如何从 c 中的 .txt 文件中读取一行数字？

Question

I'm trying to make a code that will read three numbers (13.1, 270.66, and 81.3) from the first row of a .txt file and associate them with the variables I want it to. This is what I tried:

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

int main()
{
    char filename[100];
    printf("Enter the name (including file extension) of the file you wish to use the values from: ");
    scanf("%s",filename);
    FILE *filepointer=fopen(filename,"r");
    if (filepointer==NULL){
        printf("Error: Couldn't open file: %s",filename);
    }
    double value1;
    double value2;
    double value3;
    rewind(filepointer);
    fscanf(filepointer,"%g %g %g",&value1,&value2,&value3);
    printf("Your values are: %g %g %g",value1,value2,value3);
    fclose(filepointer);
}

FYI in the .txt file the values are set us like this:13.1 270.66 81.3

Answer 1

%g in scanf() is for reading float . You should use %lg to read double instead.

Also you should:

• Stop executing rest of the program when fopen() failed.
• Check the return values of scanf() and fscanf() to check if they succeeded to read all requested things.
• Specify the maximum number of characters for %s (at most the size of destination minus one for terminating null-character) to prevent buffer overrun.

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

int main(void)
{
    char filename[100];
    printf("Enter the name (including file extension) of the file you wish to use the values from: ");
    if (scanf("%99s",filename) != 1){ /* specify the maximum length and check the result */
        puts("Error: failed to read file name");
        return 1;
    }
    FILE *filepointer=fopen(filename,"r");
    if (filepointer==NULL){
        printf("Error: Couldn't open file: %s",filename);
        return 1; /* stop execution when fopen() fails */
    }
    double value1;
    double value2;
    double value3;
    rewind(filepointer);
    /* use correct format specifier and check the result */
    if (fscanf(filepointer,"%lg %lg %lg",&value1,&value2,&value3) == 3){
        printf("Your values are: %g %g %g",value1,value2,value3);
    } else {
        puts("Error: failed to read numbers");
        fclose(filepointer);
        return 1;
    }
    fclose(filepointer);
}