8 주 강의

8주 강의 Arrays, Pointers, and Strings

One dimensional array • int grad0, grad1, grad2;  int grad[3]; • int a[size]; /* a[0], …, a[size-1] • #define N 100 int a[N]; /* a[0] .. a[99]is allocated */ for(i=1; i<100; ++i) sum += a[i];

Initialization float f[5]={0.0, 1.0, 2.0, 3.0, 4.0}; int a[100]={0}; /* all elements to 0 */ int a[]={2,3,5,-7}; /* int a[4] … */ char s[] = “abc”; /*  char s[] = {‘a’, ‘b’, ‘c’, ‘\0’} */

Pointers • &v는 변수 v의 location(주소) • int *p ::: declares p to be type pointer to int • p=0;  p=NULL; • p=&i; • p=(int *) 1776 • /* an absolute address */

Pointer 예 int a, b, *p; a=1; b=2; p=&a;

Pointer에 의한 접근의 결과

Legality of opertions

주의할 점 • 7 * * p / * q + 7 • ??? 7 * * p /* q +7 --trouble making • &3 /* illegal */ • &(k+3) /* illegal */ • register v; &v /* illegal */ • &a[0], &a[i+j+3] /* legal */ • int a[3]; &a ?????

Call by reference (address) • To change the values of variables in the calling environment • Simulating the effect of call by reference by pointers

예 • void swap(int *p, int *q) { int tmp; tmp = *p; *p = *q; *q = tmp } • int i=3, j=5; swap(&i, &j);

The simulation of call by reference • Declaring a function parameter to be a pointer • Using the de-referenced pointer in the function body • Passing an address as an argument when the function is called

The relationship between arrays and pointers • An array name itself is an address, or pointer value, and pointers as well as arrays, can be subscripted • a[i]  *(a+i) • #define N 100 int a[N], I, *p, sum=0;  p=a;  p=&a[0];  p=a+1  p=&a[1];

계속 • for (p=a; p < &a[N]; ++p) sum += *p;  for(i = 0; i < N; ++i) sum += *(a + i);  p=a; for (i=0; i < N; ++i) sum += p[i];

Pointer 연산 • int *a; short *b; float *c; double *d; a+1  주소 4 증가 b+1  주소 2 증가 c+1  주소 4 증가 d+1  주소 8 증가

Arrays as function arguments • double sum(double a[], int n) { int i; double sum = 0.0; for (i = 0; i < n; ++i) sum+=a[i]; return sum; }  double sum(double *a, int n)

수행 내용

Bubble sorting • Greedy method • 큰 것을 먼저 뒤로 보내거나, 작은 것을 먼저 앞으로 보내거나 • 257(6.7절)페이지 설명

p.258

calloc() and malloc() • stdlib.h • Dynamic memory allocation (calloc : contiguous allocation, malloc : memory allocation) • Dynamically create space for arrays, structures, unions • callc(n, el_size) • malloc(n*el_size) • 실패시 NULL을 return (memory가 부족하면)

예 • int *a, n; … a=calloc(n, sizeof(int)); • a=malloc(n*sizeof(int)); • deallocation : free(ptr) free(a); • Reallocation : realloc() • 261page 프로그램 설명

Memory map

Offsetting the pointer a = calloc(n, sizeof(double));

Offsetting the pointer a = calloc(n+1, sizeof(double)) ;

Merge sorting • Merge to arrays • Merge방법 설명 (264page 프로그램) • Merge sorting 방법 설명 (266 page) • Why power of 2? • 그러면 해결방법은

Strings • Strings are one-dimensional array of type char • A null character is a byte with all bits off (\0) • char *p = “abc”; • A string constant is treated as a pointer • “abc”[1]  *(“abc”+1) • char *p=“abcde”; • char s[] = “abcde”;  char s[] = {‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘\0’};

The memory map - 272 페이지의 word_count program 설명

String handling functions in the standard library • char *strcat(char *s1, const char *s2); • int strcmp(const char *s1, const char *s2); • char *strcpy(char *s1, const char *s2); • size_t strlen(const char *s); /* unsigned int */ • strlen()와 strcpy()의 프로그램 (page 273, 274) • Strcat() 프로그램  275page

예제

Multidimensional arrays

Two-dimensional arrays int a[3][5]; /* row major */ row major  column major

예제

Storage mapping function • int a[3][5] • a[i][j]  *(&a[0][0] + 5*i +j) • int a[I][J][K} • a[i][j][k]  *(&a[0][0][0] + I*J*i + J*j +k

Formal parameter declarations • int a[3][5] • int sum(int a[][5]) { int i,j,sum=0; for(i=0;i<3;++i) for(j=0,j<5;++j) sum +=a[i][j]; return sum; } • int a[][5]  int a[3][5]  int (*a)[5]

One dimensional array • parameter int b[]  int b[3]  int *b • char *argv[]  char *argv[3]  char **argv • Caution ::: char x[][] <=/=> char **x

Three dimensional array • int a[7][9][2]; • a[i][j][k]  *(&a[0][0][0] + 9*2*I + 2*j + k) • Parameter ::: int a[][9][2]  int a[7][9][2]  int (*a)[9][2]

Initialization • int a[2][3] = {1,2,3,4,5,6};  int a[2][3] = {{1,2,3},{4,5,6}};  int a[ ] [3] = {{1,2,3},{4,5,6}}; • 뒤의 0은 쓰지 않아도 int a[2][2][3] = { {{1,1,0}, {2,2,0}} {{3,0,0}, {4,4,0}} };  int a[][2][3] = {{{1,1}, {2}}, {3}, {4,4}}}; • int a[2][2][3] = {0};

The use of typedef • Self documentation • #define N 3] typedef double scalar; typedef scalar vector[N]; typedef scalar matrix[N][N]; scalar a; vector b; matrixc; • page 283 프로그램 설명

Arrays of pointers • Sorting strings :: a pointer of a pointer • 285page 설명

Example

Before swapping After swapping W W w[i] f o r ＼0 w[i] f o r ＼0 w[j] a p p l e ＼0 w[j] a p p l e ＼0 Swapping of two strings

숙제 • 사람의 이름을 입력하여 순서화한다 - 1수준 ::: 순서화만 - 2수준 ::: 사람 이름 대신에 주소 - 3수준 ::: 한글 이름을 입력하여 순서화

Arguments to main() • int main(int argc, char *argv[]) { int i; printf(“argc = %d\n”, argc); for (i=0;i<argc;++i) printf(argv[%d] = %s\n”, i argv[i]); return 0; } • my_echo a is an apple • argv=5, argv[0]= my_echo … argv[4] = apple

Ragged arrays • char a[2][15] = {“abc:”, “a is for apple”}; char *p[2] = {“abc:”, “a is for apple”}; • Ragged array ::: an array of pointers whose elements are used to point varying sizes

A ragged array 예

Functions as arguments #include “sum_sqr.h” double sum_square (double f(double x), int m, int n) { int k; double sum=0.0; for (k=m; k<=n;++k) sum +=f(k)*f(k); return sum; } • double sum_square(double (*f)(double), int m, int n); • sum += (*f)(k) * (*f)(k); 도 가능 • 조심 :::: double sum_square(double *f(double), int m, int n)은 안 된다 …함수 f(double)의 return 값이 double의 pointer

Function prototypes • Function prototypes double sum_square (double f(double x), int m, int n) double sum_square (double f(double), int m, int n) double sum_square (double f(double x), int, int) double sum_square (double (*f)(double x), int, int) double sum_square (double (*)(double x), int, int) double sum_square (double g(double x), int a int b) double sum_square (double f(double x0, int m, int n) double sum_square (double f(double x0, int m, int n)

Using bisection to find the root of a function • A real number x that satisfies the equation f(x) = 0  a root of f • 프로그램 설명 (298페이지)

Finding a root by bisection Finding a root

The Kepler equation • m = x – e sin(x)  y=x and y=m + e sin(x)  x – e sin(x) – m = 0

A solution of the Kepler equation y y =m+e sinx y =x x x0 Kepler equation

8 주 강의

8 주 강의

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