Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Pointers Page 76 October 6, 1997 Pointers • Pointers are variables whose values are the addresses of other variables • Basic operations“address of” (reference)“indirection” (dereference) • Suppose x and y are integers, p is a pointer to an integer: p = &x; p gets the address of xy = *p; y gets the value pointed to by py = *(&x); • Declaration syntax mimics use of variables in expressions int *p; *p is an int , so p is a pointer to an int • Unary * and & bind more tightly than most other operators y = *p + 1; y = (*p) + 1;y = *p++; y = *(p++); Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Pointer References Page 77 October 6, 1997 Pointer References • Pointer references (e.g. *p ) are variables int x, y, *px, *py;px = &x; px is the address of x*px = 0; sets x to 0 py = px; py also points to x*py += 1; increments x to 1 y = (*px)++; sets y to 1, x to 2 • Passing pointers to functions simulates passing arguments “by reference” void swap(int x, int y) {int t;t = x;x = y;y = t;}int a = 1, b = 2;swap(a, b);printf("%d %d\n", a, b); Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Pointers & Arrays Page 78 October 6, 1997 Pointers & Arrays • Pointers can “walk along” arrays int a[10], i, *p, x;p = &a[0]; p is the address of the 1st element of ax = *p; x gets a[0]x = *(p + 1) ; x gets a[1]p = p + 1; p points to a[1] , by definition p++; p points to a[2] • Array names are constant pointers p = a; p points to a[0]a++; illegal; can’t change a constant p++; legal; p is a variable • Subscripting, for any type, is defined in terms of pointers a[i] *(a + i) i[a] is legal, too! &a[i] a + ip = &a[0] &*(a + 0) &*a a • Pointers can walk along arrays efficiently p = a;for (i = 0; i < 10; i++)printf("%d\n", *p++); Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Pointer Arithmetic Page 79 October 6, 1997 Pointer Arithmetic • Pointer arithmetic takes into account the stride (size of) the value pointed to T *p;p += i increment p by i elements p -= i decrement p by i elements p++ increment p by 1 element p-- decrement p by 1 element • If p and q are pointers to the same type T p - q number of elements between p and q • Does it make sense to add two pointers? • Other operations: p < q ; <= == != >= >p and q must point to the same array; no runtime checks to insure this • Example int strlen(char *s) {char *p;for (p = s; *p; p++);return p - s;}Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Pointers & Array Parameters Page 80 October 6, 1997 Pointers & Array Parameters • Array parameters: array formal parameters are not constants, they are variables passing an array passes a pointer to the first element arrays (and only arrays) are automatically passed “by reference” void f( T a[]) {...} is equivalent to void f( T *a) {...} • String constants denote constant pointers to the actual characters char *msg = "now is the time"; char amsg[] = "now is the time";char *msg = amsg; msg points to the first character of "now is ..." • Strings can be used wherever arrays of characters are used putchar("0123456789"[i]); static char digits[] = "0123456789";putchar(digits[i]); • Is there any difference between extern char x[]; extern char *x; Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Pointers & Array Parameters, cont’d Page 81 October 6, 1997 Pointers & Array Parameters, cont’d • Copying strings: void scopy(char *s, char *t) copies t to s • Array version: void scopy(char s[], char t[]) {int i = 0;while ((s[i] = t[i]) != '\0')i++;} • Pointer version: void scopy(char *s, char *t) {while (*s = *t) { while ((*s = *t) != 0)s++;t++;}} • Idiomatic version: void scopy(char *s, char *t) {while (*s++ = *t++) while ((*s++ = *t++) != 0);} • Which one is better and why? Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Arrays of Pointers Page 82 October 6, 1997 Arrays of Pointers • Arrays of pointers help build tabular structures • Indirection ( * ) has lower precedence than []char *line[100]; same as char *(line[100]); declares an array of pointers to char (strings); declaration mimics use: *line[i] refers to the 0th character in the i th string • Arrays of pointers can be initialized char *month(int n) { static char *name[] = {"January","February",...,"December"}; assert (n >= 1 && n <= 12);return name[n-1];}int a, b;int *x[] = { &a, &b, &b, &a, NULL }; Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Arrays of Pointers, cont’d Page 83 October 6, 1997 Arrays of Pointers, cont’d • Arrays of pointers are similar to multi-dimensional arrays, but different int a[10][10]; both a[i][j]int *b[10]; b[i][j] are legal references to int s • Array a : 2-dimensional 10x10 arraystorage for 100 elements allocated at compile time a[6] is a constant ; a[i] cannot change during executioneach row of a has 10 elements • Array b : an array of 10 pointers; each element could point to an arraystorage for 10 pointer elements allocated at compile timevalues of these pointers must be initialized during execution b[6] is a variable ; b[i] can change during executioneach row of b can have a different length; “ragged array”Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: Command-Line Arguments Page 84 October 6, 1997 Command-Line Arguments • By convention, main is called with 2 arguments (actually 3!) int main(int argc, char *argv[])argc (“ arg ument c ount”) is the number of command-line arguments argv (“ arg ument v ector”) is an array of pointers to the arguments • For the command echo hello, world argc = 3argv[0] = "echo"argv[1] = "hello,"argv[2] = "world"argv[3] = NULL • NULL is the null pointer , which points to nothing; defined to be 0 • Implementation of echo : int main(int argc, char *argv[]) {int i;for(i = 1; i < argc; i++)printf("%s%c", argv[i], (i < argc-1) ? ' ' : '\n');return 0;} Copyright 1995 D. Hanson, K. Li & J.P. Singh Computer Science 217: More on argc and argv Page 85 October 6, 1997 More on argc and argv • Another (less clear) implementation of echo : int main(int argc, char **argv) {while (--argc > 0)printf("%s%c", *++argv, argc > 1
View Full Document
Unlocking...