Transcript Scheme Forms - The Bioinformatics Research Group

CS 480/680 – Comparative Languages
Forms
Writing your own procedures
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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2
Defining Functions
 You can define a symbol to be a function for reuse later.
• This is one difference between a symbol and an
ordinary variable in other languages
• (define add2
(lambda (x) (+ x 2)))
(add2 3) » 5
Scheme Forms
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  #<procedure:add2>
(define a2 add2)
(a2 3) » 5
a2  #<procedure:add2>
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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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5
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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6
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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7
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)
Each branch has an implicit
begin.
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 (> p 70)
'safe
'unsafe)
» safe
(if (< p 90)
'low-pressure)
» low-pressure
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9
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->string term)]
[(symbol? term)
(symbol->string term)]
[(null? term)
(“empty”)]
[else “unknown”]
)
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[]’s work just like ()’s in
Scheme. They are used
here to make the code
easier to read.
10
Exercises
 Write a function that expects three numerical
arguments and returns the average
 Write a function that expects a list of arguments
(any length > 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)
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