How to read data from a text file and store it in a variable in C language?

Question

I am trying to read a text file in C. The name of the file is test.txt and has the following kind of format.

Nx = 2
Ny = 4
T  = 10

I have written this C code to read the values of Nx, Ny, and T which is 2, 4, and 10 respectively.

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

void main()
{
    double Data[3];    // I'm interested in this information
    char junk1, junk2; // junk variables to avoid first two characters

    FILE * file = fopen("test.txt", "r"); // open file

    for(int i = 0; i < 3; i++) // each loop will read new line of file; i<3 for 3 lines in file
    {
        fscanf(file, "%s %s %lf\n", &junk1, &junk2, &Data[i]); //store info in Data array
        printf("%f\n", Data[i]); // print Data, just to check
    }
    fclose(file);

    int Nx; // store data in respective variables
    int Ny;
    double T;

    Nx = Data[0];
    Ny = Data[1];
    T  = Data[2];

    printf("Value of Nx is %d\n", Nx); // Print values to check
    printf("Value of Ny is %d\n", Ny);
    printf("Value of T is %f\n", T);
}

But got this as an output. This output is wrong as the values of Nx, Ny, and T are not matching with the data given above.

Please help me to solve this problem.

Answer 1

junk1 and junk2 should be arrays of char to be able to store strings.

But since it is junk you could simply not store it anywhere by using * in the fscanf conversion specifiers:

fscanf(file, "%*s %*s %lf\n", &Data[i]);

fscanf documentation: https://en.cppreference.com/w/c/io/fscanf

Answer 2

Your program makes strong assumptions about the input file:

• the file "test.txt" exists in the current directory and can be read
• it contains at least 3 settings, in the order Nx , Ny , T .

It has problems too:

• reading a string with %s into a single character variable junk1 will cause undefined behavior, same for junk2 , because fscanf() will attempt to store all characters from the string plus a null terminator at the destination address, overwriting other data with potentially catastrophic consequences.
• main has a return type int .

Here is a more generic approach:

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

int main() {
    int Nx = 0, Ny = 0;
    double T = 0;
    int has_Nx = 0, has_Ny = 0, has_T = 0;
    char buf[80];
    FILE *file;

    if ((file = fopen("test.txt", "r")) == NULL) {
        fprintf(stderr, "cannot open test.txt\n");
        return 1;
    }

    while (fgets(buf, sizeof buf, file)) {
        if (buf[strspn(buf, " ")] == '\n')  /* accept blank lines */
            continue;

        if (sscanf(buf, " Nx = %d", &Nx) == 1)
            has_Nx = 1;
        else
        if (sscanf(buf, " Ny = %d", &Ny) == 1)
            has_Ny = 1;
        else
        if (sscanf(buf, " T = %lf", &T) == 1)
            has_T = 1;
        else
            fprintf(stderr, "invalid line: %s", buf);
    }
    fclose(file);

    // Print values to check
    if (has_Nx)
        printf("Value of Nx is %d\n", Nx);
    if (has_Ny)
        printf("Value of Ny is %d\n", Ny);
    if (has_T)
        printf("Value of T is %g\n", T);
    return 0;
}