CS/CE 218 Lecture -*- Outline -*-

connection: still on details of C, now we'll look at yet another ADT
the program counter!  The operations on the program counter are better
known as statements...

* control flow in C

** statement syntax (section 3.1)
	Q: How are semi-colons used differently in C than in Pascal?
	semi-colon is a terminator, not a separator as in Pascal

*** blocks (section 3.1)
	Q: What's the equivalent of Pascal's begin-end in C?

	{ } takes role of begin-end
		but no semi-colon after }
	so
		begin
		  s1;
		  s2
		end;
		s3
	becomes
		{
		  s1;
		  s2;
		}
		s3

	Another difference from Pascal:
		blocks can be used to localize declarations
		-------------
		{
		  double z;
		  z = f(x,23.0 + 9.3*y);
		  x = z - 42.56;
		}
		{
		  int z;
		  for (z = 0; z < MAX; z++) {
			/* ... */
		  }
	        }
		-------------		

** conditionals
*** if-else, else-if (sections 3.2 and 3.3)
	Q: Is the else part of an if statement necessary?
	Q: What does the condition of an if statement test?
		if the expression is not 0
	Q: When should you use brackets for the body of an if?
		I use them all the time;
			might need to add more statements
			makes syntax I have to remember simpler
			avoids ambiguity
		But I violate this rule if the whole if goes on one line.

*** switch (section 3.4)
	like Pascal's case statement in many respects, but different in others.
	Q: What is a constant expression?
	---------------
	enum {even=0, odd=1};

	switch (n % 2) {
	case even :
	  n = n / 2;
	  break;
	case odd :
	  n = (3*n + 1) / 2;
	  break;
	}
	---------------

	Q: What is break used for in a switch statement?

	example: translation of phone numbers in letters
	----------------
	switch (c) {
	case 'a': case 'b': case 'c' :
		putchar('1');
		break;
	/*...*/
	case 'w': case 'x': case 'y' :
		putchar('9');
		break;
	case 'q': case 'z':
		fprintf(stderr, "%c is illegal\n", c);
		exit(1);
		break;
	default:
		putchar(c);
		break;
	}
	----------------
	Q: Is the break needed after the last case?
		no, but it's a good idea

	It's terribly tricky to fall through after executing some code...
		should be loudly commented

** loops

*** while and for loops (section 3.5)
	Q: The semantics of a for loop are explained on p. 60 using
		a translation to a while loop.  Is there any difference
		between the translation and the original?
		Yes, behavior of continue
			continue in a for goes to the increment step,
			not to the loop head
	Can leave out parts of the for
		for (; i<20; i++) stmt
		for (;;) stmt is an infinite loop
	Q: What initialization should go in the for loop?
		only those related directly to the loop itself.

	    bad example:
		for (x=20, y=30, i=0; i<NUM; printf("%d\n", i), i++) ;
	
	Q: How would your write an infinite loop using while?

	develop the following, then show slide
------------------
#include <ctype.h>

int atoi(const char s[])
     /* ensures result is the number represented by s */
{
  int i, n, sign;
 
  for (i = 0; isspace(s[i]); i++) {
    ;
  }
  sign = (s[i] == '-') ? -1 : 1;
  if (s[i] == '+' || s[i] == '-') {
    i++;
  }
  for (n = 0; isdigit(s[i]); i++) {
    n = 10 * n + (s[i] - '0');
  }
  return sign * n;
}
------------------
	note: leading white space allowed,
	      also s itself is left alone

*** do-while (section 3.6)
	Q: How is do-while like repeat-until in Pascal?  How is it different?
		similar to repeat-until in Pascal,
		but sense is different
----------------
int pathlen(int n)
     /* requires: n is odd and n>1 */
     /* ensures: iterate(T, result, n)==1 */
{
  int count = 0;

  do {
    n = T(n);
    count++;
  } while (n > 1);
  /* n == 1 */
  return count;
}
----------------
	Q: Can you do one or more of exercises 3-4, 3-5, and 3-6 on page 64?

*** break and continue (section 3-7)
	Q: Is there any difference between break and continue in C and in
		the Bourne shell?
		not much: break also works on switch and do
	break exits the smallest enclosing loop
	----------------
	for (i = 0; i < MAX_X; i++) {
	  for (j = 0; j < MAX_Y; j++) {
		/* ... */
		break;
		/* ... */
	  }
	  some_stuff(i);
	}
	----------------
	Q: What does a continue inside switch mean?
		next iteration of smallest enclosing while or for.

** comma operator (pages 62-63)
	The comma operator allows an expression to be broken up into parts
		and still have a value
	-------------------
	for (i=0, j=0; i<MAX_X; (j < MAX_Y-1) ? j++ : (i++, j=0))
	  a[i][j] = b[i][j];
	-------------------
	Q: In ``e1 , e2'', which expression is evaluated first?
		e1, then e2
	Q: What is the value of e1, e2?
		it's the value of e2
	Q: What is an equivalent way to write: x=2, x ?
		x = 2, since the value is 2
	Q: Is the comma in f(x=2,y) a comma operator?  In g((x=2,y))?
		no, yes