Forms Writing your own procedures

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FormsWriting your own procedures
CS 480/680 Comparative Languages
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Calling Functions

• Recall that a scheme form is a list(function
  arg1 arg2 arg3 )( 2 5 3) (number? 3) etc.
• You can write your own function using the special
  form lambda
• (lambda (x) ( x 2)) is a function, just like
• ((lambda (x) ( x 2)) 5) 7

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Defining Functions

• You can define a symbol to be a function for
  re-use later.
• This is one difference between a symbol and an
  ordinary variable in other languages
• (define add2
• (lambda (x) ( x 2)))
• (add2 3) 5

The parameter x acts like a local variable within
the function definition.
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First Class Variables

• In Scheme, procedures are first class variables
• You can assign to them, pass them as parameters,
  and otherwise use them just like any other
  variable

(add2 3) 5 add2 ? ltprocedureadd2gt (define a2
add2) (a2 3) 5 a2 ? ltprocedureadd2gt
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Parameters

• Procedures can have multiple parameters
• (define area
• (lambda (length breadth)
• ( length breadth)))
• (area 3 6) 18
• (area 3) ? procedure area expects 2 arguments,
  given 1 3
• There are several ways to create procedures that
  expect a variable number of arguments

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Variable numbers of arguments

• Three ways to specify parameters
• List of parameters (lambda (a b) )
• Function expects exactly two arguments
• Single symbol (lambda a )
• All parameters are collected into a list and
  assigned to a
• Dotted pair (lambda (a b c . d) )
• Must have at least three arguments
• First three are assigned to a, b, and c
• The remaining arguments are collected as a list
  and assigned to d

See args.scheme.
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Sequencing

• Generally, a function definition calls a single
  function and returns a single value
• Another way to say this is that the definition
  part of lambda accepts a single form
• The arguments to the function might be other
  functions
• Begin groups together a set of subforms to be
  executed in sequence. The return value is that
  of the last subform.
• Lambda actually includes an implicit begin

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Parameters and values

• In Scheme, parameters are passed by value

(define add2 (lambda (someval) (begin (set!
someval ( 2 someval)) (display someval)
(newline)) ) ) (define a 5) (add2 a) ? 7 a ? 5
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Conditionals
(if test-expression then-branch
else-branch) If test-expression evaluates to
true (ie, any value other than f), the then
branch is evaluated. If not, the else branch is
evaluated. The else branch is
optional. (define p 80) (if (gt p 70)
'safe 'unsafe) safe (if (lt p 90)
'low-pressure) low-pressure
Each branch has an implicit begin.
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Flexible conditionals

• The cond form can have as many tests as needed
• The first one that evaluates to something other
  than f is evaluated

(cond (number? term) (number-gtstring
term) (symbol? term) (symbol-gtstring
term) (null? term) (empty) else
unknown )
s work just like ()s in Scheme. They are
used here to make the code easier to read.
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Exercises

• Write a function that expects three numerical
  arguments and returns the average
• Write a function that expects a list of arguments
  (any length gt 3) and returns the third item in
  the list
• Write a function that expects at least three
  arguments. The function should print the first
  two arguments to stdout, and return the third
  argument (all the rest should be thrown away)