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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.
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
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