Returning values from functions You can return a value from a function by using the built- in function : ( return-from Function_name value) For example: (defun plus (x y) (setq answer (+ x y)) (return-from plus answer) ) (setq result (plus 10 20))

Block You can also group expressions together in a structure called block. For example: (block justablock (format t “ is ~D” ( )) ) A block is used to demarcate a group of expressions that achieves a certain result. You can return values from blocks just as you do with functions.

nil block The blocks in the previous slides are called named blocks. You can also have blocks which are labeled nil. For example, loops are default labeled nil. Here is another example of a nil block. (block nil (print “This is a nil block”) (return “End of block”) ) To return a value from a nil block, use (return value)

Funcall An example of this would be: (funcall #’+ 1 99) Notice that #’ is used to denote a function. In this case + is the required function. Funcall calls its first argument on its remaining arguments. This is an example of a function that takes another function as one of its arguments.

Apply An example of this function is as follows: (apply #’ ‘(5 6)) The above is similar to ( ) This is similar to funcall except that its final argument should be a list. The elements of that list are treated as additional arguments to funcall.

When to use these? These two functions are very useful when their first argument is a variable. This means you may have more than one function that is applicable on a set of arguments. For example, to find the shortest distance in a network, you may have several possible algorithms to perform the search. These algorithms are written as functions. Hence, you can use apply or funcall to specify the desired function (algorithm) to perform the search.

Mapcar This is another interesting function that takes a function of one argument and a list. It works by –applying the function to each element of the list, and –collects the results in a list and return them. For example, (mapcar #’sqrt ‘( )) (mapcar #’first ‘((a b) (c d) (e)))

Lambda If you want to create a temporary function and do not want to name it, then use lambda. For example: #’(lambda (x) (* 8 x)) To call the above defined lambda function, do as follows: (funcall * 5)