/gradfuns.lua b/src/gradfuns.lua
index b9c7b3b..0f20da6 100644
--- a/src/gradfuns.lua
+++ b/src/gradfuns.lua
@@ -152,7 +152,12 @@ functions.set = {
       return nil
    end,
    function(g, ans, x, k, v)
-      return g[k]
+      local gk = getValue(g[k])
+      if type(gk) == 'number' then
+        return gk
+      else
+        return torch.clone(gk)
+      end
    end,
 }
 
diff --git a/test/test.lua b/test/test.lua
index 54b31c8..1d807db 100644
--- a/test/test.lua
+++ b/test/test.lua
@@ -1687,6 +1687,11 @@ local tests = {
          return torch.sum(xc)
       end
       tester:assert(gradcheck(f4,{x=torch.randn(10,10),y=torch.randn(3)}), "Incorrect gradient")
+      local f5 = function(params)
+         params.x[2] = params.y*2.0
+         return torch.sum(params.x)
+      end
+      tester:assert(gradcheck(f5,{x=torch.randn(10,10),y=torch.randn(10)}), "Incorrect gradient")
    end,
 
    ScalarSigmoid = function()