[MXNET-1416] Fix inception inference example for potential index out of range error.

cpp-package/example/inference/inception_inference.cpp

@@ -302,13 +302,11 @@ void Predictor::PredictImage(const std::string& image_file) {
 
   // The output is available in executor->outputs.
   auto array = executor->outputs[0].Copy(Context::cpu());
-
   /*
    * Find out the maximum accuracy and the index associated with that accuracy.
    * This is done by using the argmax operator on NDArray.
    */
   auto predicted = array.ArgmaxChannel();
-
   /*
    * Wait until all the previous write operations on the 'predicted'
    * NDArray to be complete before we read it.
@@ -317,7 +315,7 @@ void Predictor::PredictImage(const std::string& image_file) {
    */
   predicted.WaitToRead();
 
-  int best_idx = predicted.At(0, 0);
+  int best_idx = predicted.At(0);
   float best_accuracy = array.At(0, best_idx);
 
   if (output_labels.empty()) {

cpp-package/include/mxnet-cpp/ndarray.h

@@ -321,6 +321,12 @@ class NDArray {
    */
   size_t Offset(size_t c, size_t h, size_t w) const;
   /*!
+  * \brief return value of the element at (index)
+  * \param index  position
+  * \return value of one dimensions array
+  */
+  mx_float At(size_t index) const;
+  /*!
   * \brief return value of the element at (h, w)
   * \param h height position
   * \param w width position

cpp-package/include/mxnet-cpp/ndarray.hpp

@@ -375,11 +375,15 @@ inline void NDArray::Save(const std::string &file_name,
 }
 
 inline size_t NDArray::Offset(size_t h, size_t w) const {
-  return (h * GetShape()[1]) + w;
+  auto const shape = GetShape();
+  CHECK_EQ(shape.size(), 2) << "The NDArray needs to be 2 dimensional.";
+
+  return (h * shape[1]) + w;
 }
 
 inline size_t NDArray::Offset(size_t c, size_t h, size_t w) const {
   auto const shape = GetShape();
+  CHECK_EQ(shape.size(), 3) << "The NDArray needs to be 3 dimensional.";
   return h * shape[0] * shape[2] + w * shape[0] + c;
 }
 
@@ -391,6 +395,13 @@ inline mx_float NDArray::At(size_t c, size_t h, size_t w) const {
   return GetData()[Offset(c, h, w)];
 }
 
+inline mx_float NDArray::At(size_t index) const {
+  auto shape = GetShape();
+  CHECK_EQ(shape.size(), 1) << "The NDArray needs to be 1 dimensional.";
+  CHECK_LT(index, shape[0]) << "Specified index is out of range.";
+  return GetData()[index];
+}
+
 inline size_t NDArray::Size() const {
   size_t ret = 1;
   for (auto &i : GetShape()) ret *= i;