1. Forms Writing your own procedures CS 480/680 – Comparative Languages

2. Scheme Forms2 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

3. Scheme Forms3 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.

4. Scheme Forms4 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  # (define a2 add2) (a2 3) » 5 a2  #

5. Scheme Forms5 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…

6. Scheme Forms6 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.

7. Scheme Forms7 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

8. Scheme Forms8 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

9. Scheme Forms9 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 (> p 70) 'safe 'unsafe) » safe (if (< p 90) 'low-pressure) » low-pressure Each branch has an implicit begin.

10. Scheme Forms10 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”] ) []’s work just like ()’s in Scheme. They are used here to make the code easier to read.

11. Scheme Forms11 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)