C 编程 - 关于 while 循环再次提示用户问题

Question

In this example, I am trying to code a program which let users solve qudratic equations. At the end, should if they press y/Y, they can restart the program. If they pressed n/N, the program will exit and if they press any other, the program should prompt them again for either ay/Y/N/n.

Unfortunately, I can't seem to run this logic at the end properly. Any ideas why? Thanks

#include <ctype.h> //in order to use toupper
#include <stdio.h> // * Solution of a*x*x + b*x + c = 0 *
#include <math.h>

int main(void)
{
double a, b, c, root1, root2;
char do_again;
while (do_again == 'Y');
printf("Input the coefficient a => ");
scanf("%lf", &a);
printf("Input the coefficient b => ");
scanf("%lf", &b);
printf("Input the coefficient c => ");
scanf("%lf", &c);
if (a == 0)
{
printf("You have entered a = 0.\n");
printf("Only one root: %8.3f", -c/b);
}
else 
{
    root1 = (- b + sqrt(b*b-4*a*c))/(2*a);
    root2 = (- b - sqrt(b*b-4*a*c))/(2*a);
    printf("The first root is %8.3f\n", root1);
    printf("The second root is %8.3f\n", root2);
}
printf("Solve again (y/n)? ");
fflush(stdin);
do_again = toupper(getchar());
if (do_again !='Y' && do_again !='N' )  
    printf("Please try again");
    do_again = toupper(getchar());
}

Answer 1

Two problems:

1. The statement

   while (do_again == 'Y');

   is equivalent to

   while (do_again == 'Y') { // Empty body }

   The loop body is the single semicolon at the end of the line

2. When you use the variable do_again it's uninitialized . It's value will be indeterminate . You need to initialize all variables before you use them.

These two problems together means that either you have an infinite loop (if do_again just happens to be equal to 'Y' ), or no loop at all.

---

There's also other problems in your code, for example

if (do_again !='Y' && do_again !='N' )  
    printf("Please try again");
    do_again = toupper(getchar());

which is equivalent to

if (do_again !='Y' && do_again !='N' )  
    printf("Please try again");
do_again = toupper(getchar());

That is, only the printf call is inside the if , the assignment is always and unconditionally performed.

You also lack error checking. What happens if, for example, one of the scanf calls fails one way or another?