Read a table file to arrays in C-CodePudding

In Matlab is one command, but I need to write some simple function in C (or C actually, but it doesn't matter). My goal now is to create three "double" arrays, that are taken form a table file. The file looks like this (QEOS.txt):

T rho P
11.605 2.51188643150958e-13 4.28547303990553e-16
11.605 3.16227766016838e-13 4.28547303990553e-16
11.605 3.98107170553497e-13 4.28547303990553e-16
11.605 5.01187233627271e-13 4.28547303990553e-16
11.605 6.30957344480194e-13 4.28547303990553e-16
11.605 7.94328234724282e-13 4.28547303990553e-16
11.605 1e-12 4.28547303990553e-16
11.605 1.25892541179417e-12 4.28547303990553e-16
14.6098294038713 2.51188643150958e-13 5.45304484892599e-16
14.6098294038713 3.16227766016838e-13 5.45304484892599e-16
14.6098294038713 3.98107170553497e-13 5.45304484892599e-16
14.6098294038713 5.01187233627271e-13 5.45304484892599e-16
14.6098294038713 6.30957344480194e-13 5.45304484892599e-16
...........

And I want the first column to become a variable "T", the second to be "rho" and the third "P". Shouldn't be hard at all, right? So I have:

#include <stdio.h>
#include <stdlib.h>
 
struct threeNum{
    double n1, n2, n3;
};
 
int main(){
    int n;
    struct threeNum num;
    double P,T,rho;
    FILE *fptr;
    fptr = fopen("QEOS.txt","r");
     for(n = 1; n < 25;   n){
         fread(&num, sizeof(struct threeNum), 1, fptr);
         printf("n1: %f \tn2: %f \tn3: %f  \n", num.n1, num.n2, num.n3);
     }
     fclose(fptr);
     
     T=num.n1;
     rho=num.n2;
     P=num.n3;
     return 0;
}

The "25" is just ab example. In any case, the code prints 0.000's and random garbage. What's wrong? P.S: And I get "** stack smashing detected **: terminated Aborted (core dumped)"

CodePudding user response：

As pointed out in the comments you must use text based functions like fscanf, as opposed to binary ones such as fread, when reading formatted text.

You should always test the return values of library functions as most of them can fail with dramatic results (program crashes or misleading output).

Note that

T=num.n1;
rho=num.n2;
P=num.n3;

has no effect in this program.

Suggestion: use fgets and sscanf for more control, reading line by line.

A quick example:

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

int main(void) {
    double P, T, rho;
    FILE *fp = fopen("QEOS.txt", "r");

    if (!fp) {
        perror("fopen");
        return EXIT_FAILURE;
    }

    char line[512];

    if (!fgets(line, sizeof line, fp) || !strstr(line, "T rho P")) {
        fprintf(stderr, "Invalid or missing header.\n");
        return EXIT_FAILURE;
    }

    while (fgets(line, sizeof line, fp)) {
        if (3 == sscanf(line, "%lf%lf%lf", &T, &rho, &P)) {
            printf("n1: %f\tn2: %e\tn3: %e\n", T, rho, P);
        }
    }

    fclose(fp);
}