Is anything wrong with Eval function? I get value of pn =1 for all x values.

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Muhtasim Ahmed 2018-4-8

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/393494-is-anything-wrong-with-eval-function-i-get-value-of-pn-1-for-all-x-values

评论： Are Mjaavatten 2018-4-9

function HW_4_8()
    y= @(x)(1/(1+25*x.^2));
    %x=linspace(-1,1,41);
    for i=0:5
        x=cos(i*pi/40);
    end
    fprintf('Coefficient of the newton interpolation\n\n');
    a= Coef(x,y);
    for i=1:length(a)
        fprintf('%5.0f\t%15.10f\n',i-1,a(i));
    end
    xp=linspace(-1,1,200);
    fprintf('Approximate value of y(x)\n');
    fprintf('    n           x           p(x)');
    fprintf( '          y(x)              diff\n');
    for i=1:length(xp)
        p=xp(i);
        funval=(1./(1+25*p.^2));
        %  fplot(funval,[-1,1]);
        pn= Eval(x,a,p);
        D=abs(funval-pn);
        % disp(D);
        % fplot(diff,[-1,1]);
        fprintf('%5.0f\t%5.10f\t%15.10f\t%15.10f\t%15.10f\n',i,p,pn,funval,D);
    end
    %fplot(D,[-1,1]);
end
function[a]=Coef(x,y)
    mx=length(x);
    my=length(y);
    % if mx ~= my
    %     error('must be same lenght');
    % end
    T=zeros(mx,mx);
    T(:,1)=(my)';
    for j=2:mx
        for i=1:(mx-j+1)
            T(i,j)=(T(i+1,j-1)-T(i,j-1))/(x(i+j-1)-x(i));
        end
    end
    a=T(1,:);
end
function pn=Eval(x,a,p)
    m=length(a);
    sum=0;
    for i=1:m
        temo=1;
        for j=1:i-1
            temo=temo*(p-x(j));
        end
        sum =sum+a(i)*temo;
    end
    pn=sum;
end