How to add total of multiple repeats in C? - c

Question

I am writing a simple C program which takes data from user and does some maths. Here is my code:
#include <stdio.h>
int main(void) {
int semester_1,grade_1,grade_2,grade_3,subtotal,total_marks,average;
printf("Enter number of semester you to check");
scanf("%d", &semester_1);
while (semester_1 > 0) {
printf("Enter marks for first subject");
scanf("%d", &grade_1);
printf("Enter marks for second subject");
scanf("%d", &grade_2);
printf("Enter marks for third subject");
scanf("%d", &grade_3);
subtotal = grade_1 + grade_2 + grade_3;
total_marks = subtotal / 300 * 100;
printf("Your average this semester is %d", total_marks);
semester_1--;
}
average = semester_1 / 100 * total_marks;
printf("Your final average for all semesters is %d", average);
}
The problem with this code is that when I run the program returns 0 for final average for all semesters.
I wanted to get the final average for all semesters. Lets say if user enters 3 for numbers of semester they want to check and then they will be enter marks 3 times and then final average will be displayed, but it only gives 0.

Answer 1

#include <stdio.h>
int main(void) {
int semester_1,grade_1,grade_2,grade_3,subtotal,total_marks,average;
printf("Enter number of semester you to check");
scanf("%d", &semester_1);
while (semester_1 > 0) {
printf("Enter marks for first subject");
scanf("%d", &grade_1);
printf("Enter marks for second subject");
scanf("%d", &grade_2);
printf("Enter marks for third subject");
scanf("%d", &grade_3);
subtotal = grade_1 + grade_2 + grade_3;
total_marks = subtotal / 300.0 * 100.0;
printf("Your average this semester is %d", total_marks);
semester_1--;
}
average = semester_1 / 100.0 * total_marks;
printf("Your final average for all semesters is %d", average);
}
Rounding errors

Answer 2

It is probably because here:
total_marks = subtotal / 300 * 100;
subtotal is less than 300 * 100. And since both the operands of / are of type int, integer division is performed resulting in total_marks becoming 0.
Fix it by changing the type of total_marks to float, or more preferably, double. Then, cast one of the operands of / to float if you changed total_marks's type to float or double if you changed total_marks's type to double. The cast makes sure that integer division is not performed and floating-point division is performed.
You might need to do the same with average.
Fixed Code:
#include <stdio.h>
int main(void) {
int semester_1, grade_1, grade_2, grade_3, subtotal; /* Better to use an array */
double total_marks, average;
printf("Enter number of semester you to check");
scanf("%d", &semester_1);
while (semester_1 > 0) {
printf("Enter marks for first subject");
scanf("%d", &grade_1);
printf("Enter marks for second subject");
scanf("%d", &grade_2);
printf("Enter marks for third subject");
scanf("%d", &grade_3);
subtotal = grade_1 + grade_2 + grade_3;
total_marks = (double)subtotal / 300 * 100; /* Note the cast */
printf("Your average this semester is %f", total_marks); /* Note the change in the format specifier */
semester_1--;
}
average = (double)semester_1 / 100 * total_marks; /* Note the cast */
printf("Your final average for all semesters is %f", average); /* Note the change in the format specifier */
}

Answer 3

Your code has several issues. The first is the one pointed out by Cool Guy: dividing small integer by bigger integer will lead to the result being zero due to integer truncation.
The second is that you aren't keeping a running total, and you're decrementing the number of semesters for your loop counter.
You should add a new variable that stores the cumulative sum of each semester, and you should save the initial value of semester_1
(Also, style-wise,
for (int i = 0; i < num_semesters; i++)
is much more readable than (and preserves the value of num_semesters)
while(semester_1 > 0)
)