How to evaluate a polynomial at a specific x-value?

Question

Use map to evaluate a given polynomial at a specific x-value.

Input:

• p: A list of coefficients for increasing powers of x
• x: The value of x to evaluate

Output: Number representing the value of the evaluated polynomial

Example: poly_eval([1, 2, 3], 2) = 1(2)^0 + 2(2)^1 + 3(2)^2 = 17

Answer 1

def poly_eval(coeff_list, x):
    total = 0
    for i, coeff in enumerate(coeff_list):
        total += coeff * x**i
    return total

or if you really want to use map :

def poly_eval(coeff_list, x):
    n = len(coeff_list)
    return sum(map(lambda coeff, x, y: coeff*x**y, coeff_list, [x]*n, range(n)))

Answer 2

This is actually an interesting question. Since the answer is relatively simple and the pen and paper solution is known by everybody the real thing is kind of overlooked.

As mentioned, normally most people would approach like how it's done by pen and paper. However there is a better way which is more suitable for coding purposes, known as the Ruffini Horner method. This is a perfect case for reducing.

Write your polynomial in an array. So y = x^3-7x+7 would be var y = [1,0,-7,7].

Then a simple function;

var calcP = (y,x) => y.reduce((p,c) => p*x+c);

That's it.