logical-operators | 易学教程

Logical operators (“and”, “or”) in DOS batch

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问题 How would you implement logical operators in DOS Batch files? 回答1: You can do and with nested conditions: if %age% geq 2 ( if %age% leq 12 ( set class=child ) ) or: if %age% geq 2 if %age% leq 12 set class=child You can do or with a separate variable: set res=F if %hour% leq 6 set res=T if %hour% geq 22 set res=T if "%res%"=="T" ( set state=asleep ) 回答2: The IF statement does not support logical operators ( AND and OR ), cascading IF statements make an implicit conjunction. IF Exist File1.Dat

Why is my c != 'o' || c != 'x' condition always true? [duplicate]

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This question already has an answer here: Why non-equality check of one variable against many values always returns true? 3 answers I have this loop statement, which I'll express using C-like syntax (C, C++, Java, JavaScript, PHP, etc. all use similar syntax): while (c != 'o' || c != 'x') { c = getANewValue(); } I want it to run until I get a 'o' or 'x' , but it never exits, even when c is 'o' or 'x' . Why not? I've also tried using if : if (c != 'o' || c != 'x') { // Show an error saying it must be either 'o' or 'x' } but that also always shows the error message, even when c is 'o' or 'x' .

Is (4 > y > 1) a valid statement in C++? How do you evaluate it if so?

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Is that a valid expression? If so, can you rewrite it so that it makes more sense? For example, is it the same as (4 > y && y > 1) ? How do you evaluate chained logical operators? The statement (4 > y > 1) is parsed as this: ((4 > y) > 1) The comparison operators < and > evaluate left-to-right . The 4 > y returns either 0 or 1 depending on if it's true or not. Then the result is compared to 1. In this case, since 0 or 1 is never more than 1 , the whole statement will always return false . There is one exception though: If y is a class and the > operator has been overloaded to do something

Logical Operators, || or OR?

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问题 I remember reading a while back in regards to logical operators that in the case of OR , using || was better than or (or vice versa). I just had to use this in my project when it came back to me but I can\'t remember which operator was recommended or if it was even true. Which is better and why? 回答1: There is no "better" but the more common one is || . They have different precedence and || would work like one would expect normally. See also: Logical operators ( the following example is taken

Why is my c != 'o' || c != 'x' condition always true? [duplicate]

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问题 This question already has answers here : Why non-equality check of one variable against many values always returns true? (3 answers) Closed last year . I have this loop statement, which I\'ll express using C-like syntax (C, C++, Java, JavaScript, PHP, etc. all use similar syntax): while (c != \'o\' || c != \'x\') { c = getANewValue(); } I want it to run until I get a \'o\' or \'x\' , but it never exits, even when c is \'o\' or \'x\' . Why not? I\'ve also tried using if : if (c != \'o\' || c !

Javascript AND operator within assignment

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问题 I know that in JavaScript you can do: var oneOrTheOther = someOtherVar || \"these are not the droids you are looking for...\"; where the variable oneOrTheOther will take on the value of the first expression if it is not null , undefined , or false . In which case it gets assigned to the value of the second statement. However, what does the variable oneOrTheOther get assigned to when we use the logical AND operator? var oneOrTheOther = someOtherVar && \"some string\"; What would happen when

Python's equivalent of && (logical-and) in an if-statement

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问题 Here\'s my code: def front_back(a, b): # +++your code here+++ if len(a) % 2 == 0 && len(b) % 2 == 0: return a[:(len(a)/2)] + b[:(len(b)/2)] + a[(len(a)/2):] + b[(len(b)/2):] else: #todo! Not yet done. :P return I\'m getting an error in the IF conditional. What am I doing wrong? 回答1: You would want and instead of && . 回答2: Python uses and and or conditionals. i.e. if foo == 'abc' and bar == 'bac' or zoo == '123': # do something 回答3: Two comments: Use and and or for logical operations in Python

Is (4 > y > 1) a valid statement in C++? How do you evaluate it if so?

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问题 Is that a valid expression? If so, can you rewrite it so that it makes more sense? For example, is it the same as (4 > y && y > 1) ? How do you evaluate chained logical operators? 回答1: The statement (4 > y > 1) is parsed as this: ((4 > y) > 1) The comparison operators < and > evaluate left-to-right. The 4 > y returns either 0 or 1 depending on if it's true or not. Then the result is compared to 1. In this case, since 0 or 1 is never more than 1 , the whole statement will always return false .

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

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问题 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

python and / or operators return value [duplicate]

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问题 This question already has an answer here: How do “and” and “or” act with non-boolean values? 8 answers I was watching a 2007 video on Advanced Python or Understanding Python, and at 18\'27\" the speaker claims \"As some may know in Python and and or return one of the two values, whereas not returns always a boolean.\" When has this been the case? As far as I can tell, and and or return booleans, too. 回答1: The and and or operators do return one of their operands, not a pure boolean value like