Percentage calculation problems in C - c

Question

I am making a program that counts how many passing grades there are based on the conditions. However, when I try calculating the percentage of passing scores, that is, if (passGrade >= 0) { printf("Percent of passing grades are %d! \n", (inputGrade/passGrade)*100); return 0; the program does not show the result, and instead shows up a zero, I did debug the program, and the passGrade, which is the number of total grades I entered in this line, shows up its value printf("You have entered %d passing grades! \n", passGrade); but when it comes to the next line, it just shows up a -1. I did look online for the problem, and nothing shows up.
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
int main()
{
int inputGrade, passGrade, percentGradePass;
inputGrade = 0;
passGrade = 0;
/* printf("Please type a grade (-1) to exit: ");
scanf("%i", &inputGrade);
printf("You typed: %i \n", inputGrade); */
while (inputGrade != -1)
{
printf("Please type a grade (-1) to exit: ");
scanf("%d", &inputGrade);
printf("You typed: %d \n", inputGrade);
if (inputGrade == -1)
{
inputGrade = -1;
}
else if (inputGrade >= 70 && inputGrade <= 100) {
passGrade = passGrade + 1;
}
else {
inputGrade = 0;
}
}
printf("You have entered %d passing grades! \n", passGrade);
if (passGrade >= 0) {
printf("Percent of passing grades are %d! \n", (inputGrade/passGrade)*100);
return 0;
}
}
Help would be much appreciated.

Answer 1

There are a number of issues with your code. Some issues are critical while others rather represents "strange/unnecessary" code.
Example with comments inline:
while (inputGrade != -1)
{
printf("Please type a grade (-1) to exit: ");
scanf("%d", &inputGrade); // ALWAYS, ALWAYS... check the return value of scanf
printf("You typed: %d \n", inputGrade);
if (inputGrade == -1)
{
inputGrade = -1; // Unnecessary! inputGrade is already -1 so no need to
// assign it again
}
else if (inputGrade >= 70 && inputGrade <= 100) {
passGrade = passGrade + 1;
}
else {
inputGrade = 0; // Unnecessary! inputGrade will get a new value from scanf
// in the next loop
}
}
So your loop could simply be:
while (inputGrade != -1)
{
printf("Please type a grade (-1) to exit: ");
if (scanf("%d", &inputGrade) != 1)
{
// Invalid input, terminate program
exit(1);
}
printf("You typed: %d \n", inputGrade);
if (inputGrade >= 70 && inputGrade <= 100)
{
passGrade = passGrade + 1;
}
}
Another potential issue is this:
(inputGrade/passGrade)*100
Since both variable have the type int the result of the division will be truncated to nearest integer, e.g. 6/10 will result in 0 instead of 0.6 and the whole expression will consequently become 0. To avoid that you can do the multiplication first (i.e. (100 *inputGrade)/passGrade) or use a floating point type instead of integer types.
That said, it seems that there is a basic misunderstanding in your algorithm. You say:
I try calculating the percentage of passing scores
To calculate the percentage you want
100 * NumberOfPassingGrades / TotalNumberOfGrades
but that is not what you are doing. Instead of using inputGrade in the calculation, you need to count the number of grades and use that in the calculation.
Something like:
int totalGrade = 0;
while (1)
{
printf("Please type a grade (-1) to exit: ");
if (scanf("%d", &inputGrade) != 1)
{
// Invalid input, terminate program
exit(1);
}
if (inputGrade != -1)
{
// Stop the loop
break;
}
printf("You typed: %d \n", inputGrade);
if (inputGrade >= 70 && inputGrade <= 100)
{
passGrade = passGrade + 1;
}
totalGrade = totalGrade + 1;
}
printf("You have entered %d passing grades! \n", passGrade);
printf("You have entered %d grades in total \n", totalGrade);
if (totalGrade > 0)
{
printf("Percent of passing grades are %d! \n", (100 * passGrade)/totalGrade);
}

Answer 2

what does 'the program does not show the result' means?
when your program reaches '(inputGrade/passGrade)*100)'
1/ you should check if 'passGrade' is 0 or not because if it is '0' there will be a run-time error.
2/ and also, since while loop is finished when 'inputGrade == -1', so you need to convert it to 'plus' before divide it
3/ lastly, since 'scanf("%d", &inputGrade);' means inputGrade = 'input' so, you dont need to write 'inputGrade = -1;' in your 'if' case