10.4 Iteration

Iteration means executing part of a program repetitively. For example, you might want to repeat some computation once for each element of a list, or once for each integer from 0 to n. You can do this in Emacs Lisp with the special form while:

Special Form: while condition forms…

while first evaluates condition. If the result is non-nil, it evaluates forms in textual order. Then it reevaluates condition, and if the result is non-nil, it evaluates forms again. This process repeats until condition evaluates to nil.

There is no limit on the number of iterations that may occur. The loop will continue until either condition evaluates to nil or until an error or throw jumps out of it (see Nonlocal Exits).

The value of a while form is always nil.

(setq num 0)
     ⇒ 0

(while (< num 4)
  (princ (format "Iteration %d." num))
  (setq num (1+ num)))
     ⊣ Iteration 0.
     ⊣ Iteration 1.
     ⊣ Iteration 2.
     ⊣ Iteration 3.
     ⇒ nil

To write a repeat-until loop, which will execute something on each iteration and then do the end-test, put the body followed by the end-test in a progn as the first argument of while, as shown here:

(while (progn
         (forward-line 1)
         (not (looking-at "^$"))))

This moves forward one line and continues moving by lines until it reaches an empty line. It is peculiar in that the while has no body, just the end test (which also does the real work of moving point).

The dolist and dotimes macros provide convenient ways to write two common kinds of loops.

Macro: dolist (var list [result]) body…

This construct executes body once for each element of list, binding the variable var locally to hold the current element. Then it returns the value of evaluating result, or nil if result is omitted. For example, here is how you could use dolist to define the reverse function:

(defun reverse (list)
  (let (value)
    (dolist (elt list value)
      (setq value (cons elt value)))))

Macro: dotimes (var count [result]) body…

This construct executes body once for each integer from 0 (inclusive) to count (exclusive), binding the variable var to the integer for the current iteration. Then it returns the value of evaluating result, or nil if result is omitted. Here is an example of using dotimes to do something 100 times:

(dotimes (i 100)
  (insert "I will not obey absurd orders\n"))