defun vs defmath

defun is the normal way to define lisp functions in emacs.

They can be useful to use with org tables if you’re writing long expressions, logical operations, string manipulation, etc.

;; define a calc function and evaluate it with `C-x C-e`
(defun firstletter(n)
    (substring n 0 1)
    )

You can now use it in a table

| abdulaziz | a |
| khalid    | k |
#+TBLFM: $2='(firstletter $1)

defmath on the other hand, is designed to work with the calculator. The good thing about them is that you can use the calculator’s functions within the function definition. There are other uses for it like the ability to use the stack as arguments to functions, but here we are interested in their use in org tables:

;; define a calc function
(defmath myfact (n)
  (if (> n 0)
      (* n (myfact (1- n)))
    1))

now you can write an algebraic expression

myfact(5) => 

# and press `C-x * e` on the previous line to get
myfact(5) => 120

or you can use it in a table

| 4 |  24 |
| 2 |   2 |
| 3 |   6 |
| 5 | 120 |
#+TBLFM: $2=myfact($1)

و سلامتكم