警告:“对"D"的乘积及其转置进行成对处理，以确保结果为Hermetian." [英] Warning: "Parethesize the multiplication of 'D' and its transpose to ensure the result is Hermetian."

问题描述

如您在上面的屏幕快照中所见，我的Matlab m文件代码中具有以下表达式:
K = P * D * D' * P;
其中，P是nxn矩阵，D是nx1列向量(如果有关系，n = 4).

As you see in the screen shot above, I have the following expression in my Matlab m-file code:
K = P * D * D' * P;
Where, P is an nxn matrix, and D is a nx1 column vector (n=4, if it matters).

为什么会收到此警告消息?
如果我在那里使用或不使用括号，会有什么变化?

Why am I getting this warning message?
What changes if I use or don't use parenthesis there?

推荐答案

浮点算术不是关联的.因此，通常a * (b * c)不一定会得到与(a * b) * c相同的结果.

Floating-point arithmetic is not associative. So in general, a * (b * c) won't necessarily give the same result as (a * b) * c.

您编写的语句等效于((P * D) * D') * P，因此编译器警告您，如果您依赖于D * D'的埃尔米特对称性，则应强制其准确计算出该值.

Your statement as written is equivalent to ((P * D) * D') * P, so the compiler is warning you that if you're relying on the Hermitian symmetry of D * D', you should force it to calculate exactly that.

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