C - Weird problems when reading a .csv file into a 2d array of strings

Question

I have a large .csv data file I am trying to read into a multidimensional array (each value in the .csv file is a string) but I am having some unknown problems with the tokenization. My while loop used for tokenizing each line is as follows

while (!feof (st))
{
    fgets (row, CHAR, st);
    token = strtok (row, ",");
    
    while (token != NULL)
    {
        for (int row = 0; row < ROWS; row ++)
            for (int col = 0; col < COLS; col ++)
                strcpy (data [row][col], token);
        token = strtok (NULL, ",");
    }
}

What it does is until the end of the file, it gets the next line in the .csv file, detects the comma which separates each value, and then for each value from that line, it places it into the next empty spot in the array. It then repeats the process for each row.

However when I test print the data array, rather than being filled with the data from the .csv file, it is filled with "1" which is the very last value in the .csv file (in the last column of the last row of the file). On top of that, the array is full of empty space and garbage at the bottom. I'm not even sure how that is possible as I declared the exact size of the array from the beginning. (I also made a bit of code at the beginning of the program to clean the .csv file to remove extra rows from the bottom of the program.)

Can anybody guide me on the right direction? I feel like I am very close as when I print the tokenized values rather than attempting to place them into the array, I am able to see all the values correctly printed out.

To begin with, your usage of `while (!feof (st))` is [wrong](https://stackoverflow.com/questions/5431941/why-is-while-feoffile-always-wrong). You should check if reading succeeded after trying to read and before using what are "read". — MikeCAT, Mar 22 '23 at 17:33
*when I test print the data array ... it is filled with "1" which is the very last value in the .csv file.* You appear to be copying each token to every array element. — Weather Vane, Mar 22 '23 at 17:38
Your inner `for` loops replicate the `while (!feof())` and `while (token!=NULL` loop. The first loops over `row` and the latter iterates over `col`. You should not mix 2 different loop mechanisms to iterate over the same data in nested loops. — Gerhardh, Mar 22 '23 at 17:51
The inner `for` loops are copying the same token into every array element. When the program is done, they'll all contain the last token in the file. — Barmar, Mar 22 '23 at 18:40

score 1 · Accepted Answer · edited Mar 22 '23 at 20:49

You're reading a token with ftok(), then looping through all the array elements, copying the token into each element. So at the end, they all contain the last token. You should just copy each token into a single element.

int rownum = 0;
while (fgets (row, CHAR, st))
{
    int colnum = 0;
    token = strtok(row, ",\n");
    
    while (token != NULL)
    {
        strcpy(data [rownum][colnum++], token);
        token = strtok (NULL, ",\n");
    }
    rownum++;
}

Other issues:

Don't use !feof() as the while condition. Test the result of fgets() instead. See Why is “while( !feof(file) )” always wrong?
The string read by fgets() includes the newline terminator. Either remove this first (see Removing trailing newline character from fgets() input) or include it in the delimiter string in strtok() so you don't copy it into the array.
Avoid reusing variable names. You used row for both the line read from the file and the index into the array. While they don't actually interfere in your code because they're in different scopes, it's confusing to readers. When I refactored the loops to move int row = 0; out of the loop, they ended up conflicting, so I renamed them to rownum and colnum.

Thank you for your help, I see what I did wrong now. I appreciate it! — Jacob Mohamed, Mar 23 '23 at 01:08

C - Weird problems when reading a .csv file into a 2d array of strings

1 Answers1