How to rotate the matrix by 90 degrees?

Question

Algorithms

I have a gradient plotted as a matrix of bits:

vector<vector<int>> make_gradient(int height, int width)
{
  assert(height > 0 && width > 0);

  int cf = height / 255;
  int color = 0;
  vector<vector<int>> result(height, vector<int>(width));
  for (int i = 0; i < height; i += cf)
  {
    for (int j = 0; j < cf; ++j)
    {
      fill(result[i + j].begin(), result[i + j].end(), color % 255);
    }
    color++;
  }
  stable_sort(result.begin(), result.end());
  return result;
}

int main(int argc, char *argv[])
{
ofstream file;

  file.open(argv[1]);

  if (!file)
  {
    cout << "can't open file" << endl;
    return 0;
  }

  file << "P5" << "\n";

  file << numrows << " " << numcols << "\n";

  file << 255 << "\n";

  vector<vector<int>> pixmap;

      for_each(pixmap.begin(), pixmap.end(), [&](const auto& v) {
    copy(v.begin(), v.end(), ostream_iterator<char>{file, ""});
  });

  file.close();
}

The gradient goes from top to bottom, i.e. the image is vertical. How to change the algorithm so that it builds a horizontal image (gradient from left to right)? I swapped height and width in cycles, but it didn't help.

Answer 1

If I understand correctly, the construct in the nested loop
fill(......
fills the entire line with the given value. So it should be replaced with filling the column. Probably, one more nested loop will be needed to go through all the lines and write to given cell value.You
should get something like this:
vector> make_gradient(int height, int width)
{
assert(height > 0 && width > 0);
int cf = width / 255;
int color = 0;
vector> result( height, vector(width));
for (int i = 0; i < height ; i += cf)
{
for (int j = 0; j < cf; ++j)
{
for (Int z =0, z < width, z++)
{
result[z][i+j] = color % 255:
}
}
color++;
}
stable_sort(result.begin(), result.end());
return result;
}
int main(int argc, char *argv[])
{
ofstream file;
file.open(argv[1]);
if (!file)
{
cout << "can't open file" << endl;
return 0;
}
file << "P5" << "\n";
file << numrows << " " << numcols << "\n";
file << 255 << "\n";
vector> pixmap;
for_each(pixmap.begin(), pixmap.end(), [&](const auto&
v) { copy(v.begin(), v.end(), ostream_iterator{file, ""});
});
file.close();
}
I hope I didn't make a mistake with the coordinates anywhere :)