CODEWITHSUNDEEP
c
loisir2022
loisir2022

Reputation: 163

Use strtok in a for loop

To loop over a string str I used: 

for (tok = strtok(str, ";"); tok && *tok; tok = strtok(NULL, ";"))  
{  
   //do stuff  
}

I would like to understand how this loop works. It seems to me: 

(1) tok = strtok(str, ";"); //initialization of tok with the first token in str    
(2) tok = strtok(NULL, ";"); // go to the next token in str? how does this work? 
(3) tok && *tok;  //this stops the loop when tok =NULL or *tok=NULL

I would appreciate your help!

Upvotes: 7

Views: 4848

Answers (4)

SheetJS
SheetJS

Reputation: 22925

Here's a sample strtok implementation: http://bxr.su/o/lib/libc/string/strtok.c#strtok

As you see in the code, it uses a static character pointer internally (pretty much every version I've seen store a pointer, either as a global variable or as a static variable as in the case above). This version calls the reentrant strtok_r (and the side effect of the line if (s == NULL && (s = *last) == NULL) is to use the last pointer if NULL is passed)

Upvotes: 4

jh314
jh314

Reputation: 27802

You can find more about strtok here. It has some examples of how to use it.

Quoting from the link, the str parameter in strtok(str, delim):

Notice that the contents of this string are modified and broken into smaller strings (tokens). Alternativelly, a null pointer may be specified, in which case the function continues scanning where a previous successful call to the function ended.

Your 3 guesses are correct.

Upvotes: -1

Code-Apprentice
Code-Apprentice

Reputation: 83557

(2) tok = strtok(NULL, ";"); // go to the next token in str? how does this work?

That's exactly how strtok() works. By sending NULL as the first parameter, you signal that strtok() should continue with the string which was sent to it during the last call. If you want to know the exact implementation details, you will need to look at the source code for strtok(). Most likely it uses a static local variable.

Upvotes: 1

DevZer0
DevZer0

Reputation: 13535

if you read the manpages for strtok it states

The strtok() function parses a string into a sequence of tokens. On the first call to strtok() the string to be parsed should be specified in str. In each subsequent call that should parse the same string, str should be NULL.

Upvotes: -1

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