Lisp - loops

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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.

Loops

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 −

Example

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

main.lisp

; define a function accepting a flag
(defun demo-function (flag)
   ; print the block name
   (print 'entering-outer-block)
   ; start a block
   (block outer-block
      ; print the block name   
      (print 'entering-inner-block)
      (print (block inner-block
         ; if flag is true        
         (if flag
            ; return 3		 
            (return-from outer-block 3)
            ; return 5			
            (return-from inner-block 5)
         )
         (print 'This-wil--not-be-printed))
      )
      ; print block name 
      (print 'left-inner-block)
      ; print block name	  
      (print 'leaving-outer-block)
   t)
)
; call the function
(demo-function t)
; terminate printing
(terpri)
; call the function
(demo-function nil)

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

ENTERING-OUTER-BLOCK 
ENTERING-INNER-BLOCK 

ENTERING-OUTER-BLOCK 
ENTERING-INNER-BLOCK 
5 
LEFT-INNER-BLOCK 
LEAVING-OUTER-BLOCK
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