Searching and Sorting

Transcription

1 Introduction to Programming ( 數 ) Lecture 10 Spring 2005 May 13, 2004 NCCU C prog. 1

2 Topics More on Strings, Arrays, Pointers Static variables Structure: struct NCCU C prog. 2

3 Review of String Basics Strings = arrays of chars with NULL ( \0 ) termination. o char a[ ] = "xyz ; o char a[ ] = {'x', 'y', 'z', '\0'}; /* same thing */ Strings are often handled through pointers o Char *cp = A string ; Strings I/O o scanf, fscanf, gets, fgets, puts, fputs String Operations (<string.h>) o strlen, strcpy, strcat, strcmp, NCCU C prog. 3

4 string declaration and memory layout char a[3]; char *p; char *p = "abc"; char a[] = "abc"; p??? p a??? a?????? a b c \0 a b c \0 NCCU C prog. 4

5 Review: Things You Can and Can't Do You can't use = to assign one string variable to another (use library functions strcpy etc.) You can't use == to directly compare strings (use library functions strcmp etc.) You can directly scanf or printf strings (use %s) Many Functions in <string.h> NCCU C prog. 5

6 String Assignment: Dangers #include <string.h>... char medium[ ] = Four score and seven ; char big[1000] ; char small[5] ; strcpy(big, medium) ; strcpy(big, Bob ) ; strcpy(small, big) ; strcpy(small, medium) ; /* looks like trouble... */ NCCU C prog. 6

7 Cont d: strcpy results medium: Four score and seven\0 big: big: small: small: Four score and seven\0?????... Bob\0 score and seven\0?????... Bob\0? Four score and seven\0 Major Pitfall: overrunning allowed length NCCU C prog. 7

8 Ex: Reading strings from a file main(void) /* #define LINE_LEN 80 #define NAME_LEN 40 */ { char line[line_len], inname[name_len], outname[name_len]; FILE *inp, *outp; char *status; int i = 0; } printf("name of input file> "); scanf("%s", inname); printf("name of output file> "); scanf("%s", outname); inp = fopen(inname, "r"); outp = fopen(outname, "w"); for (status = fgets(line, LINE_LEN, inp); status!= 0; //NULL status = fgets(line, LINE_LEN, inp)) { if (line[strlen(line) - 1] == '\n') line[strlen(line) - 1] = '\0'; fprintf(outp, "%3d>> %s\n", ++i, line); } return (0); NCCU C prog. 8

9 File used as input In the early 1960s, designers and implementers of operating systems were faced with a significant dilemma. As people's expectations of modern operating systems escalated, so did the complexity of the systems themselves. Like other programmers solving difficult problems, the systems programmers desperately needed the readability and modularity of a powerful high-level programming language. Output file 1>> In the early 1960s, designers and implementers of operating 2>> systems were faced with a significant dilemma. As people's 3>> expectations of modern operating systems escalated, so did 4>> the complexity of the systems themselves. Like other 5>> programmers solving difficult problems, the systems 6>> programmers desperately needed the readability nd 7>> modularity of a powerful high-level programming language. NCCU C prog. 9

10 見 列 NCCU C prog. 10

11 Char Pointers <> String A Char pointer does NOT hold the space for character strings! char *sp; printf("\n enter a string: "); scanf( %s, s); /* ERROR */ printf("\n *sp = %s", sp); How to fix it? NCCU C prog. 11

12 constant and variable strings char *p, a[3], b[] = "abc"; p = "xyz"; /* OK. Assigns pointer to p */ a = "xyz"; /* NO, a is a constant */ a[] = "xyz"; /* NO, syntax error */ p = b; /* OK. b (pointer) assigned to p */ a[] = p; /* NO, involves copying all elements, use strcpy()*/ a[] = b[]; /* NO, same problem as above */ NCCU C prog. 12

13 Arrays, Pointers, & Strings An array name is a read-only pointer to the 0 th element of the array int a[10], *ip; ip = &a[0]; /* or ip = a; */ But a = ip; is error! Pointer arithmetic: ip+1 points to a[1] ; *(a+1) points to a[1], too. 列 參數 參數 Pointer NCCU C prog. 13

14 Arrays, Strings, and Pointers /* Passing an array to a function == passing the beginning address of the array*/ #include <stdio.h> #include <string.h> void mysub ( char [ ] ) ; /* mysub(char *) */ int main ( ) { char name[20] = "Richard J. Freuler" ; mysub (name) ; } NCCU C prog. 14

15 Arrays, Strings, and Pointers void mysub ( char text[ ] ) /* mysub(char *text) */ { int len, k ; len = strlen (text) ; printf ("%d\n", len ) ; for (k = 0 ; k < len ; k++) printf ("%c", text [k] ) ; } /*Program Output */ blanks 18 RichardJ. Freuler NCCU C prog. 15

16 Arrays/Pointers An array name is a read-only pointer to the 0 th element of the array. An array parameter can be declared as an array or a pointer; an array argument can be passed as a pointer. int strlen(char s[]) { int n = 0; while (s[n]!= 0) n++; return n; } Could be written: while (s[n]) int strlen(char *s) { int n = 0; while (*s++!= 0) n++; return n; } Could be written: while (*s) NCCU C prog. 16

17 An Interesting String Example void guess_me (char *s, char *r) { } while (*s++ = *r++); What does it do? How to use it? char str1[length], str2[length] = abcd ; guess_me(str1, str2); NCCU C prog. 17

18 Array of Strings & Array of Pointers NCCU C prog. 18

19 Arrays of Strings A string is a 1-D array of characters, so a 2-D array of characters would be an array of strings char names[4][10] = {"Robert","Adam", "Charles","Benjamin"}; 4 省略 R o b e r t \0 \0 \0 \0 A d a m \0 \0 \0 \0 \0 \0 C h a r l e s \0 \0 \0 B e n j a m i n \0 \0 This will declare an array of 4 strings with each string having a maximum of 10 characters names[0] names[1] names[2] names[3] for (i=0; i<4; i++) printf( %s, names[i]); NCCU C prog. 19

20 Arrays of strings 例 度 串 度不 array of (char *), 不 (char [ ][ ]). char *s[] = {"one", "two", "three"}; This is an array of pointers that point to arrays of characters, thus it is a 2-D array. S o n e \0 t w o \0 t h r e e \0 NCCU C prog. 20

21 In memory we have: char *s[] = {"one", "two", "three"}; printf( %c %c %c, s[1][1], *s[1], **s); Output: w t o s[0] s[1] s[2] o n e \0 t w o \0 t h r e e \0 NCCU C prog. 21

22 Array of strings vs. 2-D array of char char *s[] = {"ab", "cd"}; Is not the same as char t[][3] = {{'a','b','\0'}, {'c','d','\0'}}; Former is an array of pointers pointing to strings. Latter is linear sequence of characters. What s the difference? Recall that array names are constant pointers. NCCU C prog. 22

23 The memory picture s[0] s[1] a b \0 c d \0 t[0] t[1] a b \0 c d \0 char *s[] = {"ab", "cd"}; /* s[0], s[1] are variables*/ s[0] = new string ; is OK, char t[][3] = {{'a','b','\0'}, {'c','d','\0'}}; /* t[0], t[1] are constants */ t[0] = new string ; is error! NCCU C prog. 23

24 1-D Array of Strings char *days[] = { }; M O N D A Y \0 T U E S D A Y \0 W E D N E S D A Y \0 T H U R S D A Y \0 F R I D A Y \0 S A T U R D A Y \0 S U N D A Y \0 NCCU C prog. 24

25 Command-line arguments, Call-by-reference NCCU C prog. 25

26 Command-line arguments winword.exe mypaper.doc The command-line: msword.exe mypaper.doc Two command-line arguments: (1) winword.exe, (2) mypaper.doc (command-line arguments are character strings) (1) winword.exe - Name of the program for Windows to run. (Microsoft Word for Windows) (2) mypaper.doc - Name of the file for Word to open. The program winword.exe uses the command line argument mypaper.doc. NCCU C prog. 26

27 Can our C programs use arguments on the command line? Example: A filename hard-coded into the program: histogram This program histogram will ALWAYS read from data_in.txt. NCCU C prog. 27

28 Can our C programs use arguments on the command line? Example: A filename hard-coded into the program: histogram This program histogram will ALWAYS read from other_data.txt. NCCU C prog. 28

29 Better: A filename provided by the user on the command-line. histogram new_data.txt How do we get command-line arguments into our C programs? Parameters to function NCCU C prog. 29

30 int main( int argc, char *argv[ ] ) argc: argument count - Number of command-line arguments argv[ ]: argument vector - Array containing the command line strings msword.exe mypaper.doc 1 2 = 2 Note: argc 1 always! (why?) is msword.exe is mypaper.doc

31 Command-Line Arguments argc and argv o When main is called to begin execution, it is called with two arguments argc and argv argc : The first (conventionally called argc) is the number of commandline arguments the program was invoked with argv : The second (conventionally called argv) is a pointer to an array of character strings that contain the arguments, one per string. Example: if echo is a program and executed on phoenix prompt, such as C:\> echo hello world argv argc 3 pointer array null e c h o \0 h e l l o \0 w o r l d \0 char *argv[] : array of char pointers (strings) NCCU C prog. 31

32

33

34

35 void usage( char *program_name ) { printf( Usage: %s <name>\n, program_name ); printf( where <name> is the name of an input data file.\n ); }

36 Pass-by-Address (Output Parameters) Call-By-Reference NCCU C prog. 36

37 Pointers as function parameters call by reference x = 0 x = Contents Address NCCU C prog. 37

38 Call by reference: output parameters 1. Define a formal parameter as a pointer. 2. Use a pointer for the corresponding function call argument. Note: A function can have both input and output parameters. NCCU C prog. 38

39 Input Output

40 swap main s n1 111 main s n2 222 swap s temp swap s a swap s b n1=111 n2=222 n1=222 n2=111 NCCU C prog. 40

41 Character Handling Library Character handling library o Includes functions to perform useful tests and manipulations of character data o Each function receives a character (an int) or EOF as an argument The following slide contains a table of all the functions in <ctype.h> NCCU C prog. 41

42 Character Handling Library Prototype int isdigit( int c ) int isalpha( int c ) int isalnum( int c ) int isxdigit( int c ) int islower( int c ) int isupper( int c ) int tolower( int c ) int toupper( int c ) int isspace( int c ) int iscntrl( int c ) int ispunct( int c ) int isprint( int c ) int isgraph( int c ) Description Returns true if c is a digit and false otherwise. Returns true if c is a letter and false otherwise. Returns true if c is a digit or a letter and false otherwise. Returns true if c is a hexadecimal digit character and false otherwise. Returns true if c is a lowercase letter and false otherwise. Returns true if c is an uppercase letter; false otherwise. If c is an uppercase letter, tolower returns c as a lowercase letter. Otherwise, tolower returns the argument unchanged. If c is a lowercase letter, toupper returns c as an uppercase letter. Otherwise, toupper returns the argument unchanged. Returns true if c is a white-space character newline ('\n'), space (' '), form feed ('\f'), carriage return ('\r'), horizontal tab ('\t'), or vertical tab ('\v') and false otherwise Returns true if c is a control character and false otherwise. Returns true if c is a printing character other than a space, a digit, or a letter and false otherwise. Returns true value if c is a printing character including space (' ') and false otherwise. Returns true if c is a printing character other than space (' ') and false otherwise. NCCU C prog. 42

43 Part 1: Case Conversion Fuction /* * Converts the lowercase letters of its string argument to uppercase * leaving other characters unchanged. */ char * string_toupper(char *str) /* input/output - string whose lowercase letters are to be replaced by uppercase */ { int i; for (i = 0; i < strlen(str); ++i) if (islower(str[i])) str[i] = toupper(str[i]); } return (str); NCCU C prog. 43

44 Upper/Lower case Source Code NCCU C prog. 44

45 String-to-Number & Number-to-String Conversions string 17 int (17) // string double ( ) NCCU C prog. 45

46 String to int Conversion #include <ctype.h> int stringtointeger (char *s, int *n) { int i = 0, r = 0; while (isdigit (s[i])) { int d = s[i] - '0'; r = r * 10 + d; ++i; } if (s[i] == '\0' && i > 0) { *n = r; return 1; } else { return 0; } char str[] = 3129 ; int k, status; } Status = stringtointeger(str, &k); NCCU C prog. 46

47 Diagram Your Approach... We will build up the string left-to-right We will add one digit each step offset: workarea string: \0 value now: 3129 value now: 129 r = r * 10 + d; value now: 29 r = r * 10 + d; value now: 9 r = r * 10 + d; value now: r = r * 10 + d; done, return the string NCCU C prog. 47

48 String Conversion Functions Conversion functions o In <stdlib.h> (general utilities library) Convert strings of digits to integer and floating-point values Prototype double atof( const char *nptr ) int atoi( const char *nptr ) long atol( const char *nptr ) double strtod( const char *nptr, char **endptr ) long strtol( const char *nptr, char **endptr, int base ) unsigned long strtoul( const char *nptr, char **endptr, int base ) Description Converts the string nptr to double. Converts the string nptr to int. Converts the string nptr to long int. Converts the string nptr to double. Converts the string nptr to long. Converts the string nptr to unsigned long. char str[] = 3129 ; int k; k = atoi(str); NCCU C prog. 48

49 1 /* Example of 2 Using atof */ 3 #include <stdio.h> 4 #include <stdlib.h> 5 6 int main() 7 { 8 double d; 9 10 d = atof( "99.0" ); // convert a string to double 11 printf( "%s%.3f\n%s%.3f\n", 12 "The string \"99.0\" converted to double is ", d, 13 "The converted value divided by 2 is ", 14 d / 2.0 ); 15 return 0; 16 } The string "99.0" converted to double is The converted value divided by 2 is NCCU C prog. 49

50 Converting numbers to strings Can be done with sprintf() function <stdio.h> The printf() function converts values to characters and writes them to the output buffer The sprintf() function converts values to characters and then writes them to a string stored in memory #include <stdio.h> Output: void main(void) { Pay is $ char paycheck[40]; double hours = 36.7, rate = 12.36; sprintf(paycheck, "Pay is $%.2f.", hours * rate); puts(paycheck); } NCCU C prog. 50

51 Reading from a String using sscanf The sscanf function allows us to read from a string argument using scanf rules First argument of sscanf is string to read from, remaining arguments are as in scanf Example: char buffer[100] = A ; sscanf(buffer, %c%d%f, &ch, &inum, &fnum); /* puts A in ch, 10 in inum and 50.0 in fnum */ ch A inum 10 fnum 50.0 NCCU C prog. 51

52 Static Variables NCCU C prog. 52

53 流 next_number 不 流 int next_number(); int next_number () { int counter = 0; counter ++; return counter; Local variables. Doesn t work! } number = next_number(); // gets 1 number = next_number(); // still gets 1 NCCU C prog. 53

54 流, cont d next_number 不 流 int next_number(); int counter = 0; // global variable 數 int next_number_1 () { counter ++; return counter; } number = next_number(); // gets 1 number = next_number(); // gets 2 NCCU C prog. 54

55 流, cont d Static variable 來 next_number 不 流 int next_number () { static int counter = 0; counter ++; return counter; 行 Static variable. It works well! } number = next_number(); // gets 1 number = next_number(); // gets 2 NCCU C prog. 55

56 Static Variables Local variables 參數 (parameters) 行 行 不 Static variables (scope) 行 int next_number () { static int counter = 0; counter ++; return counter; scope } counter = 100; Error! counter : not defined. Out of scope. NCCU C prog. 56

57 Structure in C 料 不 Arrays NCCU C prog. 57

58 Structure in C ( 料 ) Array: ( ) 料 Structure: 不 料 錄 (record) 例 料 ( 串 ) 數 年 ( 數 ) 料 欄 (field) Example: struct sdata /* sdata, sdata tag */ { char name[15]; /* 欄 */ char id[10]; int math; int eng; }; struct sdata student; /* sdata 數 student */ strcpy(student.name, ); student.math = 99; /* 數 */ NCCU C prog. 58

59 -1 ( ) : struct { 料 欄 1; 料 欄 2;.. 料 欄 n; }; Example: struct sdata { char name[15]; char id[10]; int math; int eng; }; struct sdata students[50], grad, s; struct 數 1, 數 2,, 數 m; NCCU C prog. 59

60 2 ( ) : struct { 料 欄 1; 料 欄 2;.. 料 欄 n; } 數 1, 數 2,, 數 m; Example: struct sdata { char name[15]; char id[10]; int math; int eng; } students[50], grad, s; NCCU C prog. 60

61 3 ( ) : struct { 料 欄 1; 料 欄 2;.. 料 欄 n; } 數 1, 數 2,, 數 m; 不 (No Tag) Example: struct { char name[15]; char id[10]; int math; int eng; } students[50], grad, s; 數 NCCU C prog. 61

62 數 Field Selection 利 數 (.) 來 數 欄 數. 欄 例 : struct sdata student; /* 數 */ o o o o student.name student.id student.math student.eng NCCU C prog. 62

63 數 #include <stdio.h> int main(void) /* 欄 欄 */ { struct sdata /* 數 */ { char name[15]; int math; } student; printf( Student s name: ); /* 數 欄 */ scanf( %s, student.name); printf( Math score: ); /* 數 欄 */ scanf( %d,&student.math); printf( *****Output*****\n ); /* 數 */ printf( %s s Math score is %d\n, student.name, student.math); return 0; } NCCU C prog. 63

64 數 行 : Student s name: David Wang Math score: 88 *****Output***** David Wang s Math score is 88 NCCU C prog. 64

65 數 Example: struct sdata { char name[16]; int math; int eng; }; struct sdata s1, s2; 念 sizeof(struct sdata)= 24 bytes 數 來 欄. strcpy(s1.name, John ); strcpy(s2.name, Mary ); s1 J o h n \0 s2 M a r y \0 NCCU C prog. 65

66 串 Example: struct sdata { char *name; int math; int eng; }; struct sdata s1, s2; s1.name = John ; s1.math = 99; s1.eng = 98; J o h n \0 NCCU C prog. 66

67 Accessing Char Ptr Fields mystruct.myint = 42; mystruct.mychar = 'a'; mystruct.mystr = "foobar"; strcpy(mystruct.mystr, "foobar"); struct { int myint; char mychar; char mystr[20]; } mystruct; NCCU C prog. 67

68 欄 ANSI C 欄 不 不 欄 // store a date as four // separate data fields: // month, day, and year // month_name struct date { int month; int day; // date: No! int year; char month_name[4]; }; struct sdata { char *name; int math; int eng; int year; }; NCCU C prog. 68

69 數 // store a date as four // separate data fields: // month, day, and year // month_name struct date { int month; int day; int year; char month_name[4]; }; I. All at once (best for small structure variables): date today = {5, 14, 2004, May }; // each number is assigned to the // corresponding field in the structure // variable II. One field at a time: date todays_date; todays_date.month = 5; todays_date.day = 14; todays_date.year = 2004; strcpy(todays_date.month_name, May ); NCCU C prog. 69

70 Valid Operations o o o o Structure Operations Assigning a structure to a structure of the same type Taking the address (&) of a structure Accessing the members of a structure Using the sizeof operator to determine the size of a structure 不 o o You CAN T use == or!= to compare structs You CAN T use printf() / scanf() on entire structs: Assignment statements OK. Example: struct date d3 = d2; // d2 d3 NCCU C prog. 70

71 typedef: A way of giving meaningful names to existing types. typedef old_type new_type_name; Can be used to rename all current types. typedef int bool; char, int, float, double, enum, struct, and pointers to these. Mostly used with struct type typedef struct { char name[15]; double diameter; int moons; double orbit_time, rotation_time; } planet_t; NCCU C prog. 71

72 Typedef or Struct Tag 兩 typedef struct { char * dept; char * title; int number; int section; } course_t; /* 'course_t' is type*/ course_t c1, c2, c3; struct course { char * dept; char * title; int number; int section; }; /* 'struct course' is type*/ struct course c1, c2, c3; NCCU C prog. 72

73 Nested structs typedef struct { int x, y; } point_t; typedef struct { point_t p1, p2; } line_t; int main(void) { line_t ln1; ln1.p1.x = 10; /* line starting point */ ln1.p1.y = 5; ln1.p2.x = 30; /* line ending point */ ln1.p2.y = 10; } NCCU C prog. 73

74 Pointers and Structures 數 來 ( ) 來 數 欄 struct card { char *face; char *suit; }; struct card onecard, deck[ 52 ]; struct card *cptr; cptr = &onecard; cptr face = 10 ; cptr suit = Heart ; printf( "%s", cptr suit ); cptr->suit is equivalent to (*cptr).suit NCCU C prog. 74

75 Using Structures With Functions Passing structures to functions o o o Pass entire structure or pass individual members Both pass call by value It is not a good idea to pass a structure to or return from function. 率 量 The better way is passing a pointer to the structure to the functions and returning a pointer from function. To pass structures call-by-reference ( ) o o Pass its address Pass a pointer pointing to the structure Returning a structure value from a function call NCCU C prog. 75

76 Using Structures With Functions (cont d) Example of passing and returning the whole structure typedef struct { : int hour, minute, second } time_t; time_t new_time(time_t time_of_day, /* input - time to be updated */ int elapsed_secs) /* input - seconds since last update */ { int new_hr, new_min, new_sec; } new_sec = time_of_day.second + elapsed_secs; time_of_day.second = new_sec % 60; new_min = time_of_day.minute + new_sec / 60; time_of_day.minute = new_min % 60; new_hr = time_of_day.hour + new_min / 60; time_of_day.hour = new_hr % 24; return (time_of_day); NCCU C prog. 76

77 Using Structures With Functions (cont d) Example of passing address thru pointer int day_of_year(struct date *pd) // 年 { int i, day, leap; day = pd -> day; leap = pd->year%4 ==0 && pd->year %100 ==0 pd->year%400 ==0; for (i=1; i<pd -> month; i++) day += day_tab[leap][i]; return (day); } o The declaration struct date *pd; says that pd is a pointer to a structure of the type date o If p is a pointer to a structure, then p-> member_of_structure refers to the particular members, like pd -> year o p-> member_of_structure is equivalent to (*p).member_of_structure o Notice:. has higher precedence than * ; *pd.year is wrong, since pd.year is not a pointer. o Both -> and. associate from left to right. So p -> q -> member are (p->q)->member. Example: emp.birthday.month are (emp.birthday).month NCCU C prog. 77

78 Example Figure 11.3 Function Comparing Two Structured Values for Equality #include <string.h> /* * Determines whether or not the components of planet_1 and planet_2 match */ int planet_equal(planet_t planet_1, /* input - planets to */ planet_t planet_2) /* compare */ { return (strcmp(planet_1.name, planet_2.name) == 0 && planet_1.diameter == planet_2.diameter && planet_1.moons == planet_2.moons && planet_1.orbit_time == planet_2.orbit_time && planet_1.rotation_time == planet_2.rotation_time); } NCCU C prog. 78

79 Example planet_t get_planet(void) { planet_t planet; } Figure 11.6 Function get_planet Returning a Structured Result Type /* * Gets and returns a planet_t structure */ scanf("%s%lf%d%lf%lf", planet.name, &planet.diameter, &planet.moons, &planet.orbit_time, &planet.rotation_time); return (planet); typedef struct { char name[15]; double diameter; int moons; double orbit_time, rotation_time; } planet_t; NCCU C prog. 79

80 Example: Complex Numbers typedef struct { double real, imag; } complex_t; Functions for complex numbers: int scan_complex(complex_t *c); void print_complex(complex_t c); complex_t add_complex(complex_t c1, complex_t c2); complex_t subtract_complex(complex_t c1, complex_t c2); complex_t multiply_complex(complex_t c1, complex_t c2); complex_t divide_complex(complex_t c1, complex_t c2); complex_t abs_complex(complex_t c); NCCU C prog. 80

81 Input a Complex Number /* * Complex number input function returns standard scanning error code * 1 => valid scan, 0 => error, */ int scan_complex(complex_t *c) /* output - address of complex variable to fill */ { int status; } status = scanf("%lf%lf", &c->real, &c->imag); if (status == 2) // 讀 兩 return 1; else return 0; NCCU C prog. 81

82 Adding Two Complex Numbers complex_t add_complex(complex_t c1, complex_t c2) /* input - values to add */ { complex_t csum; csum.real = c1.real + c2.real; csum.imag = c1.imag + c2.imag; } return (csum); NCCU C prog. 82

83 Printing a Complex Number void print_complex(complex_t c) /* input - complex number to display */ { // (3 + 5i) (6-8i) double a, b; char sign; a = c.real; b = c.imag; printf("("); if (fabs(a) <.005 && fabs(b) <.005) { // 數 數 printf("%.2f", 0.0); } else if (fabs(b) <.005) { printf("%.2f", a); } else if (fabs(a) <.005) { printf("%.2fi", b); } else { if (b < 0) sign = '-'; else sign = '+'; printf("%.2f %c %.2fi", a, sign, fabs(b)); } printf(")"); NCCU C prog. 83 }

84 Main() for Complex Numbers int main(void) { complex_t com1, com2; /* Gets two complex numbers */ printf("enter the real and imaginary parts of a complex number\n"); printf("separated by a space> "); scan_complex(&com1); printf("enter a second complex number> "); scan_complex(&com2); } /* Forms and displays the sum */ printf("\n"); print_complex(com1); printf(" + "); print_complex(com2); printf(" = "); print_complex(add_complex(com1, com2)); NCCU C prog. 84

85 Partial Implementation of Type and Operators for Complex Numbers (cont d) NCCU C prog. 85

86 Example: Comparing Two Structures Suppose we have defined a date structure as follows. How do we compare two date structures? typedef { int year, month, day; } date_t; int compare_date(date_t d1, d2) { if (d1.year==d2.year) { // 年 if (d1.month==d2.month) { // if (d1.day == d2.day) return 0; // else if (d1.day < d2.day) return -1; else return 1; } else if (d1.month < d2.month) return -1; else return 1; } else if (d1.year < d2.year) return -1; else return 1; } NCCU C prog. 86

87 What happens in terms of memory? struct { int myint; char mychar; char mystr[20]; }; NCCU C prog. 87

88 What happens in terms of memory? struct { int myint; char mychar; char mystr[20]; }; NCCU C prog. 88

89 What happens in terms of memory? struct { int myint; char mychar; char mystr[20]; }; NCCU C prog. 89

90 What happens in terms of memory? struct { int myint; char mychar; char mystr[20]; }; NCCU C prog. 90

91 Still use base+offset concept Compiler keeps track of offsets into structure of each member in "some sort" of symbol table member offset myint 0 mychar 4 mystr 5 (Sum of sizes of all previous elements) Question: Assume "mystruct" is located at location 1000 What will be address of myint, mychar and mystr? 1000, 1004, 1005 NCCU C prog. 91

92 Size of a Struct? sizeof(mystruct) = Σ (sizes of all elements) Could also write: sizeof(struct foo) Note: Some compilers will allocate more space due to alignment issues (space vs. time) Wasteful of space but more efficient Therefore... Always use sizeof!!! NCCU C prog. 92