problem with CSV input

This is a discussion on problem with CSV input within the C Programming forums, part of the General Programming Boards category; I am an extreme newbie to C and am trying to write a "simple" program that opens a csv text ...

  1. #1
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    problem with CSV input

    I am an extreme newbie to C and am trying to write a "simple" program that opens a csv text file of integers, parses them, and then sums them. I wrote what I thought should work, but it does not actually retrieve the integer values to sum. The only token it reads is the filename. Could someone help????

    Here is what I have.



    Code:
    #include <stdio.h>
    
    #include <stdlib.h>
    
    #include <string.h>
    
    int main()
    
    {
    
       int x;
    
       int sum;
    
       char filename[] = "data.txt";
    
       FILE *file = fopen(filename,"r");
    
       char *ptr;  
    
       ptr = strtok(filename, ",");
    
       sum = 0;
    
       if (file == NULL)
    
          {
    
          printf("File could not be opened.\n");
    
          getchar ();
    
          }
    
       else
    
       {
    
           while (ptr != NULL)
    
           {
    
           x = atoi(ptr);
    
           printf("The string is: %d\n", ptr);
    
           
    
           sum = sum + x;
    
           ptr = strtok(NULL, ",");
    
           }
    
           fclose(file);
    
       }         
    
       printf ("The sum of the numbers is: %d\n",sum);
    
       getchar ();
    
    }

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    So, strtok doesn't read from a file. Maybe fgets is what you're thinking of?

  3. #3
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    that could be why it doesn't read the values. I know nothing about C and am constructing the code from examples I find...still trying to understand the structure and everything. I have been trying to figure out how to use fgets, but have not had any success.

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    So you should read what it does. You can go to cplusplus.com (despite the name), or you can type "man fgets" into your shell/Google, depending on your OS.

  5. #5
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    yeah...been reading up on it, but trying to decipher the language...looking at a couple examples, too

  6. #6
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    this might help.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    void read_csv_line(char *s);
    
    int main()
    {
        char s[80];
        FILE *fp = fopen("test.txt", "r");
        while (fgets(s, 80, fp)) read_csv_line(s);
        fclose(fp);
        return EXIT_SUCCESS;
    }
    
    void read_csv_line(char *s)
    {
        char *t = strtok(s, ",");
        while (t)
        {
            printf("%s\n", t);
            t = strtok(NULL, ",");
        }
    }

  7. #7
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    That helped a lot. Is there any way to do the same thing without restricting the size? Ultimately it's supposed to be able to handle any file without knowing how large the data set is.
    Last edited by cmiller4; 02-05-2009 at 09:56 AM.

  8. #8
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    If you are referring to the number 80 - it is the line length not the file size. The solution given by Meldreth is unassuming about the file size.

  9. #9
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    the only limit in my example was the line size. the number of lines in the file is unbound. you can remove the line limit, but it's harder because you have to make a growing array and that means using your own input function.
    Code:
    #define THRESHOLD 16
    
    char *getline(FILE *fp)
    {
        char *s = NULL;
        int sz = 0, cap = 0;
        char c;
        while ((c = fgetc(fp)) != EOF)
        {
            if (sz == cap)
            {
                cap += THRESHOLD;
                s = realloc(s, cap+1);
            }
            if (c == '\n') break;
            s[sz++] = c;
        }
        if (s) s[sz] = '\0';
        return s;
    }
    be careful with that one though, i wrote it fast just now so it might be buggy and it definitely wastes memory.

  10. #10
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    Thanks...I'll play around with it. I'm not too worried about memory size right now, since I'm just learning and my primary objective is trying to figure out how to make it work. Efficiency is phase II.

  11. #11
    CSharpener vart's Avatar
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    s = realloc(s, cap+1);

    FAQ explains why this could be a bad idea - if realloc fails the original pointer is lost

    also there is no check for return value of realloc at all

    and fgetc returns int - so c should be declared as int - to distinguish EOF and 0xFF character
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  12. #12
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    Quote Originally Posted by cmiller4 View Post
    Thanks...I'll play around with it. I'm not too worried about memory size right now, since I'm just learning and my primary objective is trying to figure out how to make it work. Efficiency is phase II.
    Assuming you are not trying to write a CSV reader to take ALL possible CSV files (and your current code doesn't look like it handles for examle quotes), then I'd suggest you just increase the maximum length of a line to some large value, e.g. 8000 - you can check if fgets read the whole line by checking if the last item is a newline:
    Code:
    size_t len;
    fgets(str, ...); 
    len = strlen(str);
    if (str[len-1] == '\n')  
        // We got a whole line
    else
       // Line too long to fit in buffer.
    Trying to cope with extremely long lines when a "large enough" value can be found is a waste of effort.

    Sure, if you are writing a commercial piece of software to handle CSV files, you obviously should cope with extremely long lines (and it may actually be better to read a chunk of data into a buffer and scanning each character in that case, rather than relying on strtok).

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  13. #13
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    Smile

    Thanks so much for all of the valuable information. I am definitely learning more about this process...the books I have don't speak layman's English, so you all have been a huge help.

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