Code is ignoring IF command in C, Issues with FOR

Question

I had posted on here before, but I was never able to get the help I needed. I'm working on a school project and I can not get my program to work properly.

The program should prompt the user to enter the number of gallons used and the number of miles driven for each of the 3 tanks of gas. The program should then calculate and display the miles per gallon obtained for each tank. Once processing is complete for the 3 tanks, the program will calculate the overall mileage(total gallons / total miles) and display a friendly "Goodbye" message.

The issue i am having is that I can not get it to display to OVERALL Millage. it ends after looping 3 times. I know different loop statements need conditions to be met, but I cant get the FOR loop to work properly. Im getting really frustrated, cause I know this should not be this hard.

Code

#include <stdio.h>
int main(void) 
{
    int miles,i=3;
    float gallons, mg, overall = 0, avg = 0;
    while(i>0) 
    {
        printf("Enter the gallons used: ");
        scanf("%f", &gallons);
        printf("Enter miles driven: ");
        scanf("%d", &miles);
        mg = miles/gallons;
        printf("The miles/gallon for this tank was : %f\n", mg);
        overall += miles;
        avg += gallons;i--;
    }

    if(gallons == 0) 
    {
        printf("\n\n The overall miles/gallon was: %f\n", overall/avg);
        exit(0);
    }
    return 0;
}

Answer 1

If I read your code correctly, then what is preventing the overall mileage from being printed is the following final if statement:

if (gallons == 0)

If you remove it, then the overall mileage should print. Use this pattern:

while (i > 0)
{
    // your while loop here
}

printf("\n\n The overall miles/gallon was: %f\n", overall/avg);
exit(0);

Answer 2

This if (if (gallons == 0) {})block is out of while loop. First, you need to move the if loop inside while loop. and this if condition should be for variable i as follow and not for gallons.

if (i == 0) 
{
    printf("\n\n The overall miles/gallon was: %f\n", overall/avg);
}

In this case, after 3 iteration, value of i will be 0 so it will enter into the if block and calculate and print the overall miles/gallon.

Answer 3

Adding to Tim Biegeleisen's answer:

mg = miles/gallons;  

What if gallons equals to 0? e.g. 0 miles for 0 gallons
This will lead to floating point exception.

A simple if-else can solve this problem!

if(!gallons)
    mg = 0;  
else
    mg = miles/gallons;