What's wrong with the folow expression?

f is my imput function and i have to find the first and the second derivate.

I tried to define

df=diff(f); 

f2=diff(f,2);

f1=inline (f);

df1=inline(df);

df2=inline (f2);

but appears

Error using inline (line 51)
Input must be a string." Error in convergefinal (line 11)"
df1=inline(df);

What could it be?

Answer 1

[Walter Roberson]

diff(f) would invoke f with no arguments, fetch its return value, and apply the numeric diff() function to it, where the numeric diff() function is (X(2:end) - X(1:end-1)).

If you have the symbolic toolbox, you could try

syms t
df = diff(f(t),t);
f2 = diff(df,t);
f1 = inline(char(f(t)));
df1 = inline(char(df));
df2 = inline(char(f2));

Or more directly,

syms t
ft = f(t);
df = diff(ft,t);
f2 = diff(df, t);
f1 = @f;
df1 = matlabFunction(df, 'vars', t);
df2 = matlabFunction(f2, 'vars', t);

f

[Mariana]

thank you