Tokenizing strings in C

Question

I have been trying to tokenize a string using SPACE as delimiter but it doesn't work. Does any one have suggestion on why it doesn't work?

Edit: tokenizing using:

strtok(string, " ");

The code is like the following

pch = strtok (str," ");
while (pch != NULL)
{
  printf ("%s\n",pch);
  pch = strtok (NULL, " ");
}

Answer 1

Do it like this:

char s[256];
strcpy(s, "one two three");
char* token = strtok(s, " ");
while (token) {
    printf("token: %s\n", token);
    token = strtok(NULL, " ");
}

Note: strtok modifies the string its tokenising, so it cannot be a const char*.

Answer 2

Here's an example of strtok usage, keep in mind that strtok is destructive of its input string (and therefore can't ever be used on a string constant

char *p = strtok(str, " ");
while(p != NULL) {
    printf("%s\n", p);
    p = strtok(NULL, " ");
}

Basically the thing to note is that passing a NULL as the first parameter to strtok tells it to get the next token from the string it was previously tokenizing.

Answer 3

strtok can be very dangerous. It is not thread safe. Its intended use is to be called over and over in a loop, passing in the output from the previous call. The strtok function has an internal variable that stores the state of the strtok call. This state is not unique to each thread - it is global. If any other code uses strtok in another thread, you get problems. Not the kind of problems you want to track down either!

I'd recommend looking for a regex implementation, or using sscanf to pull apart the string.

Try this:

char strprint[256];
char text[256];
strcpy(text, "My string to test");
while ( sscanf( text, "%s %s", strprint, text) > 0 ) {
   printf("token: %s\n", strprint);
}

Note: The 'text' string is destroyed as it's separated. This may not be the preferred behaviour =)

Answer 4

I've made some string functions in order to split values, by using less pointers as I could because this code is intended to run on PIC18F processors. Those processors does not handle really good with pointers when you have few free RAM available:

#include <stdio.h>
#include <string.h>

char POSTREQ[255] = "pwd=123456&apply=Apply&d1=88&d2=100&pwr=1&mpx=Internal&stmo=Stereo&proc=Processor&cmp=Compressor&ip1=192&ip2=168&ip3=10&ip4=131&gw1=192&gw2=168&gw3=10&gw4=192&pt=80&lic=&A=A";

int findchar(char *string, int Start, char C) {
    while((string[Start] != 0)) { Start++; if(string[Start] == C) return Start; }
    return -1;
}

int findcharn(char *string, int Times, char C) {
   int i = 0, pos = 0, fnd = 0;

    while(i < Times) {
       fnd = findchar(string, pos, C);
        if(fnd < 0) return -1;
        if(fnd > 0) pos = fnd;
       i++;
   }
   return fnd;
}

void mid(char *in, char *out, int start, int end) {
    int i = 0;
    int size = end - start;

    for(i = 0; i < size; i++){
        out[i] = in[start + i + 1];
    }
    out[size] = 0;
}

void getvalue(char *out, int index) {
    mid(POSTREQ, out, findcharn(POSTREQ, index, '='), (findcharn(POSTREQ, index, '&') - 1));
}

void main() {
   char n_pwd[7];
   char n_d1[7];

   getvalue(n_d1, 1);

   printf("Value: %s\n", n_d1);
} 

Answer 5

You can simplify the code by introducing an extra variable.

#include <string.h>
#include <stdio.h>

int main()
{
    char str[100], *s = str, *t = NULL;

    strcpy(str, "a space delimited string");
    while ((t = strtok(s, " ")) != NULL) {
        s = NULL;
        printf(":%s:\n", t);
    }
    return 0;
}

Answer 6

When reading the strtok documentation, I see you need to pass in a NULL pointer after the first "initializing" call. Maybe you didn't do that. Just a guess of course.

Answer 7

Here is another strtok() implementation, which has the ability to recognize consecutive delimiters (standard library's strtok() does not have this)

The function is a part of BSD licensed string library, called zString. You are more than welcome to contribute :)

https://github.com/fnoyanisi/zString

char *zstring_strtok(char *str, const char *delim) {
    static char *static_str=0;      /* var to store last address */
    int index=0, strlength=0;       /* integers for indexes */
    int found = 0;                  /* check if delim is found */

    /* delimiter cannot be NULL
    * if no more char left, return NULL as well
    */
    if (delim==0 || (str == 0 && static_str == 0))
        return 0;

    if (str == 0)
        str = static_str;

    /* get length of string */
    while(str[strlength])
        strlength++;

    /* find the first occurance of delim */
    for (index=0;index<strlength;index++)
        if (str[index]==delim[0]) {
            found=1;
            break;
        }

    /* if delim is not contained in str, return str */
    if (!found) {
        static_str = 0;
        return str;
    }

    /* check for consecutive delimiters
    *if first char is delim, return delim
    */
    if (str[0]==delim[0]) {
        static_str = (str + 1);
        return (char *)delim;
    }

    /* terminate the string
    * this assignmetn requires char[], so str has to
    * be char[] rather than *char
    */
    str[index] = '\0';

    /* save the rest of the string */
    if ((str + index + 1)!=0)
        static_str = (str + index + 1);
    else
        static_str = 0;

        return str;
}

As mentioned in previous posts, since strtok(), or the one I implmented above, relies on a static *char variable to preserve the location of last delimiter between consecutive calls, extra care should be taken while dealing with multi-threaded aplications.

Answer 8

int not_in_delimiter(char c, char *delim){

    while(*delim != '\0'){
            if(c == *delim) return 0;
            delim++;
    }
    return 1;
}

char *token_separater(char *source, char *delimiter, char **last){

char *begin, *next_token;
char *sbegin;

/*Get the start of the token */
if(source)
  begin = source;
else
  begin = *last;

sbegin = begin;

/*Scan through the string till we find character in delimiter. */
while(*begin != '\0' && not_in_delimiter(*begin, delimiter)){
       begin++;
}

/* Check if we have reached at of the string */
if(*begin == '\0') {
/* We dont need to come further, hence return NULL*/
   *last = NULL;
    return sbegin;
}
/* Scan the string till we find a character which is not in delimiter */
 next_token  = begin;
 while(next_token != '\0' && !not_in_delimiter(*next_token, delimiter))    {
    next_token++;
 }
 /* If we have not reached at the end of the string */
 if(*next_token != '\0'){
  *last = next_token--;
  *next_token = '\0';
   return sbegin;
}
}

 void main(){

    char string[10] = "abcb_dccc";
    char delim[10] = "_";
    char *token = NULL;
    char *last = "" ;
    token  = token_separater(string, delim, &last);
    printf("%s\n", token);
    while(last){
            token  = token_separater(NULL, delim, &last);
            printf("%s\n", token);
    }

}

You can read detail analysis at blog mentioned in my profile :)