/*
  This is an incomplete program to compute the GPA of a set of courses.
  gpa.in contains sample input (plain text file)
  gpa.out contains sample output (plain text file)
  compile in a Unix shell: gcc gpa.c  (creates executable called a.out)
  run in a Unix shell: ./a.out  (input from keyboard, output to screen)
  run in a Unix shell: ./a.out <gpa.in >gpa.out
*/

// First part of a C program usually contains pre-processor directives:

// include header files (contain function prototypes) for used libraries
#include <stdio.h>   // for basic I/O: puts, getchar, printf
#include <ctype.h>   // for character related functions: isspace, isdigit
#include <string.h>  // for string functions: 

// create symbolic constants to avoid "magic numbers"
#define GSIZE 3

// function declarations are called prototypes
// every function must be declared or fully defined before it is called
double getpoints(char []);  // no parameter names are needed

int main(void) {
    char grade[GSIZE];
    int i, ch;
    int credits;
    double points;
    double totalpoints = 0, totalcredits = 0;

    puts("enter grades and credits: ");

    // repeat until end of input
    while ((ch  = getchar()) != EOF) {
        // get grade
        i = 0;
        while (!isspace(ch)) {
            grade[i++] = ch;
            ch = getchar();
        }
        grade[i] = '\0';  // strings must be terminated with special null character
        points = getpoints(grade);

        // skip whitespace
        while (isspace(ch)) {
            ch = getchar(); 
        } 

        // read credits, one character at a time, build integer
        credits = 0;
        while (isdigit(ch)) {
            credits = credits * 10 + ch - '0';
            ch = getchar();
        }

        // go to next line
        while (ch != '\n')
            ch = getchar();

        printf("grade is: %-2s  credits = %d  points = %4.1f \n", 
                grade, credits, points);

        // missing parts to actually use the grade to compute the GPA!

    } // while not EOF

	return 0;  // 0 indicates normal termination, no errors
} // main

/* Converts a letter grade to its corresponding point value.
 */
double getpoints(char g[]) {
    double points = 0;
    g[0] = toupper(g[0]);
    switch (g[0]) {
    case 'A': points = 4; break;
    case 'B': points = 3; break;
    case 'C': points = 2; break;
    case 'D': points = 1; break;
    default: points = 0;
    }

    if (g[1] == '+' && 'B' <= g[0] && g[0] <= 'D')
        points += .3;
    if (g[1] == '-' && 'A' <= g[0] && g[0] <= 'C')
        points -= .3;
    return points;
}