Emacs Lisp: Loop

While Loop

Most basic loop in elisp is with while.

(while test body)

, where body is one or more lisp expressions.

Tip: There is no for-loop.

(let ((x 0))
  (while (< x 4)
    (print (format "number is %d" x))
    (setq x (1+ x))))

Example: While Loop Over a List

One common form to loop thru a list is using the while function. In each iteration, pop is used to reduce the list.

(let ((xx '(a b c)))
  (while xx
    (message "%s" (pop xx))
    (sleep-for 1)))

Example: While Loop Over a Vector

;; iterate over a vector with while-loop

(let (xx ii)
  (setq xx [0 1 2 3 4 5])
  (setq ii 0)

  (while (< ii (length xx))
    (insert (format "%d" (aref xx ii)))
    (setq ii (1+ ii))))

; inserts 012345

dotimes: Loop Fixed Number of Times with Index

dotimes

• (dotimes (VAR COUNT) BODY)
• (dotimes (VAR COUNT RESULT) BODY)

• Loop a fixed number of times.
• Evaluate BODY with VAR bound to successive integers running from 0, inclusive, to COUNT, exclusive.
• Return nil or RESULT
• RESULT means, evaluate RESULT after loop to get the return value.

Tip: dotimes is useful when you need to go ever a list and also get the current element's index.

(dotimes (i 4)
  (insert (number-to-string i)))
;; inserts 0123, return nil

(let ((xx [3 4 5]))
  (dotimes (i (length xx))
    (insert
     (number-to-string
      (elt xx i)))))
;; inserts 345

;; example. dotimes over a vector, with conditional exit

(let ((xx [10 20 30 40 50]))
      (catch 'TAG743
        (dotimes (i 99)
          (message "i %s %s" i (aref xx i))
          (if (eq (aref xx i) 30)
              (throw 'TAG743 "got it")
            nil))))

;; prints
;; i 0 10
;; i 1 20
;; i 2 30

;; return
;; "got it"