polish code in reshape_op

paddle/operators/reshape_op.cc

@@ -36,10 +36,13 @@ class ReshapeOp : public framework::OperatorWithKernel {
     auto x_dims = ctx->GetInputDim("X");
 
     std::vector<size_t> neg_dims_idx;
+    // set some dimension to -1 if it is unknown
+    const int unknown_size = -1;
     for (size_t i = 0; i < shape.size(); ++i) {
-      PADDLE_ENFORCE(shape[i] > 0 || shape[i] == -1,
-                     "Each dimension of Attr(shape) must be positive or -1.");
-      if (shape[i] == -1) {
+      PADDLE_ENFORCE(shape[i] > 0 || shape[i] == unknown_size,
+                     "Each dimension of Attr(shape) must be positive or %d.",
+                     unknown_size);
+      if (shape[i] == unknown_size) {
         neg_dims_idx.push_back(i);
         PADDLE_ENFORCE(neg_dims_idx.size() <= 1,
                        "Only one dimension of Attr(shape) can be unknown.");
@@ -53,8 +56,7 @@ class ReshapeOp : public framework::OperatorWithKernel {
       // dim infer
       shape[neg_dims_idx[0]] = in_size / (-capacity);
       // recalculate capacity
-      capacity = std::accumulate(shape.begin(), shape.end(), 1,
-                                 std::multiplies<int>());
+      capacity = shape[neg_dims_idx[0]] * (-capacity);
     }
     // capacity check
     PADDLE_ENFORCE(capacity == in_size,
@@ -98,9 +100,9 @@ the tensor X into a 2-D tensor:
 
     [[1, 2, 3, 4]]
 
-One dimension in the target shape can be set -1, and the real dimension 
-will be infered from the original shape of Input(X) and other
-dimensions in the target shape.
+One dimension in the target shape can be set -1, representing that its
+size is unknown. In this case, the real dimension will be infered from 
+the original shape of Input(X) and other dimensions in the target shape.
 )DOC");
   }
 };