(a) Arrays. Definition

Transcription

1 Part 3: Aggregate Data Types (a) Arrays Definition An array is a sequence of objects of a given type Therefore it is not a type of its own It is rather an organizational concept Array elements can be accessed individually A character string is a sequence of char objects Arrays in C A simple array definition in C looks like this: type name[size]; The name is the variable s name The size must be a constant expression An array variable is a name for such a sequence The name stands for the first element s address Elements can be accessed by their distance to the first element When accessing elements, pointer operations are used Either use * for the first, or [dist] for any element Accessing an arrays 5 th element: ar[4] Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 49

2 Initializing an Array Alternative array definition: type name[[size]] = val1, val2, val3,...; With no size the number of initializers is used for the size size must be equal or less to the number of initializers int ar[4] = 0, 1, 2, 3 ; Array Size The array s boundaries are not protected! Access beyond the limits may or may not lead to crashes! sizeof() can be used to obtain an array s size in Bytes But how do we get the number of elements? Determining an array s number of elements: int ar[2]; printf ("The number of bytes is %d\n", sizeof (ar)); printf ("The number of elements is %d\n", sizeof (ar) / sizeof (ar[0])); Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 50

3 Passing Arrays as Function Arguments An array can t be passed on as an argument! Use a pointer with the first element s address instead Problem: The information on the array s size is lost The size must be passed as an additional argument #include <stdio.h> void print_week (int *ar, int size); int main (void) enum days Su, Mo, Tu, We, Th, Fr, Sa, Days ; int week[days] = 1, 2, 3, 4, 5, 6, 7 ; print_week (week, sizeof(week) / sizeof(*week)); void print_week (int *ar, int size) if (size <= 0) return; printf ("Day no %d\n", ar[0]); print_week (&ar[1], size - 1); Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 51

4 Arrays and Pointers The array name ist just a name for the first element s address An array does therefore not have a modifyable lvalue But: whenever accessing elements in an array we use pointer operations; for this a temporary pointer is being created Rule of thumb: When accessing array members or when having an array on the right side of an assignment an array behaves like a pointer. Else it does not! int ar[5] = 1, 2, 3, 4, 5 ; int *ptr; /* legal */ ptr = &ar[1]; ptr[0] = ptr[3]; ar[3] = ptr[0]; /* illegal (no modifyable lvalue) */ ar = ptr; Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 52

5 A Special Case: Character-Strings Character Strings are special arrays Technically they are Arrays of char But to be a character string it must be terminated by a special character: \0 So every string has n visible and 1 invisible characters #include <stdio.h> void print_str (char *str, int size); int main (void) char str[] = "hallo"; print_str (str, sizeof (str) / sizeof (*str)); void print_str (char *str, int size) if (size <= 0) return; printf ("chr: %c, ord: %d\n", str[0], str[0]); print_str (&str[1], size - 1); Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 53

6 Creating Character Strings A character string can be created like an array of char Alternatively a string constant can be used for initialization: char str[] = "hello"; This will create an array of 6 characters! Also this can only be done using a constant Examples: /* legal: */ char str1[] = H, e, l, l, o, \0 ; char str2[] = "Hello"; char *ptr = "Hello"; /* illegal (not a string constant): */ char str3[] = ptr; /* legal: */ str1[0] = h ; str2[1] = e ; /* illegal (pointer points to constant object): */ ptr[3] = x ; Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 54

7 Initializing Character Strings We already know two ways of doing this: 1. Initialising in the definition (e.g. char s[] = "abc"; ), this creates a writable character string 2. Initialising pointers (e.g. have a char *cp and do a cp = "abc" ), this creates a reference to a string constant In other cases it is difficult, simple assignment won t do because an array has no modifyable lvalue! Solution: Copy data into the array Copying Strings The strcpy() utility is declared in string.h Its prototype is: char *strcpy(char *dest, char *src) It copies the string at address src to address dest It stops copying after having copied the \0 character It does not (cannot) check for enough memory space at dest Can easily lead to protection faults! Safe: char *strncpy(char *dn, char *sc, size t n) Only up to n characters are copied But: if there is no \0 within the first n characters, it will not be copied! Thus after a strncpy() the string should always be terminated by manyally adding a \0! Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 55

8 A safer strncpy() char * safe_strncpy (char *s2, char *s1, size_t n) char *ret = strncpy (s2, s1, n); s2[n-1] = \0 ; return ret; /*... */ char s1[] = "hallo", s2[4]; safe_strncpy (s2, s1, sizeof (s2)); Other String Utilities The standard library provides a lot of string functions They are all declared in string.h Operation inlude: Appending strings (strcat()) Search for characters (strchr()) Search for substrings (strstr())... (see documentation!) They all operate on pointers The user must make sure there is enough memory space Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 56

9 A better scanf() The scanf() function makes error recovery difficult A better solution is to use strings and sscanf() The data is read into a string using fgets() The sscanf() function works like scanf() but reads data from a string instead of standard input The return value is the number of successfully read variables #include <stdio.h> /* for sscanf() and fgets() */ void read_two_ints (int *i1, int *i2) char inbuf[256]; printf ("Enter two integers!\n"); /* read <= 256 characters from the standard * input (keyboard) */ fgets (inbuf, sizeof (inbuf), stdin); /* if operation failed, try again */ if (sscanf (inbuf, "%d %d", i1, i2)!= 2) printf ("That was not correct!\n"); read_two_ints (i1, i2); Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 57

10 Command Line Arguments When running a program from the command line we can pass arguments to it, like./program foo bar These arguments are accessable in main() if it is defined this way: int main (int argc, char **argv) argv is a pointer to the first entry in an array of strings In argv[0] we have the program s name, like./program In the following elements we find the command line arguments argc is the number of arguments in that array int main (int argc, char **argv) if (argc > 1) printf ("First arg: \"%s\"\n", argv[1]); printf ("Last arg: \"%s\"\n", argv[argc-1]); return 0; Note that the backslashes used before the double quotes quote them, so that they can be printed. Note also that the format code for strings in printf() is %s. Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 58

11 Part 3: Aggregate Data Types (b) Structs Data Structures in C A struct is a bundle of objects of arbitrary types Therefore it is not a type of its own It is like arrays an organizational concept Struct elements can be accessed individually A struct definition looks like this: struct [name] member-definitons [names]; The first name can be used as a name for the struct The names in the end can be used to create variables The member-definitions are an arbitrary number of variable definitions. struct birthday int year; char *month; int day; herbert; struct birthday yinka; Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 59

12 Part 3: Aggregate Data Types Initializing Structs Like arrays, structs can be initialized during definition But unlike arrays, structs can be assigned (they have modifyable lvalues) Accessing Struct Members Struct members can be accessed via their field names The syntax is like this: structname.fieldname When using a pointer to a struct we use this: ptrname >fieldname struct int year, day; char *month; hugo = 1977, 4, "February", maggie = hugo, *henry = &maggie; maggie = 1944, 1, "December" ; hugo.year = 1978; maggie.day = hugo.day; henry->day = 24; Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 60

13 Part 3: Aggregate Data Types Struct Dates and Time The standard library provides some utilities for dates and time in time.h time t time(time t *tptr) returns the time as a number struct tm* localtime(time t *t) makes it readable struct tm looks like this: struct tm int tm_sec; /* seconds past minute [0..59] */ int tm_min; /* minutes past hour [0..59] */ int tm_hour; /* hours past midnight [0..23] */ int tm_mday; /* day of month [1..31] */ int tm_mon; /* month [0..11] */ int tm_year; /* year since 1900 [>0] */ int tm_wday; /* day of week [0..6] */ int tm_yday; /* days since Jan 1 [0..365] */ int tm_isdst;/* daylight saving time[-1/0/1] */ ; time_t t = time (NULL); struct tm *tptr = localtime (&t); printf ("Today is %d/%d/%d\n", tptr->tm_year , tptr->tm_mon + 1, tptr->tm_mday); Herbert Martin Dietze <herbert@the-little-red-haired-girl.org> 61