Thread: strtok and strcmp pointer issues question

It is quite difficult for us to know what's wrong when you've left out a lot of code. Either post a small compilable snippet that demonstrates the problem or go debug it yourself.
A good idea is to use assert and put it everywhere in your program to check for things that should never be. For example, a pointer being NULL when you don't expect it to.
Carefully read the documentation of all functions and see what they take and what they return. Then put asserts to check that what you're giving it is valid and what it returns is valid.


Example:

Code:

       char *mass = NULL;
       float m;
       char *WINGSPAN = NULL;
       float wingspan;
       char *s = NULL;
       float S;

for (i=0;i<=lineno; i++) // Should probably be 0 to lineno - 1.
    {
    assert(array[i] != NULL);
       param[i]=strtok(arra[i],delims);
       param2[i]=strtok(NULL,delims);
    assert(param[i] != NULL);
    assert(param2[i] != NULL);

           printf("param[i] is %s\n",param[i]);
           printf("param2[i] is %s\n",param2[i]);

      if (strcmp("Mass*",param[i])==0) if(mass==NULL) mass=param2[i];
      if (strcmp("Wingspan*",param[i])==0) if (WINGSPAN==NULL) WINGSPAN=param2[i];
      if (strcmp("Wing area*",param[i])==0) if (s==NULL) s=param2[i];
      
    }

m=atof(mass);
printf("Mass is %f kg\n",m); 

wingspan=atof(WINGSPAN);
printf("Wingspan is %f m\n",wingspan);

S=atof(s);
printf("Wing area is %f m2\n",S);

Sorry, didn't mean to leave out that section - there are a lot of parameters so I thought it'd be simpler to just show three, but ended up snipping a bit out that I didn't mean to.

Attempt #2 at posting my code - hopefully has everything needed:

Code:

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

main ()

{
  FILE *parameters;
  char filename[100];  
  int ch=0;
  int lineno=0;
  char line[128]={'\0'};
  char arra[128][3000]={{'\0'}};
  char *mass, *WINGSPAN, *s; 
  mass=WINGSPAN=s=NULL;
  float m,wingspan,S; 

  char *param[40]; 
  char *param2[40];
  
/* Open parameters file */

  printf("Type in the name of the file containing aircraft data, e.g. param.txt\n");
  scanf("%100s",filename);
  parameters=fopen(filename,"r");
if (parameters == NULL)
  {printf("Could not open aircraft parameters file.\n"); }

else
   {

 while ((ch = fgetc(parameters)) != EOF)
 {if (ch == '\n')
  ++lineno;
  }
 fseek(parameters, 0, SEEK_SET);
 i=0;
 while(fgets(line,sizeof line,parameters) !=NULL)
   {strcpy (arra[i],line);
 i++;
   }

/* tokenise by equals sign to find parameters */

char delims[]="="; 

for (i=0;i<=lineno; i++)
	{
	   param[i]=strtok(arra[i],delims);
	   param2[i]=strtok(NULL,delims);

           printf("param[i] is %s\n",param[i]);
           printf("param2[i] is %s\n",param2[i]);
/* these prints are coming out correct */

	  if (strcmp("Mass*",param[i])==0) mass=param2[i];
	  if (strcmp("Wingspan*",param[i])==0) WINGSPAN=param2[i];
	  if (strcmp("Wing area*",param[i])==0) s=param2[i];
/* this is what appears to not be working */
	  
	}


/* converts tokens to float values */
//mass=param2[4];   this is what I don't want to use as it's too inflexible with line numbers
m=atof(mass);
//WINGSPAN=param2[6];
wingspan=atof(WINGSPAN);
//s=param2[8];
S=atof(s);
WL=param2[10];
wl=atof(WL);
// ditto again

printf("Mass is %f\n",m);
printf("Wingspan is %f\n",wingspan);
printf("Wing area is %f\n",S);

}
   return(0);
     
}

Code:

char arra[128][3000]={{'\0'}};

Is this supposed to be 128 lines each 3000 in length, or 3000 lines each 128 in length? If the latter, reverse the numbers.

Your for loop should be "< lineno", unless your final line doesn't necessarily end in a newline, in which case it's as mentioned by Elysia.

You are still printing param*[ ] before you actually see if they're null or not. Try printing out the current arra line before you tokenize it. Oh, and start checking your return values.

There's no point in actually trying to help you if you aren't going to learn from what we're telling you.


Quzah.

I'm perfectly capable of debugging it, and have done, you're right about the size of arra so I'll change that, thanks. That wouldn't be affecting the program at this stage though as the file I'm reading isn't that big.

I'm not sure anyone is understanding my question - I already know at which point it's not working, it's the strcmp bit, from my own checks that I haven't necessarily posted on here. The whole point of my thread was to ask what I was doing wrong at the strcmp stage.

Sorry if this sounds rude, I don't mean it to, but it appears you all think I'm being stupid/asking for help debugging, and that's not it at all. I just want to know how to compare tokenised strings without the program going mad on me :P and thought you might have some experience with it already.

EDIT: well, technically I am asking for help with debugging a small part of the program, but mainly for another method of doing the same thing as that part does.

But "going wrong" doesn't tell us much. We can think of a few reasons it would fail, and we have mentioned them. But as you should know, even though the problem is occurring in one part of the program doesn't mean the bug is in the part of the program. It might as well be elsewhere, and indeed, often it is.

Also, mass is not an array. So if you find "Mass*" twice in your loop, it's going to overwrite what you wrote there the first time.


Quzah.

I think a better approach would be:
Read string + space.
Store in buffer.
Read string + space.
If read string does not begin with '=', copy to buffer and repeat.
If read string begins with '=', we know we've read the entire name, so then compare it to "mass", etc, to see what it is.
Now we expect to see the value it is associated with, so read into an integer. Store the result.
Eat newline.
Repeat.

That's pretty crappy too. Why not just use fgets to read the whole line, then parse it how you need to. You could always use sscanf with something like %[^=] to peel off the first segment, or you could use strchr to find the = sign.


Quzah.

Reading can be done from an existing string, or line, or directly from the file. It doesn't matter, really.