NULL values and "0" strings in C?

Question

I'm having trouble adding a "0" string to a list of strings in C.

With my current code, I'm having trouble distinguishing between a NULL value and a "0" with my current code. I have tried to debug with the if statement (if(pch == "0")) but when I'm outputting it doesn't go into the if statement.

          printf("%c\n",next_line[0]);
          pch = strtok(next_line,",");
          printf("%s\n",pch);
          if(pch == "0"){
            printf("NULL VALUE");
            strcpy(strs[i],pch);
            i++;
            pch = strtok(NULL, ",");
          }
          while(pch != NULL){
            strcpy(strs[i],pch);
            i++;
            pch = strtok(NULL, ",");
          }
          //go on to print values

the input is: 0,1,03:48:13,9 I never make it past the first character. What can I do to keep my code? Can I maybe change

     pch = strtok(NULL, ","); 
    to pch = strtok(SOMEOTHERVALUE, ",");?

I want to copy the comma separated values above ( 0,1,03:48:13,9) into an array (strs). I'm having trouble with CSVs that start with 0. CSV's that don't have 0s (which equal NULL in C) work fine. — Colin Fausnaught
– Colin Fausnaught, Commented Mar 26, 2015 at 5:22
Why would you compare with the literal digit "0" if you're checking for a null value? It's impossible for us to figure out what's wrong with your code because you've given us no idea what it's supposed to do. It seems you want to check for a NULL (no string at all), and then copy? If there's no string because the pointer is NULL, what's to copy? — David Schwartz
– David Schwartz, Commented Mar 26, 2015 at 5:29
Hint: what does == do? It tells you if two things are equal. What is pch? It's a pointer to a part of the line. What is "0"? It's a pointer to a constant string "0". What happens when you compare pointers with ==? — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Mar 26, 2015 at 6:44
This question has nothing to do with NULL as far as I can tell. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Mar 26, 2015 at 6:44

Karthikeyan.R.S · Accepted Answer · 2015-03-26 05:26:40Z

2

You cannot able to check like this,

if(pch == "0"){
 ...
}

pch is pointer. You are comparing that with a string. So you have to make that as like this,

if ( *pch == '0' ) { 
 ...
}

Or else do like this,

if ( strcmp(pch,"0") == 0 ) {
 ...
}

It will help you to check the zero in a character string.

answered Mar 26, 2015 at 5:26

Karthikeyan.R.S

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2 Comments

Colin Fausnaught Over a year ago

Ah I see... Thank you. Now when I'm adding that to the array of strings should I use the * notation for pch too?

Karthikeyan.R.S Over a year ago

if you are using strcpy then there is no need to give that.

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