Coding a loop of polynomial with a given X to find f(x) in Java

Question

So I have trouble coding a loop of a polynomial with a given x to find f(x) in java. To put it simply, for example, I have an array of f(x) = C[0]*x^n + C[1]*x^(n-1).......C[n]n, how would I code a loop to find f(x) with the user's input of x? Attached below is my incomplete function

static double poly(double[] C, double x){
            int n=C.length-1;
            int K;
            double sum=0.0;
            //loop is supposed to go here
            return sum;
}

i hope you aren't us to do your homework ;) – ylun.ca Feb 16 '14 at 02:43 — ylun.ca, Feb 16 '14 at 02:43

shree.pat18 · Accepted Answer · 2014-02-19T13:05:26.267

0

You need to increment the variable sum by each term in the polynomial i.e.

Sum += C[i]×(x^(n-i)).

So loop from i= 0 to n with the above increment operation.

edited Feb 19 '14 at 13:05

answered Feb 16 '14 at 02:45

shree.pat18

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I do not think that `n^(x-i)` denotes the correct power of `x`. – Lutz Lehmann Feb 19 '14 at 09:43
Agreed on that. I was not using it as the operator, but as the exponent notation. – shree.pat18 Feb 19 '14 at 12:17
1

You have written n to the power of x-i, what you need is x to the power of n-i, i.e., x^(n-i). – Lutz Lehmann Feb 19 '14 at 12:32

score 0 · Answer 2 · answered Feb 16 '14 at 02:47

try this :

 static double poly(double[] C, double x){
                int n=C.length-1;
                int K;
                double sum=0.0;
                /***********/
                int i=0;
                for (k=n ,k>0;k--)
                 {
                 sum= c[i]*pow(x,k)+ sum;
                 i++;
                 }
                return sum;
    }

Lutz Lehmann · Answer 3 · 2014-02-19T12:42:10.477

0

You do this most efficiently using the Horner scheme.

C[n]+C[n-1]*x+...+C[0]*x^n 
= 
C[n]+x*(C[n-1]+x*(C[n-2]+...x*C[0]))

or as a loop

pol=C[0]
for k = 1 to n do
    pol *=x
    pol += C[k]
end for
return pol

edited Feb 19 '14 at 12:42

answered Feb 19 '14 at 09:42

Lutz Lehmann

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Coding a loop of polynomial with a given X to find f(x) in Java

3 Answers3