Add solution for problem 3

include/p3.h

@@ -0,0 +1,7 @@
+#ifndef P3_H
+#define P3_H
+
+double sum_file(const char* filename);
+
+#endif
+

input_p3.dat

@@ -0,0 +1,4 @@
+1.0
+2.2
+3.0
+5.4

src/main.c

@@ -2,6 +2,7 @@
 
 #include "p1.h"
 #include "p2.h"
+#include "p3.h"
 
 int main(void) {
 	// Problem 1
@@ -21,6 +22,11 @@ int main(void) {
 
 	printf("Average: %.2lf, Min: %.2lf, Max: %.2lf\n", avg, min, max);
 
+	// Problem 3
+	double sum_of_file = sum_file("input_p3.dat");
+
+	printf("Sum of file: %.2lf\n", sum_of_file);
+
 	return 0;
 }
 

src/p3.c

@@ -0,0 +1,48 @@
+#include "p3.h"
+
+#define _CRT_SECURE_NO_WARNINGS
+
+#include <stdio.h>
+
+double sum_file(const char* filename) {
+	FILE* infile = fopen(filename, "r");
+	
+	double sum = 0.0;
+	double value = 0.0;
+
+	// NOTE: Using feof may fail if you're not careful! For example, if the file
+	// being read from contains trailing whitespace characters, EOF may not
+	// be set until one additional read is performed. In this case, this would
+	// cause fscanf to attempt to read one additional value that does not exist.
+	// On Linux, this additional read is needed even if there is no trailing
+	// whitespace, although this may differ on other systems.
+	//
+	// As another example, if fscanf encounters an unexpected value, the position
+	// inside of the file will not be increased. Since we are attempting to read
+	// floating-point numbers, if we add an alphabetic character to the input file,
+	// fscanf will be stuck in an infinite loop trying to read invalid data and
+	// never progressing.
+	/*while (!feof(infile)) {
+		value = 0.0;
+		fscanf(infile, "%lf", &value);
+
+		// Try changing the input file and see what this prints!
+		printf("(feof) Adding value %.2lf to sum\n", value);
+
+		sum += value;
+	}*/
+
+	// A more reliable way of repeatedly reading from a file is checking
+	// the return value of fscanf. On success, fscanf will return the number
+	// of values read.
+	while (fscanf(infile, "%lf", &value) == 1) {
+		printf("(fscanf) Adding value %.2lf to sum\n", value);
+
+		sum += value;
+	}
+
+	fclose(infile);
+
+	return sum;
+}
+