comparison-operators | 易学教程

Chaining “is” operators

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问题 Does python support chaining is operators, such as the following? a = None b = None a is b is None This outputs True , some doc references would be nice. 回答1: Yes. Any operators classified as comparisons can be chained. From the language reference: Formally, if a , b , c , ..., y , z are expressions and op1 , op2 , ..., opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z , except that each expression is evaluated at most once. The

Implicit data type conversion in JavaScript when comparing integer with string using ==

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问题 The code: var num = 20; if(num == \"20\") { alert(\"It works\"); } else { alert(\"Not working\"); } The question: In C programming we have a rule name data type promotion, where when there\'s a mix of data type (example: addition of integer and floating point), the integer will first converted to floating point before the addition is being carry out. The code above will prompt me an alert box with the message \"It works\" that shows the if test condition is evaluate to true. For loosely typed

What does “===” mean?

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问题 I\'ve noticed someone using the PHP operator === which I can\'t make sense out of. I\'ve tried it with a function, and it corresponds in crazy ways. What is the definition of this operator? I can\'t even find it in the declaration of PHP operators. 回答1: $a === $b (Identical) TRUE if $a is equal to $b , and they are of the same type. (introduced in PHP 4) PHP Docs 回答2: http://www.php.net/ternary $a == $b Equal TRUE if $a is equal to $b, except for (True == -1) which is still True. $a === $b

Python's in (contains) operator returns a bool whose value is neither True nor False

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问题 As expected, 1 is not contained by the empty tuple >>> 1 in () False but the False value returned is not equal to False >>> 1 in () == False False Looking at it another way, the in operator returns a bool which is neither True nor False : >>> type(1 in ()) <type \'bool\'> >>> 1 in () == True, 1 in () == False (False, False) However, normal behaviour resumes if the original expression is parenthesized >>> (1 in ()) == False True or its value is stored in a variable >>> value = 1 in () >>>

JavaScript comparison operators: Identity vs. Equality

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问题 I\'ve been trying to understand the difference between JavaScript\'s comparison operators: identity and equality. From what I\'ve read, if you check the equality of two objects using ==, JavaScript will try to figure out if they are the same type and, if not, try to get them to that same type. However, === doesn\'t behave in the same manner. So as an example: var n = \"1\"; console.log(n==1); // outputs true console.log(n===1); // outputs false So what is the difference between these \

How to check if the string is empty?

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问题 Does Python have something like an empty string variable where you can do: if myString == string.empty: Regardless, what\'s the most elegant way to check for empty string values? I find hard coding \"\" every time for checking an empty string not as good. 回答1: Empty strings are "falsy" which means they are considered false in a Boolean context, so you can just do this: if not myString: This is the preferred way if you know that your variable is a string. If your variable could also be some

Numeric comparison difficulty in R

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问题 I\'m trying to compare two numbers in R as a part of a if-statement condition: (a-b) >= 0.5 In this particular instance, a = 0.58 and b = 0.08... and yet (a-b) >= 0.5 is false. I\'m aware of the dangers of using == for exact number comparisons, and this seems related: (a - b) == 0.5) is false, while all.equal((a - b), 0.5) is true. The only solution I can think of is to have two conditions: (a-b) > 0.5 | all.equal((a-b), 0.5) . This works, but is that really the only solution? Should I just

Numeric comparison difficulty in R

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I'm trying to compare two numbers in R as a part of a if-statement condition: (a-b) >= 0.5 In this particular instance, a = 0.58 and b = 0.08... and yet (a-b) >= 0.5 is false. I'm aware of the dangers of using == for exact number comparisons, and this seems related: (a - b) == 0.5) is false, while all.equal((a - b), 0.5) is true. The only solution I can think of is to have two conditions: (a-b) > 0.5 | all.equal((a-b), 0.5) . This works, but is that really the only solution? Should I just swear off of the = family of comparison operators forever? Edit for clarity: I know that this is a

Difference between == and === in JavaScript [duplicate]

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问题 This question already has answers here : Which equals operator (== vs ===) should be used in JavaScript comparisons? (49 answers) Closed 15 days ago . What is the difference between == and === in JavaScript? I have also seen != and !== operators. Are there more such operators? 回答1: === and !== are strict comparison operators: JavaScript has both strict and type-converting equality comparison. For strict equality the objects being compared must have the same type and: Two strings are strictly