从C中的文本文件读取大量数字

Question

I am trying to read a large amount of numerical data (doubles) from a text file. The text file has 10 columns, each with 242 numbers listed in it. The columns are separated by spaces. I am trying to specifically take the first and seventh columns and put them into an array. Currently I am trying this:

int i;
double a;
double b;
double junk;
double array[2][242]

FILE *fp;
fp = fopen("data_table.dat", "w");
for (i = 0; i <= 242; i++);
{
    fscanf(fp, "%f %f %f %f %f %f %f %f %f %f\n", a, junk, junk, junk, junk, junk, b,
    junk, junk, junk);
    array[0][i] = a;
    array[1][i] = b;
}
fclose(fp);

The thought I had was to open the text file, read one row of the doubles and save the first column and seventh column then to the array, with the rest of them being junk. However when I examine the array afterwards every single entry in the array is 6.943e-310, which is not even close to any of the data that is in my data table.

Another thing that might be worth noting is that after I run this the entire text file is empty. I can fill it again with values but then after I run the code it is empty again. Also the first entry in each column is a string of what the data in the column is (for example the first column starts of with length in nm).

Answer 1

You opened the file for writing with the "w" option in fp = fopen("data_table.dat", "w");

You want to open it for reading instead with "r" otherwise it truncates the file, which is why you see nothing in it afterwards.

Answer 2

1. (as @Ranic has posted simultaneously) You want to read from "data_table.dat", but open it with "w". That call truncates the file to size 0. Use "r" instead.
2. You have to pass pointers ( &junk , &a , &b ) to fscanf() instead of values