Differentiation with str2sym

Hello,

I am trying to differentiate usinf str2sym but i seem to get some errors.

Here is the code:

clear all
y(1) = str2sym ('x(1)');
y(2) = str2sym ('x(2)');

f = y(1) + y(2);

diff(f, y(1))

This is the error i get

Error using sym/diff (line 70)
Second argument must be a variable or a nonnegative integer specifying the number of
differentiations.

Error in prove (line 8)
diff(f, y(1))

Answer 1

[Walter Roberson]

The error message is correct. You can only differentiate with respect to an entire variable, not with respect to a function call like x(1)

x = sym('x', [1 2]);

y(1) = str2sym ('x(1)');
y(2) = str2sym ('x(2)');

y = mapSymType(y, 'x', @(e) x(children(e)));

f = y(1) + y(2);
diff(f, y(1))

[Gabriele Ansaldo]

Thanks! However, its not exactly what I meant. If I run the code you wrote and for instance i set f = y(1)^2 +y(1) this happens:

clear all
x = sym('x', [1 2]);

y(1) = str2sym ('x(1)');
y(2) = str2sym ('x(2)');

y = mapSymType(y, 'x', @(e) x(children(e)));

f = y(1)^2+ y(2);
diff(f, y(1))

This is the result

ans =
 
2*x1

where i actually expected it to be

ans =
 
2*x(1)

with the brackets around the one.

[Walter Roberson]

x = sym('x', [1 2]);
y(1) = str2sym ('x(1)');
y(2) = str2sym ('x(2)');
Y = mapSymType(y, 'x', @(e) x(children(e)));
f = Y(1)^2 + Y(2);
df = diff(f, Y(1));
df = subs(df, x, y);