CSC195, Class 16: Strings

Overview
* Representing strings
* Key string procedures
* Lab

Notes:
* I revised the structs lab to reflect questions and discussion
* Cool talk on Fermat's little theorem today at 4:30.  Why go out
  in the snow when you can hear about Math?  Extra credit for 
  going.
* Cool concert next Friday at Bob's.  Extra credit for going.
  http://www.cs.grinnell.edu/~kuper/
* Read Chapters 5 for next week
* Question from last lab.
typedef union lemur { 
  int i; 
  struct { char b1,b2,b3,b4; } bytes; 
} lemur;

(define exponent 
  (lambda (base power)
    (cond
      ((zero power) 1)
      ((even? power) (square (exponent base (/ power 2))))
      (else (* base (exponent base (- power 1)))))))

O(log_2(n))

* Question from last lab.
typedef union lemur { 
  int i; 
  struct { char b1,b2,b3,b4; } bytes; 
} lemur;

What does the code above mean? 
* Create a data structure that holds either (1) an integer
or (2) a structure.  The substructure holds four characters
and is named bytes.

Why?
* An easy way to process individual bytes in an integer.

Why is it a bad idea?
* Sometimes works
* Sometimes doesn't
* I.e. NOT PORTABLE

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Questions about strings?

A string is a NULL-TERMINATED array of characters

Libraries of string operations:
* strcpy(char *destination, char *source); 
  copies to destination from source
* strcpy(char *destination, char *source, int n);
  copies first n characters of source to destination
* strlen(char *str)
  Find the length of a string
* strcat(char *destination, char *source);
  Shoves source at the end of destination

What if source is longer than destination in strcpy?
* Explore it experimentally!
* It's up to you as programmer to decide whether it's safe.

------------------------------------------------------------

Reflection

* Write a simple program in which the two printf statements
are different

#include <stdio.h>
main()
{
  char antelope[] = "%d";
  printf("Bare: ");
  printf(antelope);
  printf("\n");
  printf("Alternative: ");
  printf("%s", antelope);
  exit(0);
} /* main() */

* Moral: The first argument to printf is *always* a pattern,
  even if it's a variable.  
  So: You can build patterns on the fly [EB].

* Why is it okay to write
  char antelope[10] = "antelope";
but not
  char antelope[8] = "antelope";

* Observation: If you initialize the string when you create it,
  you need not specify a size (but you may)

  char antelope[] = "antelope";

* For those who get to the last question.  The Name Game is a
  song popularized by Shirley Ellis.  For "Larry" it goes

    Larry Larry Bo Barry 
    Banana Fanna Fo Farry
    Fe Fi Mo Marry, Larry