如何在Matlab中评估函数的导数? [英] how to evaluate derivative of function in matlab?

问题描述

这应该非常简单.我有一个函数f(x)，我想在MATLAB中为给定的x评估f'(x).

This should be very simple. I have a function f(x), and I want to evaluate f'(x) for a given x in MATLAB.

我所有的搜索都提出了符号数学，这不是我所需要的，我需要数值微分.

All my searches have come up with symbolic math, which is not what I need, I need numerical differentiation.

例如如果我定义:fx = inline('x.^2')

我想找到f'(3)，应该是6，我不想找到2x

I want to find say f'(3), which would be 6, I don't want to find 2x

推荐答案

要获取数值差(对称差)，请计算(f(x+dx)-f(x-dx))/(2*dx)

To get a numerical difference (symmetric difference), you calculate (f(x+dx)-f(x-dx))/(2*dx)

fx = @(x)x.^2;
fPrimeAt3 = (fx(3.1)-fx(2.9))/0.2;

或者，您可以创建函数值的向量并应用 DIFF ，即

Alternatively, you can create a vector of function values and apply DIFF, i.e.

xValues = 2:0.1:4;
fValues = fx(xValues);
df = diff(fValues)./0.1; 

请注意，diff采用前向差，并且假定dx等于1.

Note that diff takes the forward difference, and that it assumes that dx equals to 1.

但是，根据您的情况，最好将fx定义为多项式，并评估函数的导数，而不是函数值.

However, in your case, you may be better off to define fx as a polynomial, and evaluating the derivative of the function, rather than the function values.

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