Pointer and Array 指標與陣列

Pointer and Array指標與陣列 double *a, b[10]; char name[80], *np;

Pointer 指標 指標為正整數(或長正整數)變數，用來存放某特定數態的位址。 double a, c, *b; // a 是 8-byte 浮點實數，b 是 double 指標 b = &a; // 將 a 的位址存入 b c = *b; // 將 b 所指向的浮點實數存為 c (*b) 0.123456789012345 e+05 &a = 8076 &b 8076 &a &c = 8084 8-byte

Scanf(“format string”, list of pointers) int a, *ap; double b, *bp; ap = &a; bp = &b; scanf(“%d %lf”, ap, bp); // same as scanf(“%d %lf”, &a, &b); printf(“a=%d b=%lf\n”, *ap, *bp); // same as printf(“a=%d b=%lf\n”, a, b);

Some Considerations double a=5.0, *ap; ap = &a; =============================== *(&a) = ? &(*ap) = ? &ap = ? printf(“ a = %lf\n”, *ap); printf(“ap = %p\n”,ap); printf(“ %p\n”,&ap); printf(“ap = %lx\n”,ap);

String = Character Array 字串 char fname[80], achr＝’c’, *ap; scanf(“%s”, fname); // fname =“myprog.c” ap = &achr; printf(“%c\n”,*ap); // c ap = fname; printf(“%c\n”, *ap); // m printf(“%c\n”, *(ap+1)); // y printf(“%c\n”,*(ap+2)); // p ……… example

Array is a pointer with fixed address 0 +1 +2 +3 +5 +8 +4 +6 c m y p r o g . c fname &achr \o …. +10 +9 +11 ap = &achr; ap = fname; ap = &fname[9] &ap

Array of various types • char sss[80]; • int na[20]; • float farry[30]; • struct {int n; double x;} sttt[5]; sttt[3].n = 1; sttt[3].x = 8.9;

Numeric Array • float X[6]; X+3; &X[3] X[3] = 5; *(X+3) = 5; pointer value location 0 -- 3 X[0] X 4 -- 7 X+1 X[1] 8 -- 11 X[2] X+2 12 -- 15 X+3 X[3] 16 -- 19 X[4] X+4 20 -- 23 X[5] X+5 ※ (x+1) 代表下一個變數、真正的位置增量看數的型態而定

Array as an argument (example) double funsum(int, double *, char *); int main() { int k, n=10; double xn[30], sum; char ss[40]; for (k = 0; k<n; k++) xn[k] = 1.0/ (double)(k+1)); sprintf(ss, “pass array dim(%d)”, n); sum = funsum(n, xn, ss); …. return 0; } double funsum(int nt, double *x, char *s) { int k; double summ; printf(“%s\n”,s); for (k=0; k<n; k++) summ = summ + x[k]; return(summ); } 結果: Pass array dim(10) Sum = 0.909091

字串列印 char ss[40]; sprintf(ss,”This is a string.”); // store 17字元 printf(“output = %s\n”,ss); ============================ sprintf(buf, format, variables); buf: 記憶體緩衝區 (字串位址) 。 format：字串，包含變數輸出格式 (與 printf 同) variables：每一 % 格式所對應之變數。

Array and pointer • 陣列變數本身是一個指標變數 (pointer) • 陣列作為幅數傳遞時，傳遞的是位址 (pass by address) 與其他變數傳值不同 (pass by value) 。 • 所以、在函數中改變陣列元素的值、會直接改變原呼叫函數中的陣列。 • 當指標變數被賦予ㄧ位址之後，用法和陣列變數完全相同。 • 如果指標指向超越陣列定義範圍，會產生 out of range 的錯誤，情況不穩定。

Honer’s method for Polynomial fx = a(n); dfx = 0.0; for (k=n ; k>0 ; k--) { dfx = dfx * x + fx; fx = fx * x + a[k-1]; } Notice: dfx 要先於 fx

f(x) f’(x) x2 x1 牛頓法解根 (example) x1 = xinit; do { fdf = poly(x, a, n); dx = fdf.x(x, a, n)/fdf.y(x, a, n); x1 -= dx; } while ( fabs(dx) > xcrit );

main Scheme 1 input n, a[0]-a[n] 2 1 3,4 Int TwoDouble Read xinit input(a) poly(x,a,n) yes |xinit|>100? Read n end Horing method no 3,4 dx = df/f x = x - dx Read a[0]-a[n] Return f&df 5,6 no |dx|<xcrit ? 5,6 Return n yes Output x 2

Project. 4 Root of polynomial • Using Newton’s method found the root of f(x) = x5 + 2x4 + 4x3 + 8x2+ 3x + 6 with xcrit = 10-8 xinit = 0.0

Pointer and Array 指標與陣列