CSC195, Class 24: Functions and Pointers

Overview:
* Detour: #include and #define directives.
* Functions that return pointers.
* Pointers to functions.
  + Contrast to Scheme.
  + Contrast to Java.
* Lab.

Notes:
* An outline for the impromptu lecture is now available.
* Homework 5 ready.
* Questions about the postconditions of permute?
  + Look at it again (on the EBoard, 22)
  + Bring questions on Monday.
* Why doesn't C see future function declarations?
  + If you call a function above where you declare it,
    C may complain about inconsistencies.

main()
{
  foo(bar);
  ...
}

void foo(int whatever)

C compiler: 
Warning: "Attempt to redeclare foo.  Who taught you to program?"

The C compiler does (more-or-less) one pass through the code.
The Java compiler does more than one.
* Uses less memory.
* Uses much less processor time.
* Uses much less disk I/O.

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The C preprocessor does an initial pass quick pass through
the code to handle some very simple things quickly.

#include filename

* Shoves the contents of filename into your file
  exactly where the #include was.
  + May also include some notes to help C remember what
    went where.
* Can include declarations
* Can include code

#define NAME expression
* Uses the text of expression wherever NAME appears in
  the program.


#define NAME(V1,V2,V3) expression

A macro
Wherever NAME(exp1,exp2,exp3) appears
  replaces V1 by exp1 in expression
  replaces V2 by exp2 in expression
  replaces V3 by exp3 in expression
  replace NAME(...) by modified expression

#define SUM(x1,x2) { x1+x2 }

printf("Sum: %d\n", SUM(34,5)):

#define SQUARE(x) (x*x)

printf("Val: %d", SQUARE(3+4));
printf("Val: %d", (3+4*3+4));

printf("Val: %d", SQUARE(++x));

#ifdef NAME
  arbitrary code
#endif

If you've defined NAME (earlier or on the command line),
includes "arbitrary code".  Otherwise, doesn't.

#ifdef DEBUGGING
  #define WARNING(str) printf("%s",str)
#else
  #define WARNING(str)
#endif

Play with some of this in lab.
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Functions can return pointers.

int *allocate_an_integer(void)
{
  return (int *) malloc(sizeof(int));
} /* allocate_an_integer(void) */
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You can make pointers to functions
* C doesn't let you pass functions around.
* If you have cases in which you'd like to pass fucntions
  around, you gotta pass around pointers to functions.
* E.g., the lessThan operation in a generalized binary
  search algorith (or equals in sequential search)

How do you write "compare is a pointer to a function of
two parameters, both of which are integers and returns
an integer?"

int search(int (*compare)(int x,int y) ...)
  (*compare)(2,3)

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

int slt(char *str1, char *str2)
{
  while ((*str1) && (*str1++ == *str2++))
    ;
  return *--str1 < *--str2;
} /* slt(char*,char*) */