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String

Particular in C, there is no string type, we use char array following \0 to represent a string or more officially, every string (string literal included) has a explicit \0 at the end. If we use pointer to present, then is a string literal since it call the global value and we can not do editing for those globals.

Generally, we can define string in multiple ways:

char arr[] = "abcd"; // Autometically assign size for 'a', 'b', 'c', 'd', '\0'
char arr[5] = {'a', 'b', 'c', 'd', '\0'}; // add null termination by ourself
char arr[5] = "abcd"; // Autometically assign '\0' to extra space
char *arr = malloc(sizeof(char)*5);
strcpy(arr, "abcd"); // or we can use strncpy(arr, "abcd", 4); and assign arr[4] = '\0';
// A bad example, not a string
char arr[4] = {'a', 'b', 'c', 'd'}; // Obviously a char array
char arr[4] = "abcd"; // no extra space for this to assign '\0'
char *arr = "abcd"; // This is a string literal where read global constant, which we can't assess to  change

size_t strlen(const char*s): return the size of string in size_t type (long int) do not count \0

char *strchr(const char*s, int c): return NULL if no such character chr(c) found in s. Else return the pointer to this character.

char *strstr(const char*s, const char*want_to_find): return NULL if no such substring want_to_find found in s. Else return the pointer to this substring. (finding ignore \0)

Convert​

long strtol(const char* s, char ** endptr, int base): read s, 'till the first none empty, then read the number(if exists), explain it by the given base, for example "100" in base 10 is 100 DEC, in 16 is 256 DEC. And leave the remain s let endptr point to (endptr!=NULL). Return the explained number

Copying & Connecting​

char *strcpy(char* d, char* s): (d should has at leastsizeof(s) +1 size) overwrite d by s, return the pointer to d. Notice, \0 is copying from s, so choose a good is necessary.

char *strncpy(char* d, char* s, size_t n): Similarly, overwrite d by s stop at n, return the pointer to d. (Notice, there is no \0 if s don't have at n index. if \0 is found and exists extra space of d, then fill all extras \0)

char* strcat(char*d, const char *s): (d should have enough space for the result) concatenates the s with d (d+c in python) . s start from the index of \0 in d (replaced), and will add \0 at the end, return a pointer to the resulting string.

char* strncat(char*d, const char *s, size_t n): Similarly, but this time with limit of concatenation. If sizeof(s) < n, then connection stop after \0 of s. Else, add \0 after concatenating n characters. Return a pointer to the resulting string.

Comparing​

int strcmp(const char*s1, const char*s2): Compare each character 'till the different or \0, return int

  • if >0, the first different of s1 is less than s2
  • if =0, s1 is equal to s2
  • if <0, the first different of s2 is less than s1

int strncmp(const char*s1, const char*s2, size_t n): Similarly, but only compare first n character