Evaluation of a polynomial in SciLab

Question

Could you please help me find a mistake in my code? I need to calculate the polynomial, with parameters the vector and the value of the variable. I am not allowed to use the power sign (^) I have got this code but it doesn't work and I don't see what I am doing wrong.

function f=veelTermEvaluatie(V,x)
        f=V(1);
        for i=2:length(V)
            for j=0:i-1
                if j=0 then x=x
                    else x=x*x
                end
                f=V(i)*x
            end
        end
    endfunction

    V=[1 2 3 4 5 6]
    x=3

Answer 1

I first refactored your code, to remove the j=0 situation, since this does not change anything since x=x it can be skipped by starting at j=1. I also added some debug printing :

function f=veelTermEvaluatie(V,x)
    f=V(1);

    for i=2:length(V)

        printf("\nI: %d --> ", i)            
        for j=1:i-1                
            x = x * x
            printf("j: %d, x=%d ",j,x)
        end

        f=V(i)*x
    end

    return f
endfunction

After that it became clear that you multiply the same x, each and every time:

I: 2 --> j: 1, x=9 
I: 3 --> j: 1, x=81 j: 2, x=6561 
I: 4 --> j: 1, x=43046721 j: 2, x=-501334399 j: 3, x=0 
I: 5 --> j: 1, x=0 j: 2, x=0 j: 3, x=0 j: 4, x=Inf 
I: 6 --> j: 1, x=Inf j: 2, x=Inf j: 3, x=Inf j: 4, x=Inf j: 5, x=Inf 

What also happens is that you update the value of f to the highest term, disregarding any other terms. I think your meaning is to return all terms

So you should create a local x which is reset for every term.

Working example

function f=veelTermEvaluatie(V,x)

    //Initialize the result to be a copy of the input
    f=V;

    // For each element in the input (except the first)
    for i=2:length(V)

        printf("\nI: %d --> ", i);
        //Initialize a local x variable to the global x
        new_x = x;

        for j=1:i-1
            // Update only the local x                 
            new_x = new_x * x;
            printf("j: %d, x=%d ",j,x);
        end

        // Multiply the value in the array with the local x value
        f(i)=V(i)*new_x;
    end

    return f
endfunction

V=[1 2 3 4 5 6]
x=3

result = veelTermEvaluatie(V,x)

disp(result)
disp(sum(result))