logical-operators | 易学教程

Logical AND, OR: Is left-to-right evaluation guaranteed?

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问题 Is left-to-right evaluation of logical operators ( && || ) guaranteed? Let\'s say I have this: SDL_Event event; if (SDL_PollEvent(&event)) { if (event.type == SDL_QUIT) { // do stuff } } Is this guaranteed to be the same as this? SDL_Event event; if (SDL_PollEvent(&event) && event.type == SDL_QUIT) { // do stuff } This can also be very important, let\'s say we have two requirements, a and b . Requirement a is much more likely to fail then b . Then it\'s more efficient to say if (a && b) than

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

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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? You would want and instead of && . David Titarenco Python uses and and or conditionals. i.e. if foo == 'abc' and bar == 'bac' or zoo == '123': # do something Two comments: Use and and or for logical operations in Python. Use 4 spaces to indent instead of 2. You will thank yourself later because your code will look pretty

R gotcha: logical-and operator for combining conditions is & not &&

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问题 Why doesn\'t subset() work with a logical and && operator combining two conditions? > subset(tt, (customer_id==177 && visit_date==\"2010-08-26\")) <0 rows> (or 0-length row.names) but they each work individually: > subset(tt, customer_id==177) > subset(tt, visit_date==\"2010-08-26\") (Want to avoid using large temporary variables - my dataset is huge) 回答1: From the help page for Logical Operators , accessible by ?"&&" : & and && indicate logical AND and | and || indicate logical OR. The

Reason for the existence of non-short-circuit logical operators

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问题 When used with boolean operands, & and | become logical operators per Section 15.22.2 of the JLS. Unlike && and || , however, these don\'t short-circuit; they always evaluate both sides. I have a silly question: Why are the less-efficient non-short-circuit logical operators ( & , | ) still there, when we have the more-efficient short-circuit logical operators ( && , || )? I mean, what is the real usage of the non-short-circuit logical operators, as opposed to with the short-circuit logical

Dealing with TRUE, FALSE, NA and NaN

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问题 Here is a vector a <- c(TRUE, FALSE, FALSE, NA, FALSE, TRUE, NA, FALSE, TRUE) I\'d like a simple function that returns TRUE everytime there is a TRUE in \"a\", and FALSE everytime there is a FALSE or a NA in \"a\". The three following things do not work a == TRUE identical(TRUE, a) isTRUE(a) Here is a solution a[-which(is.na(a))] but it doesn\'t seem to be a straightforward and easy solution Is there another solution ? Here are some functions (and operators) I know: identical() isTRUE() is.na

Logical Operators in C

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问题 I am having trouble trying to understand how logical operators work in C. I already understand how the bit-level operators work, and I also know that logical operators treat nonzero arguments as representing TRUE and zero arguments as representing FALSE But say we have 0x65 && 0x55. I do not understand why and how this operations gives 0x01. I tried to convert it to binary, but I cannot figure out how it works 回答1: The && is a logical AND (as opposed to & , which is a bitwise AND ). It cares

Simple logical operators in Bash

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问题 I have a couple of variables and I want to check the following condition (written out in words, then my failed attempt at bash scripting): if varA EQUALS 1 AND ( varB EQUALS \"t1\" OR varB EQUALS \"t2\" ) then do something done. And in my failed attempt, I came up with: if (($varA == 1)) && ( (($varB == \"t1\")) || (($varC == \"t2\")) ); then scale=0.05 fi 回答1: What you've written actually almost works (it would work if all the variables were numbers), but it's not an idiomatic way at all. (…

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

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问题 What is the difference between the & and && logical operators in MATLAB? 回答1: The single ampersand & is the logical AND operator. The double ampersand && is again a logical AND operator that employs short-circuiting behaviour. Short-circuiting just means the second operand (right hand side) is evaluated only when the result is not fully determined by the first operand (left hand side) A & B (A and B are evaluated) A && B (B is only evaluated if A is true) 回答2: && and || take scalar inputs and

!! c operator, is a two NOT?

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问题 I reading this code, and have this line switch (!!up + !!left) { what is !! operator ? two logical NOT ? 回答1: yes, it's two nots. !!a is 1 if a is non-zero and 0 if a is 0 You can think of !! as clamping, as it were, to {0,1} . I personally find the usage a bad attempt to appear fancy. 回答2: You can imagine it like this: !(!(a)) If you do it step by step, this make sense result = !42; //Result = 0 result = !(!42) //Result = 1 because !0 = 1 This will return 1 with any number (-42, 4.2f, etc.)

The written versions of the logical operators

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问题 This is the only place I\'ve ever seen and , or and not listed as actual operators in C++. When I wrote up a test program in NetBeans, I got the red underlining as if there was a syntax error and figured the website was wrong, but it is NetBeans which is wrong because it compiled and ran as expected. I can see ! being favored over not but the readability of and && or seems greater than their grammatical brothers. Why do these versions of the logical operators exist and why does seemingly no