How to memorise the cursor position and return to it?

Question

Say I was working in a big file somewhere in the middle and all of the sudden I realise that I need to edit the top of the file. I do a M-< to go to the top and make my edits. Now I want to move back to the exact point where I was working before.

Is it possible to call a function before moving to the top, memorising the current point then going somewhere and calling the same function again but this time the cursor is moved to the point which was previously memorised and the memory is erased; ready to memorise a new point.

Edit: Here is what I ended up with,

(defconst POINT-REGISTER 0
  "Register to save the current point in.")


(defcustom saved-point nil
  "Whether a point is saved in `POINT-REGISTER`."
  :type 'boolean)

(defun save-or-goto-saved-point ()
  "Interactively memorise a point and return to it."
  (interactive)
  (if saved-point
      (progn (register-to-point POINT-REGISTER)
             (setq saved-point nil))
    (progn (point-to-register POINT-REGISTER)
           (message "[<Your Name>] Point saved!")
           (setq saved-point t))))

Answer 1

C-x r SPC (M-x point-to-register) to register the position and C-x r j (M-x register-to-point) to return to it. This last operation does not erase the contents of the register, but if you use this register for a new point, its contents will be overwritten.

Improved for more comfort

Notice that this function uses the register 0. I do not know wether this register is used or not, if so, you can change it to any unused register.

(defun my-point-excursion-toggle ()
  "Saves the position of the point or return to the previous saved   position. "
  (interactive)
  (cond ((get-register 0)
         (jump-to-register 0)
         (set-register 0 nil))
        (t(point-to-register 0)
          (message "Save point %d"(point)))
        ))

You can bind this function to any key sequence for instance:

(global-set-key (kbd "C-c t") #'my-point-excursion-toggle)

Answer 2

Yes, C-SPC calls set-mark-command, which pushes the current location of point onto a stack. If called with a prefix argument (C-u C-SPC) it pops the stack and returns point to the popped location. A number of other commands also push point onto the stack. For example, a search with C-s pushes point onto the stack so that you can return to where you started the search easily.

Answer 3

You talk about calling a function. Do you want to do what you describe in Elisp or interactively?

@db48x essentially interprets your question as asking how to get back to a previous position interactively, and that answer is fine for that.

For Elisp, however, there's typically no reason to set the mark and then use it to get back to it - and that's generally avoided.

Instead, just bind a local variable to the original position, then goto-char to that position later. Or use copy-marker for the original position, and then return to that marker position later.

Please clarify your question, to say whether you want to go to a previous position by program or interactively.