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C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 32 of 40 19/03/2007 10:06 AM together, taking account of any overflow in the seconds and the minutes. The assert command is also used to ensure that the times that are input are also valid. To refer to an individual element of a structure, you use the following syntax: t1[0].hour = 12; t1[0].minutes = 0; t1[0].second = t2.second; t1[1] = t2; Alternatively, and somewhat confusingly, if "p", say, is a pointer to a structure, then you can access the elements of the structure using notation like "p->hour". For example: struct time *p; // p is a pointer to a time structure struct time t = {1,2,3.0}; // t is a time structure p = &t; // p now points to t p->hour = 6; // this statement is exactly equivalent to... t.hour = 6; // ...this one Structures can contain any type, including arrays and other structures. A common use is to create a linked list by having a structure contain a pointer to a structure of the same type, for example, struct person { char *name[80]; char *address[256]; struct person *next_person; }; where "person.next_person" is actually a pointer to a "person" struct. Sounds complicated, but it is simple when you get the hang of it. Structures are the first step towards object-oriented programming (OOP), where most of the programming effort is devoted to defining the best representation of the data, and the operations which can be performed on the data. In physics, a structure can be used to represent the state of a physical system. For example, here is a structure that might be used to represent the state of a particle in three-dimensional space: struct particle { double mass; double position[3]; double velocity[3]; }; We could then define an array of 1000 particles as simply as: struct particle p[1000]; You can take this one step further and define your own type (like "int", "double") by doing: typedef struct { double mass; double position[3]; double velocity[3]; } particle; In which case you can define the array of 1000 particles as: particle p[1000]; String functions The standard C libraries include a bunch of functions for manipulating strings (i.e., arrays of chars). Note that before using any of these functions, you should include the line "#include " in your program. This include-file defines prototypes for the following functions, as well as defining special types such as "size_t", which are operating-system dependent. char *strcat(s1, s2) Concatenates s2 onto s1. null-terminates s1. Returns s1. char *strchr(s, c) Searches string s for character c. Returns a
C programming notes file:///F:/my_docs/web_phys2020/Cprogrammingnotes.html 33 of 40 19/03/2007 10:06 AM int strcmp(s1, s2) pointer to the first occurence, or null pointer if not. Compares strings s1 and s2. Returns an integer that is less than 0 is s1 < s2; zero if s1 = s2; and greater than zero is s1 > s2. char *strcpy (s1, s2) Copies s2 to s1, returning s1. size_t strlen(s) char *strncat(s1, s2, n) int strncmp(s1, s2, n) char *strncpy(s1, s2) char *strrchr(s, c) int strstr(s1, s2) size_r strspn(s1, s2) size_r strcspn(s1, s2) char *strpbrk(s1, s2) char *strerror(n) char *strtok(s1, s2) Returns the number of characters in s, excluding the terminating null. Concatenates s2 onto s1, stopping after `n' characters or the end of s2, whichever occurs first. Returns s1. Like strcmp, except at most `n' characters are compared. Like strcpy, except at most `n' characters are copied. Like strchr, except searches from the end of the string. Searches for the string s2 in s1. Returns a pointer if found, otherwise the null-pointer. Returns the number of consecutive characters in s1, starting at the beginning of the string, that are contained within s2. Returns the number of consecutive characters in s1, starting at the beginning of the string, that are not contained within s2. Returns a pointer to the first character in s1 that is present in s2 (or NULL if none). Returns a pointer to a string describing the error code `n'. Searches s1 for tokens delimited by characters from s1. The first time it is called with a non-null s1, it writes a NULL into s1 at the first position of a character from s2, and returns a pointer to the beginning of s1. When strtok is then called with a null s1, it finds the next token in s1, starting at one position beyond the previous null. File operations defines a number of functions that are used for accessing files. Before using a file, you have to declare a pointer to it, so that it can be referred to with the functions. You do this with, for example, FILE *in_file; FILE *out_file; where "FILE" is a type defined in (it is usually a complicated structure of some sort). To open a file, you do, for example: in_file = fopen("input_file.dat", "r"); out_file = fopen("output_file.dat", "w"); Note the use of "r" to indicate read access, and "w" to indicate write access. The following modes are available: "r" read "w" write (destroys any existing file with the same name) "rb" read a binary file "wb" write a binary file (overwriting any existing file) "r+" opens an existing file for random read access, and writing to its end "w+" opens a new file for random read access, and writing to its end (destroys any existing file with the same name) Once the file is opened, you can use the following functions to read/write it:
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C programming notes - School of Physics

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