Input and Output in SchemeInput and Output in Scheme

CS 480/680 – Comparative LanguagesCS 480/680 – Comparative Languages

Input/Output 2

Basic I/OBasic I/O (display arg) writes arg to standard out The second (optional) argument to display is

the output-port to write to.• You get an output port using open-output-file

• You can assign it to a symbol:• (define i (open-output-file ”myfile”))(display ”Hello world” i)

Likewise, open-input-file opens a file for reading and returns an input port.

Input/Output 3

ReadingReading These functions return a string:

• (read-char port) – one character• (read-line port) – one line• (read port) – one S-expression

When at EOF, the read functions return an eof-object, which you can test with(eof-object? arg)

Write first-line.scheme

Input/Output 4

WritingWriting These functions write to a port:

• (write-char char port) – one character• (write expr port) – one S-expression

expr can be a string, a list, or any other S-expression

• (display expr port) – like write Write puts “” around strings, #\ in front of characters, etc. Display does not

• (newline port) – writes a #\newline to the port

Create output.scheme

Input/Output 5

Automatic Opening and ClosingAutomatic Opening and Closing Just like Ruby allows you to pass a block to

open(), Scheme has a mechanism to open a file, run a subform, and automatically close the file at the end:

(call-with-input-file "hello.txt" (lambda (i) (let* ((a (read-char i)) (b (read-char i)) (c (read-char i))) (list a b c))))

» (#\h #\e #\l) See read-print.schemeSee read-print.scheme

Input/Output 6

String PortsString Ports You can read/write from a string as though it

was a file:(define i (open-input-string "hello world"))

(read-char i)» #\h

(read i)» ello

(read i)» world

Input/Output 7

Output String PortsOutput String Ports The procedure open-output-string

creates an output port that will eventually be used to create a string: (define o (open-output-string))(write 'hello o)(write-char #\, o)(display " " o)(display "world" o)

The function get-output-string gets the accumulated string associated with the port:(get-output-string o)» "hello, world"

Input/Output 8

Loading FilesLoading Files (load ”filename”) – read each S-

expression in a file and evaluate it as a form. Files can load other files (load-relative ”filename”)

• Path is relative to the location of the current file

Input/Output 9

Eval and QuoteEval and Quote The special form (eval list) evaluates a

list as code:• (define a (+ 3 5))a » 8

Quote prevents a list from being evaluated• (define a ‘(+ 3 5))a » (+ 3 5)(eval a) » 8

Input/Output 10

Eval-stringEval-string To evaluate a string, you need to use (eval-string string)

Requires the “string.ss” library:• (require (lib “string.ss”))

See eval-string.scheme

Input/Output 11

ExercisesExercises Write a program that reads in a file of S-

expressions containing fully parenthesized mathematical expressions in prefix notation. Your program should print the result of each expression.

Input File

(+ 3 7)

(* (+ 3 7) 5)

(+ (* 2 5) (+ 2 3))

Standard Out

10

50

15