Adding an amount to a specified array in an expression.

Adam Anderson

2012 2 1

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5 次查看（30 天）

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I wrote code on the forward, backward, and central finite difference methods of a polynomial as well as the first and second true derivatives and plotted them. I was wondering if there is a simple way to name the polynomial and add the step size to x without having to write out each expression separately which I have already done. something like

   f=expression
    f1(x+h)= expressions with h added to each variable
  here is the code  
        syms xx
    f=x.^3-2.*x+4;
    der1true=diff(f)
    der2true=diff(f,2)
    x=-2:.1:2;
    h=.25;
    d1f=(((x+h).^3-2.*(x+h)+4)-(x.^3-2.*x+4))/h
    d1b=((x.^3-2.*x+4)-((x-h).^3-2.*(x-h)+4))/h
    d1c=(((x+.5*h).^3-2.*(x+.5*h)+4)-((x-.5*h).^3-2.*(x-.5*h)+4))/(2*h)
    d2f=(((x+2*h).^3-2.*(x+2*h)+4-(2*((x+h).^3-2.*(x+h)+4))+(x.^3-2.*x+4)))/h^2
    d2b=((x.^3-2.*x+4)-(2*(x-h).^3-(2.*(x-h))+4)+((x-2*h).^3-2.*(x-2*h)+4))
    d2c=(((x+h).^3-2.*(x+h)+4)-(2*(x.^3-2.*x+4))-((x-h).^3-2.*(x-h)+4))
    subplot(1,2,1)
    hold on
    plot(der1true,'-m')
    plot(d1f,'-.g')
    plot(d1b,':k')
    plot(d1c,'--r')
    hold off
    subplot(1,2,2);hold on
    plot(der2true,'-m')
    plot(d2b,':k')
    plot(d2c,'--r')
    plot(d2f,'-.g')
    hold off
    I think maybe the sub command but not sure
Thanks for any help provided