可变向量字段未在F#中更新 [英] Mutable Vector field is not updating in F#

问题描述

let gradientDescent (X : Matrix<double>) (y :Vector<double>) (theta : Vector<double>) alpha (num_iters : int) =
    let J_history = Vector<double>.Build.Dense(num_iters)
    let m = y.Count |> double
    theta.At(0, 0.0)
    let x =  (X.Column(0).PointwiseMultiply(X*theta-y)) |> Vector.sum
    for i in 0 .. (num_iters-1) do
        let next_theta0 = theta.[0] - (alpha / m) * ((X.Column(0).PointwiseMultiply(X*theta-y)) |> Vector.sum)
        let next_theta1 = theta.[1] - (alpha / m) * ((X.Column(1).PointwiseMultiply(X*theta-y)) |> Vector.sum)
        theta.[0] = next_theta0 |> ignore
        theta.[1] = next_theta1 |> ignore
        J_history.[i] = computeCost X y theta |> ignore
        ()
    (theta, J_history)

即使矩阵和向量是可变的，它们的维数也是固定的 并且创建后无法更改.

Even though matrices and vectors are mutable, their dimension is fixed and cannot be changed after creation.

http://numerics.mathdotnet.com/Matrix.html

我有theta，它是大小为2x1的向量 我正在尝试更新theta.[0]和theta.[1]迭代，但是当我在每次迭代后查看它时，它仍然是[0; 0].我知道F#是不可变的，但是我在上面从他们的网站上引用了Vectors和Matrix可变的信息，所以我不确定为什么它不起作用.

I have theta which is a vector of size 2x1 I'm trying to update theta.[0] and theta.[1] iteratively but when I look at it after each iteration it remains to be [0;0]. I know that F# is immutable but I quoted above from their website that Vectors and Matrix are mutable so I'm not sure why this isn't working.

我的直觉是它与Shadowing有关...因为我在for循环中声明了let next_theta0，但我不太确定

此外，作为后续问题.我觉得我实施此方法的方式非常糟糕.从字面上看，没有理由让我在F#中实现此功能，因为它在C#中会更容易(使用此方法)，因为它感觉不太实用"，谁能提出建议以使其更实用的方法.

Also, as a follow up question. I feel like the way I implemented this is incredibly terrible. There's literally no reason for me to have implemented this in F# when it would have been much easier in C# (using this methodology) because it doesn't feel very "functional" Could anyone suggest ways to approve on this to make it more functional.

推荐答案

F#中的破坏性更新运算符被编写为<-，因此:

The destructive update operator in F# is written <-, so:

theta.[0] <- next_theta0

您在代码中执行的操作是比较 theta.[0]和next_theta0，这是导致产生bool的操作，这就是为什么必须添加在此之后调用，以避免编译器警告.

What you're doing in your code is comparing theta.[0] with next_theta0, which is an operation that results in a bool, which is why you had to add an ignore call after it, to avoid the compiler warning.

这是一个很好的通用规则:看到编译器警告时，不要只是尝试技巧来安抚编译器.而是尝试理解为什么出现警告.很有可能，这表明存在合法问题.

Here's a good general rule: when you see a compiler warning, don't just try tricks to appease the compiler. Instead, try to understand why the warning appears. Chances are, it points to a legitimate problem.

这是一个更特定于F#的规则:使用ignore是一种代码味道. ignore是一种hack，主要用于与外部代码交互，当外部代码返回某些内容，但并不真正希望消费者使用该返回值时.

And here's a more F#-specific rule: using ignore is a code smell. ignore is a sort of hack, mostly used for interaction with external code, when the external code returns something, but doesn't really expect the consumer to use that return value.

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