Identify edge and corner element numbers

Identify the elements lying along edges and corners of a 2D (n x m) matrix

With reference to the screenshot, the code extracts the element numbers of all points lying on:
1. elements lying between P and B, into the array "top"
2. elements lying between Q and D, into the array "right"
3. elements lying between R and F, into the array "bottom"
4. elements lying between S and H, into the array "left"
corners = [topleftelement toprightelement bottomrightelement bottomleftelement];