CSC151 2007S, Class 23: Recursion with Files

Admin:
* EC for tomorrow's Thursday extra (at noon) and convocation (at 11 a.m.).
* EC G-Tones 7:30 Friday, Bucksbaum Rotunda
* EC for Jason Drwal's Meditation Study - Contact him
  * Meditating itself gives you benefit!
* Are there any questions on HW8 or HW9?

Overview:
* Remaining I/O Topic: Repetitive Prompting.
* About Files.
* Key File Operations.
* File Recursion.
* Lab.

/Context/a

(display "Please enter a value and I will square it: ")
(define val (read))
(define val-squared (* val val))
(display (string-append "The value of "
                        (number->string val)
                        " squared is "
                        (number->string val-squared)))
(newline)

You've cleaned it up.

Exercise 5: Error Checking

a. What happens in your square program if someone enters something other than a number?

b. Update your program so that it prints a friendly error message (using display) and then asks again if someone enters something other than a number.

(letrec ((kernel 
          (lambda ()
            (display "Please enter a number and I'll square it: ")
            (let ((num (read)))
              (if (number? num)
                  (let ((result (* num num)))
                    ; (display "I bet you didn't know that ")
                    ; (display "Emily thinks that ")
                    (display num)
                    (display " squared is ")
                    (display result)
                    (display ".")
                    (newline))
                  (begin
                    (display "Sam thinks that you are a bit clueless today, given that ")
                    (display "you claimed that ")
                    (display num)
                    (display " is a number.")
                    (newline)
                    (kernel)))))))
  (kernel))

/On to Files/

* Idea: Store values for future use
* Programmers need to figure out how to manipulate these thingsa

* Scheme provides mechanisms for doing so
  * But makes them a bit complex

* You don't deal directly with files; rather, you deal with ports
  * We convert files to ports, which give a partial view of the file
  * Use input ports for input
  * Use output ports for output
* Why ports?
  * Multiple input from the same file
  * Models other things, too, such as Internet connections

* Conversion
  (open-input-file "filename")
  (open-output-file "filename")
* Input operations
  (read INPUT-PORT)
  (read-char INPUT-PORT)
* Output operations
  (display value OUTPUT-PORT)
  (write value OUTPUT-PORT)
  (newline OUTPUT-PORT)

(read) reads the next Scheme value:
* If the next thing in the file starts with a character,
  read a symbol
* Else If the next thing in the file startws with a digit,
  read a number
* Else If the next thing in the file is an open paren
  read a list
* Else If the next thing in the file is "
  read a string
...

Why close your input ports and output ports?
* Because our system limits how many open ports you can have
  * And although it's about 1000, you can run out