fgets（）在C中将最后一行打印两次

Question

I was supposed to open a file from a command-line argument and print the values in it. This is my code:

#include <stdio.h>
int main(int argc, char* argv[])
{
  float num;
  char const* const filename=argv[1];
  FILE* file=fopen(filename,"r");
  char line[256];
  int j=0;

  while(fgets(line,sizeof(line),file)){

    for( j=0; j<2; j++ )
    {
      if(j == 0)
      {
           fscanf(file, "%f", &num);
           printf("%f \t", num);
      }
      else if(j == 1)
      {
          fscanf(file, "%f", &num);
          printf("%f \n", num);
      }
    }
  }
  fclose(file);
}

This is what I'd like to get as output:

1 1
2 2
3 3

This is what I'm actually getting:

1 1
2 2
3 3
3 3

I don't get what's happening here.

Answer 1

I done two changes in your code.

change

fscanf(file,"%f",&num);

into

sscanf(line,"%f",&num);// here

you are reading the input in loop, but you are getting the value from the file pointer. so first line will be skipped. Then make the test case in while opening the file stream.

if ( file == NULL)  { 
     perror("fopen");
     return;      
}

Try this code, I done the above changes only,

 #include <stdio.h>
 int main(int argc, char* argv[])
 {
    float num;
    char const* const filename=argv[1];
    FILE* file=fopen(filename,"r");
    if ( file == NULL)  { 
       perror("fopen");
      return;      
    }
    char line[256];
    int j=0;
    while(fgets(line,sizeof(line),file) != NULL){
            for(j=0; j<2;j++)
            {
                    if(j==0)
                    {
                            sscanf(line,"%f",&num);
                            printf("%f \t",num);
                    }
                    if(j==1)
                    {
                            sscanf(line,"%f",&num);
                            printf("%f \n",num);
                    }
            }
    }
    fclose(file);
 } 

output:

 130.000000     130.000000
 210.000000     210.000000
 650.000000     650.000000
 324.000000     324.000000

Answer 2

Your issue is that you read the line with fgets then begin reading on the next line with fscanf . In other words after fgets (line, LMAX-1, file) The file position indicator is at the end of line 1. You then fscanf(file, "%f", &num); (which skips the '\\n' at the end of line 1 and read the value for line 2). You want:

while (fgets (line, LMAX-1, file)) {

    for (j = 0; j < 2; j++) {
        if (j == 0) {
            sscanf (line, "%f", &num);
            printf ("%f \t", num);
        } else if (j == 1) {
            sscanf (line, "%f", &num);
            printf ("%f \n", num);
        }
    }
}

Then output becomes:

./bin/dbllast dat/dbllast.txt
1.000000        1.000000
2.000000        2.000000
3.000000        3.000000