APL中的条件函数 [英] Conditional function in APL

问题描述

在任何APL方言中，是否有条件函数的符号或着名成语？

我确定我错过了某些东西，因为这是一个基本的语言元素。在其他语言中，它被称为条件运算符，但我会在这里避免使用这个术语，因为APL运算符是完全不同的东西。

例如C朋友有 x？ T：F

LISP有（如果x TF）

Python有 T if x else F

等等。

我知道现代APL有：如果和朋友，但它们是控制程序流的命令语句：它们不返回值，不能在表达式内使用，当然不能应用于布尔数组。他们有完全不同的目的，这对我来说很好。

唯一能够做出功能选择的体面表达是 （FT）[⎕IO+ x] ，它看起来并不是特别简短或可读的，尽管它完成了工作，甚至在数组上也是如此：

 （'no''yes'）[⎕IO+（⍳5）∘。>（⍳5）] 
 no no no no no 
是否否否否否
是是否否否
是是是否否
是是是是否
  

我试图用类似于⌷的类似表达式，但在布尔数组上失败了。即使我可以，它仍然需要嵌入⎕IO或硬编码1，就可读性而言甚至更糟。

在我继续并定义自己的 if 并将其用于每一个我将永远写下的程序之前，有没有关于此的任何规范？我缺少一个明显的函数或操作符？

（是否有任何APL程序员在SO？上： - ）

解决方案

是的，这里有APL程序员（但不是很多！）。

我认为答案是没有标准。

对于标量解决方案，我使用pick：

  $ b $ / code> 

虽然对于布尔数组，我使用上面的索引：

  ft [x] 
  

我总是使用索引原点零，所以不需要添加1，并且不需要parens。



如果这些不够简单，我想你必须用一个名为if的函数来覆盖它们。这也可以让你把真假放在tf的自然排序中。


Is there a symbol or well-known idiom for the conditional function, in any of the APL dialects?

I'm sure I'm missing something, because it's such a basic language element. In other languages it's called conditional operator, but I will avoid that term here, because an APL operator is something else entirely.

For example C and friends have x ? T : F
LISPs have (if x T F)
Python has T if x else F
and so on.

I know modern APLs have :If and friends, but they are imperative statements to control program flow: they don't return a value, cannot be used inside an expression and certainly cannot be applied to arrays of booleans. They have a different purpose altogether, which is just fine by me.

The only decent expression I could come up with to do a functional selection is (F T)[⎕IO+x], which doesn't look particularly shorthand or readable to me, although it gets the job done, even on arrays:

      ('no' 'yes')[⎕IO+(⍳5)∘.>(⍳5)]
no  no  no  no  no
yes no  no  no  no
yes yes no  no  no
yes yes yes no  no
yes yes yes yes no

I tried to come up with a similar expression using squad ⌷, but failed miserably on arrays of booleans. Even if I could, it would still have to embed ⎕IO or an hardcoded 1, which is even worse as far as readability is concerned.

Before I go ahead and define my own if and use it on every program I will ever write, is there any canon on this? Am I missing an obvious function or operator?

(Are there any APL programmers on SO? :-)

解决方案

Yes, there are APL programmers on SO (but not many!).

I think the answer is that there is no standard on this.

For a scalar solution, I use "pick":

  x⊃f t

While for a Boolean array I use indexing as you do above:

f t[x]

I always use index origin zero, so there is no need to add 1, and the parens are not needed.

If these are not simple enough, I think you have to cover them with a function named "if". That will also let you put the true and false in the perhaps more natural ordering of t f.

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