如何在F#中比较x和y? [英] How do I compare x and y in F#?

问题描述

我需要有关将2个数字进行比较的匹配模式的帮助.像这样的东西:

I need help with the matching pattern that would compare 2 numbers. Something like that:

let test x y =
   match x with
   | y when x < y -> printfn "less than"
   | y when x > y -> printfn "greater than"
   | _ -> printfn "equal"

当x为0且y为200时，它以某种方式落入"_"情况.我在这里做错什么了?

Somehow it falls to the "_" case when x is 0 and y is 200. What am I doing wrong here?

推荐答案

您的代码存在的问题是您编写时:

The problem with your code is that when you write:

match x with 
| y when x < y -> (...)

..这意味着您要将x的值(match <expr> with中的<expr>)分配给名为y的新变量(| <pat> when ...中的<pat>)，然后进行比较值为x的新y(现在包含x的值)-因此它将始终返回false.您始终可以重命名绑定变量，因此您的代码与编写相同:

.. it means that you want to assign the value of x (the <expr> in match <expr> with) to a new variable named y (the <pat> in | <pat> when ...) and then compare this new y (which now contains the value of x) with the value of x - and so this will always return false. You can always rename the bound variable, so your code is the same as writing:

match x with 
| newY when x < newY -> (...)

现在您可以看到为什么它永远不匹配-因为您只是将x与自身进行比较！

Now you can see why this never matches - because you are just comparing x with itself!

如果您具有一些更复杂的结构的输入(例如元组或有区别的联合，列表，数组，选项类型等)，则模式匹配特别有用.但是，如果您只想比较数字，则使用:

Pattern matching is especially useful if you have inputs of some more complicated structure - like tuples or discriminated unions, lists, arrays, option types etc. But if you simply want to compare numbers, it is much easier to just use if:

let test x y =
  if x < y then printfn "less than"
  elif x > y then printfn "greater than"
  else printfn "equal"

在您的match中，您实际上并不需要绑定任何变量-但是John的解决方案演示了如何实现该工作-它只是说，接受变量x和y并将它们分配给new变量x和y(它们具有相同的名称).

In your match, you do not really need to bind any variables - but the solution by John demonstrates how you can make that work - it simply says, take variables x and y and assign them to new variables x and y (which just have the same name).

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