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```
void test1(Mat& img, Mat& out)
{
  int element_nums = img.rows * img.cols;
  float *ptr_img = img.ptr<float>(0);
  float *ptr_out = out.ptr<float>(0);
  for (int i = 0; i < element_nums; ++i){
    ptr_out[i] = std::exp(ptr_img[i]);
    }
};

void test2(Mat& img, Mat& out)
{
  int element_nums = img.rows * img.cols;
  float *ptr_img = img.ptr<float>(0);
  float *ptr_out = out.ptr<float>(0);
  for (int i = 0; i < element_nums; ++i){
    ptr_out[i] = ptr_img[i];
    }
};

void test3(Mat& img, Mat& out){
  int element_nums = img.rows * img.cols;
  float *ptr_img = img.ptr<float>(0);
  float *ptr_out = out.ptr<float>(0);
  float* temp = new float[img.rows * img.cols];
  for (int i = 0; i < element_nums; ++i){
    temp[i] = std::exp(ptr_img[i]*(-i));
  }
};

int main(int argc, char* argv[])
{
    clock_t start, stop;
    Mat testInput = Mat::ones(4000,4000, CV_32FC1);
  Mat out = Mat(testInput.size(), CV_32FC1);
    start = clock();
    test1(testInput, out);
    stop = clock();
    cout<<"test1:  "<<(double)(stop - start)/CLOCKS_PER_SEC*1000<<"ms"<<endl;

  start = clock();
    test2(testInput, out);
    stop = clock();
    cout<<"test2:  "<<(double)(stop - start)/CLOCKS_PER_SEC*1000<<"ms"<<endl;

  start = clock();
    test3(testInput, out);
    stop = clock();
    cout<<"test3:  "<<(double)(stop - start)/CLOCKS_PER_SEC*1000<<"ms"<<endl;
}
test1:  221.924ms
test2:  17.146ms
test3:  0.001ms
open4.1  mac 1.4 GHz Intel Core i5

why is editing mat so low?