没有if和else语句的序言 [英] Prolog without if and else statements

问题描述

我目前正在尝试学习一些基本的序言.据我了解，我想远离if else陈述，以真正理解该语言.我在执行此操作时遇到了麻烦.我有一个看起来像这样的简单函数:

I am currently trying to learn some basic prolog. As I learn I want to stay away from if else statements to really understand the language. I am having trouble doing this though. I have a simple function that looks like this:

if a > b then 1
else if
   a == b then c
else
    -1;;

这只是我想转换为序言的非常简单的逻辑.

This is just very simple logic that I want to convert into prolog.

所以在这里我很困惑.我想先检查a> b，是否输出1.我可以简单地做一下:

So here where I get very confused. I want to first check if a > b and if so output 1. Would I simply just do:

sample(A,B,C,O):-
   A > B, 1,
   A < B, -1,
   0.

这是我想出的. o是输出，但我不知道如何使1为输出.有什么想法可以帮助我更好地理解这一点吗?

This is what I came up with. o being the output but I do not understand how to make the 1 the output. Any thoughts to help me better understand this?

进一步研究之后，我想到了这一点，但这似乎并不正确:

After going at it some more I came up with this but it does not seem to be correct:

Greaterthan(A,B,1.0).
Lessthan(A,B,-1.0).
Equal(A,B,C).

Sample(A,B,C,What):-
    Greaterthan(A,B,1.0),
    Lessthan(A,B,-1.0),
    Equal(A,B,C).

我走了正确的路吗?

推荐答案

您的代码同时存在语法和语义问题.

Your code has both syntactic and semantic issues.

谓词以小写开头，逗号表示一个连词.也就是说，您可以将子句读为

Predicates starts lower case, and the comma represent a conjunction. That is, you could read your clause as

sample(A,B,C,What) if
    greaterthan(A,B,1.0) and lessthan(A,B,-1.0) and equal(A,B,C).

然后请注意，What参数是无用的，因为它没有值-称为单例.

then note that the What argument is useless, since it doesn't get a value - it's called a singleton.

一种书写析取关系的可能方法(即OR)

A possible way of writing disjunction (i.e. OR)

sample(A,B,_,1) :- A > B.
sample(A,B,C,C) :- A == B.
sample(A,B,_,-1) :- A < B.

请注意测试A < B，以保护值-1的分配.这是必要的，因为Prolog将在需要时执行 all 子句.迫使Prolog避免我们不知道不应该完成的某些计算的基本结构是cut:

Note the test A < B to guard the assignment of value -1. That's necessary because Prolog will execute all clause if required. The basic construct to force Prolog to avoid some computation we know should not be done it's the cut:

sample(A,B,_,1) :- A > B, !.
sample(A,B,C,C) :- A == B, !.
sample(A,B,_,-1).

无论如何，即使您在学习时，我也认为您应该使用if/then/else语法.

Anyway, I think you should use the if/then/else syntax, even while learning.

sample(A,B,C,W) :- A > B -> W = 1 ; A == B -> W = C ; W = -1.

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