和|有什么区别?和||在MATLAB中? [英] What's the difference between | and || in MATLAB?
问题描述
MATLAB中的|和||逻辑运算符有什么区别?
What is the difference between the | and || logical operators in MATLAB?
推荐答案
I'm sure you've read the documentation for the short-circuiting operators, and for the element-wise operators.
一个重要的区别是，逐元素运算符可以对数组进行运算，而短路运算符仅适用于标量逻辑操作数.
One important difference is that element-wise operators can operate on arrays whereas the short-circuiting operators apply only to scalar logical operands.
但是可能的关键区别在于短路问题.对于短路算子，从左到右对表达式进行求值，一旦确定最终结果，就不对其余项求值.
But probably the key difference is the issue of short-circuiting. For the short-circuiting operators, the expression is evaluated from left to right and as soon as the final result can be determined for sure, then remaining terms are not evaluated.
例如，考虑
x = a && b
如果a评估为false，则我们知道a && b评估为false，而与b评估的内容无关.因此，无需评估b.
If a evaluates to false, then we know that a && b evaluates to false irrespective of what b evaluates to. So there is no need to evaluate b.
现在考虑以下表达式:
NeedToMakeExpensiveFunctionCall && ExpensiveFunctionCall
我们认为ExpensiveFunctionCall需要很长时间才能评估.如果我们可以执行其他一些便宜的测试，从而允许我们跳过对ExpensiveFunctionCall的调用，则可以避免调用ExpensiveFunctionCall.
where we imagine that ExpensiveFunctionCall takes a long time to evaluate. If we can perform some other, cheap, test that allows us to skip the call to ExpensiveFunctionCall, then we can avoid calling ExpensiveFunctionCall.
因此，假设NeedToMakeExpensiveFunctionCall计算为false.在这种情况下，因为我们使用了短路运算符，所以不会调用ExpensiveFunctionCall.
So, suppose that NeedToMakeExpensiveFunctionCall evaluates to false. In that case, because we have used short-circuiting operators, ExpensiveFunctionCall will not be called.
相反，如果我们使用逐元素运算符并编写如下函数:
In contrast, if we used the element-wise operator and wrote the function like this:
NeedToMakeExpensiveFunctionCall & ExpensiveFunctionCall
然后将永远不会跳过对ExpensiveFunctionCall的调用.
then the call to ExpensiveFunctionCall would never be skipped.
事实上，我确实希望 MATLAB文档您已经阅读过，其中包括一个很好的例子，很好地说明了这一点:
In fact the MATLAB documentation, which I do hope you have read, includes an excellent example that illustrates the point very well:
x = (b ~= 0) && (a/b > 18.5)
在这种情况下，如果b为零，则无法执行a/b.因此，对b ~= 0的测试.使用短路运算符意味着，当b为零时，我们避免计算a/b，因此避免了会出现的运行时错误.显然，基于元素的逻辑运算符将无法避免运行时错误.
In this case we cannot perform a/b if b is zero. Hence the test for b ~= 0. The use of the short-circuiting operator means that we avoid calculating a/b when b is zero and so avoid the run-time error that would arise. Clearly the element-wise logical operator would not be able to avoid the run-time error.
有关短路评估的详细讨论，请参见上的 Wikipedia文章主题.
For a longer discussion of short-circuit evaluation, refer to the Wikipedia article on the subject.
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