LISP Tutorial on 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 −

(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)

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