/*
 * Simple example of getting input from a file -- read integers from
 * file and compute sum.
 * Uses command-line argument to specify filename.
 * File should contain integers, in the same format as what would be
 * entered interactively.
 */
#include <stdio.h>

int main(int argc, char* argv[]) {
	/* 
	 * check that user supplied two arguments 
	 * by convention argv[0] is something that identifies the
	 * program (often its filename -- e.g., a.out).
	 */
        if (argc != 2) {
                printf("usage %s infile\n", argv[0]);
                return 1;
        }
       	/* 
	 * open input file -- note that fopen returns NULL if it 
	 * could not do this, e.g., file not found
	 */
	FILE * infile = fopen(argv[1], "r");
	if (infile == NULL) {
		printf("could not open input file %s\n", argv[1]);
		return 1;
        }
	/* 
	 * read integers from input file and sum 
	 * uses scanf and stops when either it gets to the end of the file
	 * or it encounters something that is not an integer.
	 */
	int sum = 0;
	int n;
	while (fscanf(infile, "%d", &n) == 1) {
		sum += n;
	}
	/*
	 * distinguish between two reasons for ending loop -- end of file
	 * encountered, or invalid input (not integers).
	 */
	if (feof(infile)) {
		/* loop stopped because everything in the file was read */
		printf("sum is %d\n", sum);
		fclose(infile);
		return 0;
	}
	else {
		/* loop stopped because of not-integer data in file */
		/* 
		 * FIXME?  not a very helpful message, so could keep a count 
		 * of lines read in the file and print it here?
		 */
		printf("invalid input\n");
		fclose(infile);
		return 1;
	}
}