C-tutorial - Part II

1. C-tutorial - Part II Prakash Linga linga@cs.cornell.edu 4110 Upson

2. All about Strings! • Array of characters (chars) • Definition: char *str; • str is a pointer to a memory location which stores a character (the first in the string). • How we know the end of the string? The character \0 (null character) indicates the end of the string. • Where on earth is the string stored?

3. Strings… • In memory right! • So allocate memory to store a string char *str = (char *) malloc(sizeof(char)*MAXSTRLEN); • How do I store the string now? Brute-force: Store character by character. Better Approach: Use strcpy Note: What follows is mainly from SunOS man pages.

4. Sssss • strcpy strcpy(char*destn, char* source); #include <string.h> char *str = (char *) malloc(sizeof(char)*MAXSTRLEN); strcpy(str, “That was easy”); • What is the length of my string? Use strlen: int len = strlen(str);

5. Strings in Java • Strings are Objects • Creating a String is a call to one of the constructors. String str = new String(“This is ridiculously easy! Why can’t I do this in C?”); • Host of methods in the String class: length, trim, compareTo, substring, charAt, concat, indexOf , endsWith, replace to name a few.

6. Back to the Obscure World! • We have a host of functions for string manipulation in Ctoo: strcpy, strncpy, strlcpy strcat, strncat, strlcat strcmp, strncmp, strcasecmp, strncasecmp, strchr, strrchr, strstr, strlen strtok, strtok_r, strcspn, strspn, strdup, strpbrk

7. Copies char *strcpy(char *s1, const char *s2); char *strncpy(char *s1, const char *s2, size_t n); size_t strlcpy(char *dst, const char *src, size_t dstsize); strncpy copies over n bytes from the source to the destination (many questions here!!!???) strlcpy copies at most dstsize-1 characters from src to dst, truncating src if necessary. Result is always null terminated. (again many small questions!)

8. Cats • Cats are similar to copies! char *strcat(char *s1, const char *s2); char *strncat(char *s1, const char *s2, size_t n); size_t strlcat(char *dst, const char *src, size_t dstsize); Any interesting questions here? strncat: result is \0 terminated (always). This is not the case with strncpy

9. Compares intstrcmp(const char *s1, const char *s2); intstrncmp(const char *s1, const char *s2, size_t n); strcasecmp and strncasecmp are case insensitive versions of strcmp. Return values for strcmp: Returns 0 if equal; 1 if s1 > s2 and –1 otherwise. Note: Use #include<strings.h> for strcasecmp and strncasecmp

10. strchr, strrchr and strstr char *strchr(const char *s, int c); char *strrchr(const char *s, int c); char *strstr(const char *s1, const char *s2); strchr Return a pointer to the first occurrence of c in s strrchr Return a pointer to the last occurrence of c in s strstr Return a pointer to the first occurrence of string s2 in s1 (any tricky case?)

11. Tok’s char *strtok(char *s1, const char *s2); char *strtok_r(char *s1, const char *s2, char **lasts); The strtok() function can be used to break the string pointed to by s1 into a sequence of tokens, each of which is delimited by one or more characters from the string pointed to by s2. The first call (with pointer s1 specified) returns a pointer to the first character of the first token, and will have written a null character into s1 immediately following the returned token. The function keeps track of its position in the string between separate calls, so that subsequent calls (which must be made with the first argument being a null pointer) will work through the string s1 immediately following that token.

12. Strtok example #include <string.h> int main () { char str[100]; char * tmp; strcpy(str, "This is a sample string,just testing."); printf ("Splitting \"%s\" into tokens:\n",str); tmp = strtok (str," ,."); while (tmp != NULL) { printf ("%s\n",tmp); tmp = strtok (NULL, " ,."); } return 0; }

13. Output of tok program Splitting "This is a sample string,just testing." into tokens: This is a sample string just testing

14. Same example with strtok_r #include <string.h> int main () { char str[100]; char * tmp, *last; strcpy(str, "This is a sample string,just testing."); printf ("Splitting \"%s\" into tokens:\n",str); tmp = strtok_r (str," ,. " , &last); while (tmp != NULL) { printf ("%s\n",tmp); tmp = strtok_r (NULL, " ,. " , &last); } return 0; }

15. strspn and strcspn size_t strcspn(const char *s1, const char *s2); size_t strspn(const char *s1, const char *s2); The strcspn() function returns the length of the initial segment of string s1 that consists entirely of characters not from string s2. The strspn() function returns the length of the initial segment of string s1 that consists entirely of characters from string s2.

16. strdup char *strdup(const char *s1); Make a copy (allocates memory using malloc) of the parameter string and return a pointer to this copy char *strpbrk(const char *s1, const char *s2); The strpbrk() function returns a pointer to the first occurrence in string s1 of any character from string s2, or a null pointer if no character from s2 exists in s1.