LISP - Loops

There may be a situation, when you need to execute a block of code numbers of times. A loop statement allows us to execute a statement or group of statements multiple times and following is the general form of a loop statement in most of the programming languages.


LISP provides the following types of constructs to handle looping requirements. Click the following links to check their detail.

Sr.No. Construct & Description
1 loop

The loop construct is the simplest form of iteration provided by LISP. In its simplest form, it allows you to execute some statement(s) repeatedly until it finds a return statement.

2 loop for

The loop for construct allows you to implement a for-loop like iteration as most common in other languages.

3 do

The do construct is also used for performing iteration using LISP. It provides a structured form of iteration.

4 dotimes

The dotimes construct allows looping for some fixed number of iterations.

5 dolist

The dolist construct allows iteration through each element of a list.

Gracefully Exiting From a Block

The block and return-from allows you to exit gracefully from any nested blocks in case of any error.

The block function allows you to create a named block with a body composed of zero or more statements. Syntax is −

(block block-name(

The return-from function takes a block name and an optional (the default is nil) return value.

The following example demonstrates this −


Create a new source code file named main.lisp and type the following code in it −

(defun demo-function (flag)
   (print 'entering-outer-block)
   (block outer-block
      (print 'entering-inner-block)
      (print (block inner-block

         (if flag
            (return-from outer-block 3)
            (return-from inner-block 5)

         (print 'This-wil--not-be-printed))

      (print 'left-inner-block)
      (print 'leaving-outer-block)
(demo-function t)
(demo-function nil)

When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result returned is −