GPA Calculator help in C - c

Question

I am currently in the process of learning C and one of the challenge questions at the end of a chapter has me building a GPA Calculator. Here is the Challenge:
Create a student GPA average calculator. The program should prompt the user to enter up to 30 GPAs, which are stored in a single-dimension array. Each time he or she enters a GPA, the user should have the option to calculate the current GPA average or enter another GPA. Sample data for this program:
GPA: 3.5
GPA: 2.8
GPA: 3.0
GPA: 2.5
Hint: Be careful to not calculate empty array elements into your student GPA average.
I have the program appear to somewhat work but when it calculates the GPA average it calculates wrong. Will someone please take a look at my source code and let me know what I did wrong?
Source Code
#include <stdio.h>
int main()
{
float fGrades[30] = {0};
float fAverage = 0;
int x;
int y = 0;
float fSum = 0;
char cResp = '\0';
printf("\nGPA Calculater");
printf("\nYou can enter up to 30 grades");
do
{
printf("\nPlease enter a grade and press enter: ");
scanf("%f", &fGrades[x]);
printf("\nDo you want to calculate the GPA? (Y or N): ");
scanf("\n%c", &cResp);
} while (x < 30 && cResp != 'Y' && cResp != 'y');//end do while loop
for (x = 0; x < 30; x++)
{
fSum += fGrades[x];
}//end for loop
fAverage = fSum / x;
printf("\nYour final GPA is: %.2f\n", fAverage);
return 0;
}//end main function
Output I get is as follows:
GPA Calculater
You can enter up to 30 grades
Please enter a grade and press enter: 3.22
Do you want to calculate the GPA? (Y or N): n
Please enter a grade and press enter: 3.13
Do you want to calculate the GPA? (Y or N): n
Please enter a grade and press enter: 2.89
Do you want to calculate the GPA? (Y or N): n
Please enter a grade and press enter: 3.05
Do you want to calculate the GPA? (Y or N): y
Your final GPA is: 0.10

Answer 1

A few things to suggest:
initialize x to 0
in the initial do/while loop, increment x after you have scanf'd in the array
in the loop that calculates fSum, use another counter and go from counter = 0; counter < x; ++counter and use fSum += fGrades[counter];.

Answer 2

Your problem is, that you always sum up 30 elements no matter what.
You should only add as many items that have been entered by the user.
So something like:
#include <stdio.h>
int main()
{
float fGrades[30] = {0};
float fAverage = 0;
int x;
int y = 0;
int counter = 0;
float fSum = 0;
char cResp = '\0';
printf("\nGPA Calculater");
printf("\nYou can enter up to 30 grades");
do
{
printf("\nPlease enter a grade and press enter: ");
scanf("%f", &fGrades[counter]);
counter++;
printf("\nDo you want to calculate the GPA? (Y or N): ");
scanf("\n%c", &cResp);
} while (counter < 30 && cResp != 'Y' && cResp != 'y');//end do while loop
for (x = 0; x < counter; x++)
{
fSum += fGrades[x];
}//end for loop
fAverage = fSum / x;
printf("\nYour final GPA is: %.2f\n", fAverage);
return 0;
}//end main function

Answer 3

change the loop that calculated fSum to work with another variable instead of x
int i;
for ( i = 0; i < x; i++)
{
fSum += fGrades[ i ];
}//end for loop

Answer 4

There are multiple problems
1) x is not initialized, so scanf in do-while may crash your code
2) x in not incremented in do-while
3) while calculating average, you should sum only entered values, not all 30 values, else your average calculation will be wrong
Modified code as below, please check
int main()
{
float fGrades[30] = {0};
float fAverage = 0;
int x;
int y = 0;
float fSum = 0;
char cResp = '\0';
printf("\nGPA Calculater");
printf("\nYou can enter up to 30 grades");
x = 0;
do
{
printf("\nPlease enter a grade and press enter: ");
scanf("%f", &fGrades[x]);
printf("\nDo you want to calculate the GPA? (Y or N): ");
scanf("\n%c", &cResp);
x++;
} while (x < 30 && cResp != 'Y' && cResp != 'y');//end do while loop
for (y = 0; y < x; y++)
{
fSum += fGrades[y];
}//end for loop
fAverage = fSum / x;
printf("\nYour final GPA is: %.2f\n", fAverage);
return 0;
}//end main function

Answer 5

You can remove the 'for' loop to calculate the sum of all values. You can calculate the sum into the 'do while' loop. It will make your code more efficient.
#include <stdio.h>
int main()
{
float fGrades[30] = {0};
float fAverage = 0;
int x;
int y = 0;
float fSum = 0;
char cResp = '\0';
int count = 0;
printf("\nGPA Calculater");
printf("\nYou can enter up to 30 grades");
do
{
printf("\nPlease enter a grade and press enter: ");
scanf("%f", &fGrades[x]);
fSum += fGrades[x];
count++;
printf("\nDo you want to calculate the GPA? (Y or N): ");
scanf("\n%c", &cResp);
} while (x < 30 && cResp != 'Y' && cResp != 'y');//end do while loop
fAverage = fSum / count;
printf("\nYour final GPA is: %.2f\n", fAverage);
return 0;
}//end main function