matlab求解一階和二階導(dǎo)：

（1）數(shù)值方法

for i = 1?

? ? d2ydx2(i) = (y(i+2)+y(i)-2*y(i+1))/power((x(i+2)-x(i))/2,2);

end

for i = 2:n-1

? ? d2ydx2(i) = 2/(x(i+1)-x(i-1))*((y(i+1)-y(i))/(x(i+1)-x(i))-(y(i)-y(i-1))/(x(i)-x(i-1)));

end

for i = n

? ? d2ydx2(i) = (y(i)-2*y(i-1)+y(i-2))/power((x(i)-x(i-2))/2,2);

end

（2）多項(xiàng)式

y1 = polyfit(x,y,3);

x_ = linspace(0,1);

y_ = polyval(y1,x_);

x1 = x;

syms x1 f(x)

f(x1) = poly2sym(y1,x1);

f2(x1) = diff(f(x1), x1, 2);?

（3）三次樣條

pp = csape(x, y);

pp1 = fnval(fnder(pp, 1), x); %求一階導(dǎo)

pp2 = fnval(fnder(pp, 2), x); %求二階導(dǎo)

建議使用spline函數(shù)做插值，結(jié)合diff函數(shù)做高階導(dǎo)求解，當(dāng)點(diǎn)數(shù)較少時(shí)，不建議使用數(shù)值解法

matlab讀取字母、數(shù)字、標(biāo)點(diǎn)符號(hào)混合數(shù)據(jù)中的數(shù)字，使用正則表達(dá)式比較簡(jiǎn)單：

s=regexp(str,'\-?\d*\.?\d*','match');??