Printing out each line of an input file twice? - c

Question

I am writing code which very simply reads in a file and prints out what was in the file appropriately.
I have always struggled with getting such a program to terminate upon end of file and think I've found the appropriate solution, however each line is printing twice in my output, for a reason beyond me.
Here is my main file:
int main(int argc, char *argv[]) {
// insure 2 arguments given, one for a.out and one for the test file
if (argc != 2) {
// result if request fails
printf("Requires 2 arguments. Be sure to include test file location\n");
return 0;
}
FILE *fp; //open the file
fp = fopen(argv[1], "r");
char option;
int key;
int i = 0;
while (fscanf(fp, "%c %d", &option, &key) != EOF) {
printf("%d\n", key);
}
}
The key is printing twice!
Hopefully this is a simple error I'm just overlooking due to overexposure to the problem.

Answer 1

You probably want:
fscanf(fp, "%c %d\n", &option, &key);
And you also want to check the return value of fscanf to make sure it equals 2.
In the first iteration of your loop, the newline is not being consumed.
In the second iteration, the newline is consumed and put in option, and the %d does not match, and fscanf returns 1. key is unchanged which is why it gets printed again.
In the third iteration, fscanf finally returns EOF.
General rule: Always check return values to ensure they are what you expect. (You also violate this rule by failing to check the return from fopen.) At worst it does nothing; at best, it helps you debug problems like this.

Answer 2

#include <stdio.h>
int main(int argc, char *argv[])
{
if (argc != 2)
{
fprintf(stderr, "Requires 1 argument - a file name\n");
return 1;
}
FILE *fp; //open the file
if ((fp = fopen(argv[1], "r")) == 0)
{
fprintf(stderr, "Failed to open file %s\n", argv[1]);
return 1;
}
char option;
int key;
while (fscanf(fp, "%c %d", &option, &key) == 2)
printf("%d\n", key);
return 0;
}
Note the changes in error reporting, and in the file reading process. The code is still probably not quite what you want; you might get the newline after the number after the first line of input stored in option after the first line. Fixing that requires fgets() and sscanf():
#include <stdio.h>
int main(int argc, char *argv[])
{
if (argc != 2)
{
fprintf(stderr, "Requires 1 argument - a file name\n");
return 1;
}
FILE *fp; //open the file
if ((fp = fopen(argv[1], "r")) == 0)
{
fprintf(stderr, "Failed to open file %s\n", argv[1]);
return 1;
}
char buffer[1024];
while (fgets(buffer, sizeof(buffer), fp) != 0)
{
char option;
int key;
if (fscanf(fp, "%c %d", &option, &key) == 2)
printf("%d\n", key);
else
{
fprintf(stderr, "Format mismatch on %s", buffer);
fclose(fp); // Not 100% necessary here, but tidiness is important
return 1;
}
}
fclose(fp); // Not 100% necessary here, but tidiness is important.
return 0;
}
Although I closed fp before the end, it is not crucial when the program is about to exit, and return from main() is pretty much equivalent to exit(). If it was in a function other than main() though, it is very important to ensure that you free any resource you allocate, such as the file stream fp.
Warning: uncompiled code. Caveat Lector.