Homework / Exam
• Continuing K&R Chapter 6
• Exam 2 after next class
– Open Book / Open Notes
– Up through end of K&R 6.4 plus MAKE
1
Review structs
struct point {
int x;
int y;
} pt;
struct point pt1, pt2;
struct point maxpt = {320, 200};
2
Review structs
struct rect {
struct point pt1;
struct point pt2;
};
 Note: must have ; following }
struct rect box;
3
Review structs
box.pt1.x = 5;
box.pt1.y = 10;
box.pt2.x = 10;
box.pt2.y = 20;
area = (box.pt2.x – box.pt1.x)
* (box.pt2.y – box.pt1.y);
4
What can we do with a struct?
• Reference members
box.pt2.x = box.pt1.x + width;
• Assign as a unit
pt2 = pt1;
• Create a pointer to it
struct point *ppt1;
ppt1 = &pt1;
5
What can we do with a struct?
• Not legal to compare structs
if (pt1 == pt2) …
 INVALID
• Must be done as:
if (pt1.x == pt2.x && pt1.y == pt2.y) …
6
structs and Functions, K&R 6.2
/* ptinrect:
if point p in rect r, return 1 else return 0
note: slightly different from K&R example */
int ptinrect (struct point p, struct rect r)
{
return p.x >= r.pt1.x && p.x <= r.pt2.x
&& p.y >= r.pt1.y && p.y <= r.pt2.y;
}
7
Arrays of structs, K&R 6.3
• Multiple related arrays
char * keyword[NKEYS];
int keycount[NKEYS];
• Can be implemented as an array of structs
struct key {
char *word;
int count;
} keytab [NKEYS];
8
Arrays of structs
• Alternative array of structs implementation
struct key {
char *word;
int count;
};
struct key keytab[NKEYS];
9
Arrays of structs
• Initialization for an array of structs
struct key {
char *word;
int count;
} keytab[ ] = {
“auto”, 0,
…
“while”, 0
};
/* NKEYS is dynamically derived */
10
Size of structs
• sizeof is a compile-time unary operator
• Can be used to get integer (actually size_t):
sizeof object
sizeof (type name)
• Applied to structs
#define NKEYS (sizeof keytab / sizeof (struct key))
#define NKEYS (sizeof keytab / sizeof keytab[0])
11
Pointers to structs, K&R 6.4
• Declare and initialize a pointer to struct
struct point p;
struct point *pp = &p;
• Refer to members of struct p via pointer pp
(*pp).x and (*pp).y
 See precedence
• More commonly done as:
pp->x and pp->y
 See precedence
12
Pointers to structs
struct string {
int len;
char *cp;
} *p;
Expression
++p->len
*p->cp
*p->cp++
Same as
++(p->len)
*(p->cp)
*((p->cp)++)
Value / Effect
increments len
value is a char
value is a char
increments cp
13
Pointers to structs
/* ptinrect: (pointer version)
if point p in rect r, return 1 else return 0 */
int ptinrect (struct point *pp, struct rect *rp)
{
return pp->x >= rp->pt1.x && pp->x <= rp->pt2.x
&& pp->y >= rp->pt1.y && pp->y <= rp->pt2.y;
}
14