short-circuiting | 易学教程

C++ short-circuiting of booleans

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问题 I'm new to c++ and am curious how the compiler handles lazy evaluation of booleans. For example, if(A == 1 || B == 2){...} If A does equal 1, is the B==2 part ever evaluated? 回答1: No, the B==2 part is not evaluated. This is called short-circuit evaluation. Edit: As Robert C. Cartaino rightly points out, if the logical operator is overloaded, short-circuit evaluation does not take place (that having been said, why someone would overload a logical operator is beyond me). 回答2: Unless the ||

Short circuit evaluation with both && || operator

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问题 I know what is short circuit evaluation in C. a && b (operand b is not checked if a = 0) a || b (operand b is not checked if a = non zero) But I am stuck at this question int x = 0; if (5 || 2 && ++x) printf("%d", x); This outputs 0 . My first thinking goes as follows: According to precedence table , precedence is ++ , && , || (descending order) ++x : evaluated.x becomes 1. 2 && ++x evaluated. Both operands are evaluated. || is evaluated. But according to this, 1 should be printed, not 0 . My

Is the shortcircuit behaviour of Python's any/all explicit?

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问题 Prompted by the discussion here The docs suggest some equivalent code for the behaviour of all and any Should the behaviour of the equivalent code be considered part of the definition, or can an implementation implement them in a non-shortcircuit manner? Here is the relevant excerpt from cpython/Lib/test/test_builtin.py def test_all(self): self.assertEqual(all([2, 4, 6]), True) self.assertEqual(all([2, None, 6]), False) self.assertRaises(RuntimeError, all, [2, TestFailingBool(), 6]) self

Short circuit on |= and &= assignment operators in C#

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问题 I know that || and && are defined as short-circuit operators in C#, and such behaviour is guaranteed by the language specification, but do |= and &= short-circuit too? For example: private bool IsEven(int n) { return n % 2 == 0; } private void Main() { var numbers = new int[] { 2, 4, 6, 8, 9, 10, 14, 16, 17, 18, 20 }; bool allEven = true; bool anyOdd = false; for (int i = 0; i < numbers.Length; i++) { allEven &= IsEven(numbers[i]); anyOdd |= !IsEven(numbers[i]); } } When the 9 entry is hit,

PHP short circuit lazy evaluation, where is it in the php.net manual?

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问题 Sorry if this sounds like a really silly question. But I Googled the web and also Googled specifically both the php.net site and the stackoverflow.com site. I know PHP does short circuit lazy evaluation when using and, or, &&, || operators, but where is it stated loud and clear in the PHP manual??? I found only Wikipedia as the only \'trusted\' source that say PHP does lazy evaluation on these operators. 回答1: Closest thing I can find to an 'official' mention of PHP's short-circuit

What is short circuiting and how is it used when programming in Java? [duplicate]

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Possible Duplicate: Does java evaluate remaining conditions after boolean result is known Why do we usually use || not | , what is the difference? I missed my class lecture the other day and I was wondering if anyone could give an explanation what short circuiting is and maybe an example of it being used in a simple Java program. Thanks for your help! T.J. Crowder Short-circuiting is where an expression is stopped being evaluated as soon as its outcome is determined. So for instance: if (a == b || c == d || e == f) { // Do something } If a == b is true, then c == d and e == f are never

How to check for null in Twig?

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问题 What construct should I use to check whether a value is NULL in a Twig template? 回答1: Depending on what exactly you need: is null checks whether the value is null : {% if var is null %} {# do something #} {% endif %} is defined checks whether the variable is defined: {% if var is not defined %} {# do something #} {% endif %} Additionally the is sameas test, which does a type strict comparison of two values, might be of interest for checking values other than null (like false ): {% if var is

Short-circuit evaluation on C

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问题 I\'m studying C from A Book on C by Kelley-Pohl, and there\'s this exercise that I don\'t understand: int a = 0, b = 0, x; x = 0 && (a = b = 777); printf(\"%d %d %d\\n\", a, b, x); x = 777 || (a = ++b); printf(\"%d %d %d\\n\", a, b, x); They just say to imagine the output and compare it to the real one. I thought the output would have been 777 777 0 778 778 1 but it is 0 0 0 0 0 1 回答1: The && operator uses lazy evaluation. If either side of the && operator is false , then the whole expression

Java ternary (immediate if) evaluation

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问题 I can\'t find the relevant portion of the spec to answer this. In a conditional operator statement in Java, are both the true and false arguments evaluated? So could the following throw a NullPointerException Integer test = null; test != null ? test.intValue() : 0; 回答1: Since you wanted the spec, here it is (from §15.25 Conditional Operator ? :, the last sentence of the section): The operand expression not chosen is not evaluated for that particular evaluation of the conditional expression.

Is there actually a reason why overloaded && and || don't short circuit?

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问题 The short circuiting behaviour of the operators && and || is an amazing tool for programmers. But why do they lose this behaviour when overloaded? I understand that operators are merely syntactic sugar for functions but the operators for bool have this behaviour, why should it be restricted to this single type? Is there any technical reasoning behind this? 回答1: All design processes result in compromises between mutually incompatible goals. Unfortunately, the design process for the overloaded