Differentiation with str2sym

24 次查看（过去 30 天）
Gabriele Ansaldo
Gabriele Ansaldo 2020-5-11
评论： Walter Roberson 2020-6-15
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))

请先登录，再回答此问题。

回答（1 个）

Walter Roberson
Walter Roberson 2020-5-12
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))
  2 个评论
Gabriele Ansaldo
Gabriele Ansaldo 2020-5-12
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
Walter Roberson 2020-6-15
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);

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Code Generation 的更多信息

标签

产品

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by