logical-operators

SQL Logic Operator Precedence: And and Or

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问题 Are the two statements below equivalent? SELECT [...] FROM [...] WHERE some_col in (1,2,3,4,5) AND some_other_expr and SELECT [...] FROM [...] WHERE some_col in (1,2,3) or some_col in (4,5) AND some_other_expr Is there some sort of truth table I could use to verify this? 回答1: And has precedence over Or , so, even if a <=> a1 Or a2 Where a And b is not the same as Where a1 Or a2 And b, because that would be Executed as Where a1 Or (a2 And b) and what you want, to make them the same, is the

Is short-circuiting logical operators mandated? And evaluation order?

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问题 Does the ANSI standard mandate the logical operators to be short-circuited, in either C or C++? I\'m confused for I recall the K&R book saying your code shouldn\'t depend on these operations being short circuited, for they may not. Could someone please point out where in the standard it\'s said logic ops are always short-circuited? I\'m mostly interested on C++, an answer also for C would be great. I also remember reading (can\'t remember where) that evaluation order isn\'t strictly defined,

Why don't logical operators (&& and ||) always return a boolean result?

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问题 Why do these logical operators return an object and not a boolean? var _ = (obj.fn && obj.fn() ) || obj._ || ( obj._ = {} ); var _ = obj && obj._; I want to understand why it returns result of obj.fn() (if it is defined) OR obj._ but not boolean result. 回答1: var _ = ((obj.fn && obj.fn() ) || obj._ || ( obj._ == {/* something */}))? true: false will return boolean. UPDATE Note that this is based on my testing. I am not to be fully relied upon. It is an expression that does not assign true or

Boolean operators && and ||

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问题 According to the R language definition, the difference between & and && (correspondingly | and || ) is that the former is vectorized while the latter is not. According to the help text, I read the difference akin to the difference between an \"And\" and \"AndAlso\" (correspondingly \"Or\" and \"OrElse\")... Meaning: That not all evaluations if they don\'t have to be (i.e. A or B or C is always true if A is true, so stop evaluating if A is true) Could someone shed light here? Also, is there an

Java logical operator short-circuiting

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问题 Which set is short-circuiting, and what exactly does it mean that the complex conditional expression is short-circuiting? public static void main(String[] args) { int x, y, z; x = 10; y = 20; z = 30; // T T // T F // F T // F F //SET A boolean a = (x < z) && (x == x); boolean b = (x < z) && (x == z); boolean c = (x == z) && (x < z); boolean d = (x == z) && (x > z); //SET B boolean aa = (x < z) & (x == x); boolean bb = (x < z) & (x == z); boolean cc = (x == z) & (x < z); boolean dd = (x == z)

How do “and” and “or” act with non-boolean values?

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问题 I\'m trying to learn python and came across some code that is nice and short but doesn\'t totally make sense the context was: def fn(*args): return len(args) and max(args)-min(args) I get what it\'s doing, but why does python do this - ie return the value rather than True/False? 10 and 7-2 returns 5. Similarly, changing the and to or will result in a change in functionality. So 10 or 7 - 2 Would return 10. Is this legit/reliable style, or are there any gotchas on this? 回答1: TL;DR We start by

What's the difference between & and && in JavaScript?

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What's the difference between & and && in JavaScript? Example-Code: var first = 123; var second = false; var third = 456; var fourth = "abc"; var fifth = true; alert(first & second); // 0 alert(first & third); // 72 alert(first & fourth); // 0 alert(first & fifth); // 1 alert(first && second); // false alert(first && third); // 456 alert(first && fourth); // abc alert(first && fifth); // true It seems like && is a logical "and" which gives me always the second value if both are true. But what is &? (By the way, && seems to be "and" in Python; & seems to be & in Python.) & is bitwise AND This