Printing the average of combined student marks using C - c

Question

I've made this program below to calculate the average mark of a student.
Everything works well until I use -1, it is supposed to stop the program as is display the average of all students that have been entered, say Goodbye! and then terminate.
I think my calculations might be wrong though because it is printing the wrong result for the average marks of the students.
Thanks in Advance.
#include <stdio.h>
int main(void)
{
float final_mark(int a_mark1, int a_mark2, int lab_mark, int quiz_mark, int exam_mark);
int i, a_mark1, a_mark2, lab_mark, quiz_mark, exam_mark;
float average_mark = 0.0;
do
{
for (i = 0; i < 2; i++)
{
printf("Enter assignment 1 mark (-1 to quit): ");
scanf("%d", &a_mark1);
if(a_mark1 == -1)
{
average_mark += final_mark(a_mark1, a_mark2, lab_mark, quiz_mark, exam_mark);
if ((average_mark > 1 ) && (average_mark < 100 ))
{
printf("The average student mark is %.2f%% \n", average_mark);
}
printf("Goodbye! \n");
return 0;
}
printf("Enter assignment 2 mark: ");
scanf("%d", &a_mark2);
printf("Enter laboratory mark: ");
scanf("%d", &lab_mark);
printf("Enter quiz mark: ");
scanf("%d", &quiz_mark);
printf("Enter exam mark: ");
scanf("%d", &exam_mark);
printf("Student %d final mark: %.2f \n", i + 1, final_mark(a_mark1, a_mark2, lab_mark, quiz_mark, exam_mark));
}
}
while(a_mark1 != -1);
return 0;
}
float final_mark(int a_mark1, int a_mark2, int lab_mark, int quiz_mark, int exam_mark)
{
float final_mark = a_mark1 * 0.1 + a_mark2 * 0.15 + lab_mark * 0.15 + quiz_mark * 0.1 + exam_mark * 0.5;
return final_mark;
}

Answer 1

I think you need to rethink your logic a little bit. Why not use a while loop to control the flow. Then you can bail out of the program immediately if user inputs -1 right away. You should use an array to store the averages for each student, then you can loop through and find the class average as well.
-Your float final_mark function seems a little sketchy without any parenthesis.
-You should put your function prototype outside of main as well. See below changes.
#include <stdio.h>
#define MAX_STUDENTS 10 //define what the max number of students is
float final_mark(int mark1, int mark2, int lab, int quiz, int exam);
int main()
{
int i = 0, mark1 = 0, mark2 = 0,
lab = 0, quiz = 0, exam = 0;
int num_students;
float students_avg[MAX_STUDENTS] = {0}; //array to hold averages for students
float average = 0;
while (i < MAX_STUDENTS) {
printf("Enter assignment 1 mark (enter -1 to quit):\n");
scanf("%d", &mark1);
if (mark1 == -1)
break; //no more students, break out of while loop
printf("Enter assignment 2 mark: ");
scanf("%d", &mark2);
printf("Enter laboratory mark: ");
scanf("%d", &lab);
printf("Enter quiz mark: ");
scanf("%d", &quiz);
printf("Enter exam mark: ");
scanf("%d", &exam);
average = final_mark(mark1, mark2, lab, quiz, exam);
students_avg[i] = average; //add this average to array
printf("Student # %d average was %.2f\n", i, students_avg[i]);//debug info
i++;
}
num_students = i; //how many students grades did we read?
average = 0; //reset to 0 so we can use below
for (i = 0; i < num_students; i++)
average += students_avg[i];
if (num_students > 0)
printf("Class average is %.2f\n", average/num_students);
else
printf("Goodbye!\n");
return 0;
}
float final_mark(int mark1, int mark2, int lab, int quiz, int exam)
{
//we can just return the calculation
return ((mark1 * 0.1) + (mark2 * 0.15) + (lab * 0.15) + (quiz * 0.1) + (exam * 0.5));
}

Answer 2

Just erase two lines above printf("GOODBYE! \n"); you will get what you want.

Answer 3

maybe you have to initialized mark s variables to 0. and a_mark1 to 0 too if the user enter -1