我可以在MATLAB中更改符号函数的公式吗? [英] Can I change the formula of a symbolic function in MATLAB?

问题描述

我有以下代码:

syms t x;
e=symfun(x-t,[x,t]);

在这个问题中，我想解决x是t的函数，但我只知道给定t的值，因此在这里将其建模为变量.我想针对时间区分e而不丢失" x ，这样我就可以用我所知道的x'(t)代替它. 在我的另一个问题中，有人建议我写以下内容:

In the problem I want to solve x is a function of t but I only know its value at the given t,so I modeled it here as a variable.I want to differentiate e with respect to time without "losing" x,so that I can then substitute it with x'(t) which is known to me. In another question of mine here,someone suggested that I write the following:

e=symfun(exp(t)-t,[t]);

然后进行微分检查，是否可以将exp(t)替换为x'(t)的值.

and after the differentiation check if I can substitute exp(t) with the value of x'(t).

这可能吗?还有其他更整洁的方式吗?

Is this possible?Is there any other neater way?

推荐答案

我真的不确定我是否理解您的要求(而且我也不了解您的

I'm really not sure I understand what you're asking (and I didn't understand your other question either), but here's an attempt.

由于x是时间的函数，因此，通过使其成为 symfun 调用抽象"或任意"符号函数，即没有定义的符号函数.在Matlab R2014b中:

Since, x is a function of time, let's make that explicit by making it what the help and documentation for symfun calls an "abstract" or "arbitrary" symbolic function, i.e., one without a definition. In Matlab R2014b:

syms t x(t);
e = symfun(x-t,t)

返回

e(t) =

x(t) - t

采用symfun函数e的时间导数:

edot = diff(e,t)

返回

edot(t) =

D(x)(t) - 1

edot(t)的表达式是x的时间导数的函数:

the expression for edot(t) is a function of the derivative of x with respect to time:

xdot = diff(x,t)

这是抽象的symfun:

xdot(t) =

D(x)(t)

现在，我想您希望能够将xdot(xdot_given)的特定值替换为t_given处的t的e(t).您应该只使用 subs 即可执行此操作像这样:

Now, I think you want to be able to substitute a specific value for xdot (xdot_given) into e(t) for t at t_given. You should be able to do this just using subs, e.g., something like this:

sums t_given xdot_given;
edot_t_given = subs(edot,{t,xdot},{t_given, xdot_given});

如果edot的唯一随时间变化的部分是xdot部分，则可能无需替换t.

You may not need to substitute t if the only parts of edot that are a function of time are the xdot parts.

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