C provides two functions strtok() and strtok_r() for splitting a string by some delimiter. Splitting a string is a very common task. For example, we have a comma separated list of items from a file and we want individual items in an array.
strtok()
// Splits str[] according to given delimiters. // and returns next token. It needs to be called // in a loop to get all tokens. It returns NULL // when there are no more tokens. char * strtok(char str[], const char *delims);
// A C/C++ program for splitting a string // using strtok() #include <stdio.h> #include <string.h> int main() { char str[] = "Geeks-for-Geeks" ; // Returns first token char * token = strtok (str, "-" ); // Keep printing tokens while one of the // delimiters present in str[]. while (token != NULL) { printf ( "%s\n" , token); token = strtok (NULL, "-" ); } return 0; } |
Output:
Geeks for Geeks
strtok_r()
Just like strtok() function in C, strtok_r() does the same task of parsing a string into a sequence of tokens. strtok_r() is a reentrant version of strtok()
There are two ways we can call strtok_r()
// The third argument saveptr is a pointer to a char * // variable that is used internally by strtok_r() in // order to maintain context between successive calls // that parse the same string. char *strtok_r(char *str, const char *delim, char **saveptr);
Below is a simple C program to show the use of strtok_r() :
// C program to demonstrate working of strtok_r() // by splitting string based on space character. #include <stdio.h> #include <string.h> int main() { char str[] = "Geeks for Geeks" ; char * token; char * rest = str; while ((token = strtok_r(rest, " " , &rest))) printf ( "%s\n" , token); return (0); } |
Output:
Geeks for Geeks
Another Example of strtok :
// C code to demonstrate working of // strtok #include <stdio.h> #include <string.h> // Driver function int main() { // Declaration of string char gfg[100] = " Geeks - for - geeks - Contribute" ; // Declaration of delimiter const char s[4] = "-" ; char * tok; // Use of strtok // get first token tok = strtok (gfg, s); // Checks for delimeter while (tok != 0) { printf ( " %s\n" , tok); // Use of strtok // go through other tokens tok = strtok (0, s); } return (0); } |
Output:
Geeks for geeks Contribute
Practical Application
strtok can be used to split a string in multiple strings based on some separators. A simple CSV file support might be implemented using this function. CSV files have commas as delimeters.
// C code to demonstrate practical application of // strtok #include <stdio.h> #include <string.h> // Driver function int main() { // Declaration of string // Information to be converted into CSV file char gfg[100] = " 1997 Ford E350 ac 3000.00" ; // Declaration of delimeter const char s[4] = " " ; char * tok; // Use of strtok // get first token tok = strtok (gfg, s); // Checks for delimeter while (tok != 0) { printf ( "%s, " , tok); // Use of strtok // go through other tokens tok = strtok (0, s); } return (0); } |
Output:
1997, Ford, E350, ac, 3000.00,
Reference:
1) manual page strtok_r()
2) http://stackoverflow.com/questions/15961253/c-correct-usage-of-strtok-r
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