CSC151 2007S, Class 36: Characters and Strings

Admin:
* EC: Talk Thursday on video editing with Scheme at 4:30 3821
* Questions on the exam?
  * Sam does respond to email intermittently
* Reading for Thursday: Files

Overview:
* Review: Primitive Types
* Representing text.
* Characters: The basic building blocks.
* Combining characters into strings.

=Theme of Course: To Write Algorithms, We Need Basic Building Blocks=

* Types: Kinds of values we can compute with
* Procedures: Operations on those types

* Examples
  * Numbers
  * Colors
  * Strings (today's lesson, plus the simple strings we've learned in the past)
  * Lists
  * Booleans
  * Spots (User defined, rather than primitive)
  * Ums
  * Images
  * Procedures (built with lambda)

=Today: Another Common Pair of Types=

* Much of modern computing requires text processing

* Three basic types
  * Strings: Collections of letters (and other things you can type)
  * Characters: The things that make up strings
  * Files: How we store strings for reuse later

In writing algorithms that use a type, we need to know
* The general group of values that are included in the type
* How to represent the basic values in the type
  * How do I say "the letter a" or "the string a"
  * Different languages make different decisions
* What operations can be used on values in the type

Scheme: Basic Character Issues
* Use national/international standards to say what a character is
  * Traditionally: ASCII
    * Defines 128 characters
      * What you can traditionally type on a US Keyboard
      * Some hidden control characters useful for computers
  * These days: Unicode
    * Characters used in every natural language
    * Including Klingon
* ASCII is a subset of Unicode
  
How to represent characters:
* Mostly #\ followed by the character
  "The letter a" #\a
  "The chracter 1" #\1
* Sometimes #\ followed by a name
  #\space

Scheme: Strings are collections of characters
* Surround by quotation marks, don't worry about #\

=Notes from the Lab=

* Even those (integer->char 1) gives #\x, that is not "the letter x".  Rather,
  it's some character that can't be represeneted, and our implementation
  of Scheme represents all such characters as #\x.

       000   0     00    NUL ’\0’                    100   64    40    @
       001   1     01    SOH (start of heading)      101   65    41    A
       002   2     02    STX (start of text)         102   66    42    B
       003   3     03    ETX (end of text)           103   67    43    C
       004   4     04    EOT (end of transmission)   104   68    44    D
       005   5     05    ENQ (enquiry)               105   69    45    E
       006   6     06    ACK (acknowledge)           106   70    46    F
       007   7     07    BEL ’\a’ (bell)             107   71    47    G