CODEWITHSUNDEEP
c
Angus
Angus

Reputation: 12621

Using strtok to parse the string into words

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

int main(){
        char str[] = "This is a sample string";
        char *pch;
        char piece1[10] = " ";
        char piece2[10] = " ";

        printf("\n %s \n",str);
        pch = strtok(str," ");
        strcpy(piece1,pch);
        printf("\n piece1 : %s \n",piece1);
        while(pch != NULL){
                printf("\n %s \n",pch);
                pch = strtok(NULL," ");
                if(pch != NULL){
                        strcpy(piece2,pch);
                        printf("\n piece2 : %s \n",piece2);
                }
        }
        printf("\n str: %s \n",str); //prints only "This" and not the original string
        return 0;
}

I could not understand the second strtok called with NULL as 1st argument. When strtok is called with the str it splits the first word. what does the strtok(NULL," ") , do on NULL?

When I printer str, it prints the first word and not the original string it contained before tokenising.

Upvotes: 0

Views: 2587

Answers (3)

adityajones
adityajones

Reputation: 601

what strtok does behind the scenes is put \0 wherever the delimiter character is found. This is why you pass in NULL the subsequent times after the first call to strtok (it keeps track of its next starting positional state).

When you print out str it is still a pointer to the start of your original string to tokenize but the first delimiter is now a \0. printf only prints up to a \0.

Upvotes: 2

Charlie Burns
Charlie Burns

Reputation: 7044

From man strtok(), see the bold highlight.

The strtok() function is used to isolate sequential tokens in a null-ter- minated string, str. These tokens are separated in the string by at least one of the characters in sep. The first time that strtok() is called, str should be specified; subsequent calls, wishing to obtain fur- ther tokens from the same string, should pass a null pointer instead. The separator string, sep, must be supplied each time, and may change between calls.

So, it changes the string in place and it keeps a variable between calls.

Upvotes: 0

Joe
Joe

Reputation: 42627

strtok changes the input string (which is why you can't use a const/read-only string as input). Basically, at each break, it replaces the separator with \0 to terminate it.

It also stores some state globally, which is how it knows where the next item starts.

If you want to know where each word is, you need to keep track of each pointer returned from each strtok call.

Upvotes: 1

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