为什么SSI条件！= || ！=不起作用？ [英] Why SSI condition != || != doesn't work?

问题描述

我无法使用此条件表达式

 <！ -  #if expr =$ DOCUMENT_NAME！= index.html || $ DOCUMENT_NAME！= links.html - > 
  

这个没有！之前 = 完美无缺

 <！ -  #if expr =$ DOCUMENT_NAME = index.html || $ DOCUMENT_NAME = links.html - > 
  

有什么问题？我没有收到任何错误只是！= 虽然 || 但其他条件不起作用，但适用于单一条件。<这是因为 = 和！= 几乎不是同一个运营商。请注意，通过 De Morgan定律（我也在这篇旧帖子中解释），

  a！= b || c！= d 
  

相当于

  a = b&& c = d 
  

永远不会为<$​​ c $ c> x = a&& x = b 其中 a！= b 。

更改二元运算符要求更改条件也是等效的。

因此，按照上述逻辑，

  $ DOCUMENT_NAME！= index.html || $ DOCUMENT_NAME！= links.html 
  

相当于

  $ DOCUMENT_NAME = index.html&& $ DOCUMENT_NAME = links.html 
  

无法为真，因为$ DOCUMENT_NAME可以是index.html或links.html，但不是两者。

然而，第二个片段，

  $ DOCUMENT_NAME = index.html || $ DOCUMENT_NAME = links.html 
  

有效，因为上面没有提到逻辑上永不正确的谬误。当$ DOCUMENT_NAME index.html或links.html时会是如此。

---

某些语言/值会违反上述等价物......但这是另一个主题。

I can't get working this conditional expression

<!--#if expr="$DOCUMENT_NAME!=index.html || $DOCUMENT_NAME!=links.html" -->

while this one without ! before = works perfect

<!--#if expr="$DOCUMENT_NAME=index.html || $DOCUMENT_NAME=links.html" -->

What's the problem? I get no error simply != doesn't work though || with other condition but works for single condition.

解决方案

This is because = and != are hardly the same operator. Note that, by De Morgan's law (which I also explained in this old post),

a != b || c != d

is equivalent to

a = b && c = d

which is never true for x = a && x = b where a != b.

Changing the binary operator requires changing the conditionals as well to be equivalent.

Thus, by the above logic,

$DOCUMENT_NAME!=index.html || $DOCUMENT_NAME!=links.html

is equivalent to

$DOCUMENT_NAME=index.html && $DOCUMENT_NAME=links.html

which cannot be true as $DOCUMENT_NAME can be "index.html" or "links.html" but not both.

However, the 2nd snippet,

$DOCUMENT_NAME=index.html || $DOCUMENT_NAME=links.html

"works" because there is not the logical never-true fallacy mentioned above. It will be true when $DOCUMENT_NAME is either "index.html" or "links.html".

---

Some languages/values will violate the above equivalency .. but that is another topic.

这篇关于为什么SSI条件！= || ！=不起作用？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！