Thread: Program doesn't properly compute simple polynomial.

The program will ask for the user to enter a value for x, then compute the following polynomial: 3x^5 + 2x^4 - 5x^3 - x^2 + 7x - 6.

However, when I double check it with my calculator I get a wrong answer for random values of x. To simplify my problem I'm using only integers.

Code:

#include <stdio.h>

int main(void)
{
	int x, polynomial;
	
	printf("The program will compute the following polynomial: \n");
	printf("3x^5 + 2x^4 - 5x^3 - x^2 + 7x - 6 \n");
	printf("Please enter a value for x: ");
	scanf("%d", &x);
	
	polynomial = (3 * x * x * x * x * x) + (2 * x * x * x * x) - (5 * x * x * x) - (2 * x * x) + (7 * x) - 6;
	printf("Value = %d \n", polynomial);
	
	return 0;
}

I noticed that you wrote (2 * x * x) instead of (x * x) for x^2.

Well, there are power functions available in <math.h>, but they are for floating point. You could use them and then cast the return values, but you may need to be wary of floating point inaccuracy. Another way is to write your own intpow function, which can initially just loop and compute the power (and later you can make it more efficient if necessary). Anyway, the point is that you can then write:

Code:

polynomial = (3 * intpow(x, 5)) + (2 * intpow(x, 4)) - (5 * intpow(x, 3)) - intpow(x, 2) + (7 * x) - 6;

which would likely be easier to match to your math formula. (Then again, for the x^2 term I would still prefer to write (x * x), and since that was the place where you made the typo, maybe this would not have helped.)

I'd rarely use power functions to work with a polynomial expression. I'd draw out common factors. In this case (if I've done it right)

Code:

polynomial = x*(x*(x*(x*(3 * x + 2) - 5) - 1) + 7) - 6;

At most, in this case, 5 multiplications ans 6 addition/subtraction operations (and without any reliance on knowing the polynomial's roots). It's hard to see how any usage of a power function can beat that. Okay, it is necessary to think about the expression in a different way, but it is still pretty simple and unambiguous. The error from the OP would have been harder to make too.

I'd only consider using an actual power function if regularly dealing with polynomials with lots of zero coefficients (say 3*x^23 + 5*x^6 + x*x + 2).