Strings in the C/C++ Standard Library

Standard library in C

The C standard library operates on character arrays

strlen

int strlen(const char *str): returns the number of characters from str[0] to '\0'. Note that when O2 optimization is not used, the time complexity of this operation in the loop condition is \Theta(N) .

printf

printf("%s", s): uses %s to output a string (character array).

scanf

scanf("%s", s): uses %s to read a string (character array).

sscanf

sscanf(const char *__source, const char *__format, ...): reads variables from the string __source, such as sscanf(str,"%d",&a).

sprintf

sprintf(char *__stream, const char *__format, ...): outputs the content in the __format string to __stream, such as sprintf(str,"%d",i).

strcmp

int strcmp(const char *str1, const char *str2): compares str1 and str2 in lexicographic order. If str1 is lexicographically smaller, the function returns a negative value; if it's the same, the function returns 0; otherwise, it returns a positive value. Please note that don't simply assume that there are only three types which are 0, 1, -1. The values returned on different platforms all follow the positive/negative rule, but they are not necessarily all 0, 1, -1.

strcpy

char *strcpy(char *str, const char *src): copies the characters in src to str. str and src are all head pointers of the char arrays, and the value returned is str containing null terminator '\0'.

strncpy

char *strncpy(char *str, const char *src, int cnt): copies at most cnt characters to str. If src terminates and the number is smaller than cnt, write null characters to str until a total of cnt characters are written.

strcat

char *strcat(char *str1, const char *str2): concatenates str2 to the end of str1, replaces '\0' at the end of str1 with *str2, and return str1.

strstr

char *strstr(char *str1, const char *str2): if str2 is a substring of str1, the function returns the address of the first occurrence of str2 in str1; if str2 is not a substring of str1, it returns NULL.

strchr

char *strchr(const char *str, int c): the function finds and returns the position where the character c first appears in the string str. If the character is not found, it returns NULL.

strrchr

char *strrchr(const char *str, char c): the function finds and returns the position where the character c last appears in the string str. If the character is not found, it returns NULL.

C++ standard library

The C++ standard library operates on string objects and also provides compatibility with character arrays.

std::string

  • The right side of the assignment operator = can be const string/string/const char*/char* 。
  • The access operator [cur] returns a reference to the position of cur.
  • The access function data()/c_str() returns a const char* pointer with the same content as string.
  • The capacity function size() returns the number of characters in a string.
  • There are other functions like find() for finding and returning the position of the character.
  • std::string overloads the comparison logic operator, and the time complexity is \Theta(N) .
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