tiny in C

[AdinaG]

Hi everyone,

I'm very new to programming - sorry if this seems obvious!

I'm trying to solve quadratics using an if-else statement. I'm using the following program:

/*
solve quadratic equation illustrating if-else statements.
*/
#include<stdio.h>
#include<catam.h>
double const tiny=1.0E-20;	/*small nonzero number*/
double a,b,c;			/*coeffs of x*x, x and 1 */
double r,i;			/*real & imaginary parts of solution */
double tmp1,tmp2;		/*temporary real variables */
int main(void)
{
	a = 0;
	b = 0;
	c = 0;
	do
		{
			printf("Enter a, b and c   ");
			fflush(stdout);
			scanf("%1f %1f %1f",&a,&b,&c);
			if (fabs(a) <= tiny)
				{
					printf("Not a quadratic, a is too close to zero\n");
				}
		}
	while (fabs(a) <= tiny);
	tmp1 = b*b-4*a*c;
	if (tmp1 >= 0)
		{
			tmp1 = sqrt(tmp1)/(2*a);
			tmp2 = -b/(2*a);
			r = tmp2 + tmp1;
			printf("Solution 1 is %14.4f\n",r);
			r = tmp2 - tmp1;
			printf("Solution 2 is %14.4f\n",r);
		}
	else 
		{
			r = -b/(2*a);
			i = sqrt(-tmp1)/(2*a);
			printf("Solution 1 is %14.4f + %14.4f i\n",r,i);
			printf("Solution 2 is %14.4f - %14.4f i\n",r,i);
		}
	return 0;
}

It compiles seemingly happily, but whatever values I enter for a, b and c, I get "Not a quadratic, a is too close to zero" and asked to enter a, b and c again. Can anyone see why?

Thanks!

[wje_lf]

What happens if you output the values of a, b, and c immediately after reading them in?

[AdinaG]

Ok, I did this by adding the lines

printf("%4d  %4d  %4d\n",a,b,c);
fflush(stdout);

before the first if. Was this the correct thing to do? (sorry, I only started programming last week, so am still unsure...)

The result:

Enter a, b and c   1 1 1
1065353216     0  1065353216
Not a quadratic, a is too close to zero
Enter a, b and c

Seems a little odd...

Thanks!

[wje_lf]

Your main problem, I believe, is that the variable type doesn't match the format, either for fscanf() or, in the revised version, the printf().

Compile your program with the

-Wall

switch. When it tells you about the mismatches, read the scanf man page with an eye for the picky details.

[rclark]

Make sure your scanf is lf (double) not 1f (float) .... Big difference . That's 'l' as in Larry ....

[wje_lf]

Make sure your scanf is lf (double) not 1f (float) .... Big difference . That's 'l' as in Larry ....

True. And rclark is correct to point this out, because the difference looks subtle, but is quite imporant. But I didn't want to spell it out, for two reasons:

1. I want him to get familiar with the documentation.
2. That's not the only format string problem in his code.