operators

问题 This question already has an answer here : Python commutative operator override (1 answer) Closed 2 years ago . I have a simple class that helps with mathematical operations on vectors (i.e. lists of numbers). My Vector can be multiplied by other instances of Vector or a scalar ( float or int ). In other, more strongly typed, languages I would create a method to multiply two vector s and a separate method to multiply a vector by and int / float . I'm still pretty new to Python and am not sure

问题 Why does the following behave unexpectedly in Python? >>> a = 256 >>> b = 256 >>> a is b True # This is an expected result >>> a = 257 >>> b = 257 >>> a is b False # What happened here? Why is this False? >>> 257 is 257 True # Yet the literal numbers compare properly I am using Python 2.5.2. Trying some different versions of Python, it appears that Python 2.3.3 shows the above behaviour between 99 and 100. Based on the above, I can hypothesize that Python is internally implemented such that

问题 probably pretty easy, but I wonder what the ~= operator means in lua, as in if x ~= params then Searched but found nothing. 回答1: the ~= is not equals It is the equivalent in other languages of != 回答2: The operator ~= is exactly the negation of equality (==). See docs. 回答3: During compilation it gets translated to not a == b . 来源： https://stackoverflow.com/questions/34713203/what-does-lua-operator-mean

问题 What is the difference between IN and ANY operators in SQL ? 回答1: SQL> SQL> -- Use the ANY operator in a WHERE clause to compare a value with any of the values in a list. SQL> SQL> -- You must place an =, <>, <, >, <=, or >= operator before ANY. SQL> SELECT * 2 FROM employee 3 WHERE salary > ANY (2000, 3000, 4000); For In Operator SQL> -- Use the IN operator in a WHERE clause to compare a value with any of the values in a list. SQL> SELECT * 2 FROM employee 3 WHERE salary IN (2000, 3000, 4000

问题 As far as I know, the in operator in Python can't be chained or at least I couldn't find any info on it, here is my problem Here is the code arr = [1, True, 'a', 2] print('a' in arr in arr) # prints False print(('a' in arr) in arr) # prints True What I don't understand is the first print, I know in the second the first in returns True and then it check if True is in arr , but what about the first one? Does it check if 'a' is in arr and then if arr is in arr ? 回答1: The premise is false; the in

问题 As far as I know, the in operator in Python can't be chained or at least I couldn't find any info on it, here is my problem Here is the code arr = [1, True, 'a', 2] print('a' in arr in arr) # prints False print(('a' in arr) in arr) # prints True What I don't understand is the first print, I know in the second the first in returns True and then it check if True is in arr , but what about the first one? Does it check if 'a' is in arr and then if arr is in arr ? 回答1: The premise is false; the in

问题 Ruby does not support incrementing variables like variable++ . I saw this behaviour that: 2 ++ 4 gives 6 . In fact, any number of + signs between two variables are treated as one single + . How does ruby manage that? And since ruby does that, can it be taken as the reason for the unavailability of the ++ operator? 回答1: This: 2 ++ 4 is parsed as: 2 + (+4) so the second + is a unary plus. Adding more pluses just adds more unary + operators so: 2 ++++++ 4 is seen as: 2 + (+(+(+(+(+(+4)))))) If

问题 I want to do something to a couple of variables using operators in quick succession. I don't think what I want to do is important as such; my question is more about the fundamentals of JavaScript evaluation. In the three examples below, I try to use addition to change the values of two variables. However, not all perform as I (perhaps naïvely) expected. JSFiddle here. OPERATIONS AS THREE SEPARATE STATEMENTS var a = 9, b = 2; a += b; b += a; a += b; // a === 24, b === 13 OPERATIONS SEPARATED

问题 I want to do something to a couple of variables using operators in quick succession. I don't think what I want to do is important as such; my question is more about the fundamentals of JavaScript evaluation. In the three examples below, I try to use addition to change the values of two variables. However, not all perform as I (perhaps naïvely) expected. JSFiddle here. OPERATIONS AS THREE SEPARATE STATEMENTS var a = 9, b = 2; a += b; b += a; a += b; // a === 24, b === 13 OPERATIONS SEPARATED

问题 I am confused about the evaluation time of sizeof operator. When does the sizeof operator get evaluated? Does its evaluation time (compile-time or runtime) depend on the language (C? C++?)? Can we use sizeof in case of objects created at runtime in C++? 回答1: In almost all cases, sizeof is evaluated based on static type information (at compile-time, basically). One exception (the only one, I think) is in the case of C99's variable-length arrays (VLAs). 回答2: Almost always compile time. But the