3. Function

• Primitive functions written in C. Main reason: portability

• Function anatomy:

  • function name
  • arguments
  • documentaion (optional)
  • an expression to make the function interactive (optional)
  • function body

(defun multiply-by-seven (number)
  "Multiply NUMBER by seven."
  (* 7 number))

(multiply-by-seven 3)
21

;; make function interactive
(defun multiply-by-seven (number)
  "Multiply NUMBER by seven."
  (interactive "p") ; p - pass the prefix argument to the function
  (message "The result is %d" (* 7 number)))

• Emacs has a dozen of interactive codes (see more)

• Install function for temprary use, C-x C-e. To use code permanently, put function in .emacs initialization file.

• Special form: let which is used to bind a symbol to a value, or to create a local variable (visible only in let expression). In Emacs Lisp, scoping is dynamic, not lexical.

• let structure:

(let ((variable value)
      (variable value)
      ...)
  body...)

(let ((zebra "stripes")
      (tiger "fierce"))
  (message "One kind of animal has %s and another is %s."
           zebra tiger))

• Uninitialized variables return nil

(let ((birch 3)
      pine
      fir
      (oak 'some))
  (message
   "Here are %d variables with %s, %s, and %s value."
   birch pine fir oak))
"Here are 3 variables with nil, nil, and some value."

• if statement

(setq my-name "anchu")
(if (equal my-name "anchu")
    (message "My name is %s" my-name))
"My name is anchu"

• if-then-else expression

(defun type-of-animal (characteristic)  ; Second version.
  "Print message in echo area depending on CHARACTERISTIC.
     If the CHARACTERISTIC is the string \"fierce\",
     then warn of a tiger; else say it is not fierce."
  (if (equal characteristic "fierce")
      (message "It is a tiger!")
    (message "It is not fierce!")))

(type-of-animal "fierce")
"It is a tiger!"

(type-of-animal "striped")
"It is not fierce!"

• nil means empty list and false

• Special form: save-excursion. It saves the location of point, executes the body of the function and then restores points to previous position. Template:

(save-excursion
  first-expression-in-body
  second-expression-in-body
  third-expression-in-body
  ...
  last-expression-in-body)

4. Basic operation

;; Arithmetic
(+ 20 30)
50

(- 100 80)
20

(+ 1 2 3 4 5)
15

(* 1 2 3 4 5)
120

(/ 100 20)
5

(> 10 1)
t

(< 2 8)
t

(< 8 2)
nil

(= 2 4)
nil

(equal 2 2)
t

(exp -1)
0.36787944117144233

(log 10)
2.302585092994046

(sin pi)
1.2246467991473532e-16

(tan 90)
-1.995200412208242

;; Comparision
(equal (list 1 2 3 4) (list 1 2 3 4))
t

(eq 'x 'x)
t

(string= "a" "a")
t

(string= "a" "A")
nil

(string-equal "z" "z")
t

(string< "a" "b")
t

(string-lessp "a" "b")
t

(equal '(1 2) (list 1 2))
t

(eq '(1 2) (list 1 2))
nil

• Type predicates

  • null: test if argument is nill
  • numberp
  • stringp
  • symbolp
  • atom: test if argument is not a list or pair
  • listp
  • consp: test if it is a pair
  • vectorp: test if it is a vector
  • bufferp: is arg a buffer object
  • windowp: is arg a window object
  • framep: is arg a frame object
  • processp: is arg a process

• Get object type with type-of

(type-of 1)
integer

(type-of "12")
string

(type-of '(1 2 3))
cons

(type-of '(a . "e"))
cons

(type-of (current-buffer))
buffer

(type-of 'current-buffer)
symbol