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. Cpck the following pnks to check their detail.

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.psp 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)
   t)
)
(demo-function t)
(terpri)
(demo-function nil)

ENTERING-OUTER-BLOCK 
ENTERING-INNER-BLOCK 

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