How to use strtok()

Question

I'm writing a C program to study the usage of function strtok(). Here is my code:

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

main() {
    char abc[100] = "ls &";
    char *tok;

    tok = strtok(abc, " ");
    while (tok != NULL) {
        printf("%s", tok);
        tok = strtok(NULL, " ");
    }
    printf("\n\n\n\n\n%s", tok);
    return 0;
}

It is printing the following output:

ls&




(null)

But I want it to print & at the second printf statement. How do I do it? I need this part for my homework project.

Answer 1

1. Make sure you can identify the limits of what you print when you're printing.
2. Output newlines at the end of printed messages; the information is more likely to appear in a timely manner if you do that.
3. Don't print NULL pointers as strings; not all versions of printf() will behave nicely — some of them dump core.

Code:

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

int main(void)
{
    char abc[] = "ls &";
    char *tok;
    char *ptr = abc;

    while ((tok = strtok(ptr, " ")) != NULL)
    {
        printf("<<%s>>\n", tok);
        ptr = NULL;
    }
    return 0;
}

Or (optimized, courtesy of self.):

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

int main(void)
{
    char abc[] = "ls &";
    char *tok = abc;

    while ((tok = strtok(tok, " ")) != NULL)
    {
        printf("<<%s>>\n", tok);
        tok = NULL;
    }
    return 0;
}

Output:

<<ls>>
<<&>>

You can choose your own marker characters, but when not messing with XML or HTML, I find the double angle brackets reasonably good for the job.

You can also use your loop structure at the cost of writing a second call to strtok() (which is a minimal cost, but might be argued to violate the DRY principle: Don't Repeat Yourself):

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

int main(void)
{
    char abc[] = "ls &";
    char *tok = strtok(abc, " ");

    while (tok != NULL)
    {
        printf("<<%s>>\n", tok);
        tok = strtok(NULL, " ");
    }
    return 0;
}

Same output.

---

Revised requirement

I would like to add a printf() statement outside the while loop and print '&' outside. I need it since I want to compare it later with another variable in the program. Is there any way to do so?

Yes, there is usually a way to do almost anything. This seems to work. It also works sanely if there are more tokens to parse, or if there's only the & to parse, or if there are no tokens. Clearly, the body of the outer loop could be made into a function if you so wished; it would be sensible to do so, even.

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

int main(void)
{
    char tests[][16] =
    {
        "ls -l -s &",
        "ls &",
        "&",
        "    ",
        ""
    };

    for (size_t i = 0; i < sizeof(tests)/sizeof(tests[0]); i++)
    {
        printf("Initially: <<%s>>\n", tests[i]);
        char *tok1 = strtok(tests[i], " ");
        char *tok;

        while ((tok = strtok(NULL, " ")) != NULL)
        {
            printf("Loop body: <<%s>>\n", tok1);
            tok1 = tok;
        }
        if (tok1 != NULL)
            printf("Post loop: <<%s>>\n", tok1);
    }

    return 0;
}

Output:

Initially: <<ls -l -s &>>
Loop body: <<ls>>
Loop body: <<-l>>
Loop body: <<-s>>
Post loop: <<&>>
Initially: <<ls &>>
Loop body: <<ls>>
Post loop: <<&>>
Initially: <<&>>
Post loop: <<&>>
Initially: <<    >>
Initially: <<>>

Note how the markers pay for themselves in the last two examples. You couldn't tell those apart without the markers.