operator-keyword

问题 Why is the "IN" operator so slow when used with subquery? select * from view1 where id in (1,2,3,4,5,6,7,8,9,10) order by somedata; executes in 9ms. select * from view1 where id in (select ext_id from aggregate_table order by somedata limit 10) order by somedata; executes in 25000ms and seems to use sequential scan on the view ( view1 ) instead of index scan on primary keys returned by subquery as in does in first query. The subquery select ext_id from aggregate_table order by somedata limit

问题 This question already has answers here : Closed 9 years ago . Possible Duplicate: Where should non-member operator overloads be placed? While browsing on SO, I often find questions or answer that involves overloading/defining a std::ostream& operator<<(std::ostream& os, const Foo& foo) or a Foo operator+(const Foo& l, const Foo& r) . While I know how and when (not) to write these operators, I'm confused about the namespace thing. If I have the following class: namespace bar { class Foo {}; }

问题 Is there a reason why passing a reference to a std::map as const causes the [] operator to break? I get this compiler error (gcc 4.2) when I use const: error: no match for ‘operator[]’ in ‘map[name]’ Here's the function prototype: void func(const char ch, std::string &str, const std::map<std::string, std::string> &map); And, I should mention that there is no problem when I remove the const keyword in front of std::map . If I've been instructed correctly, the [] operator will actually insert a

问题 I'm trying to understand how the comma operator (,) works in JavaScript, it seems to have a different behaviour when it's not put between parenthesis. Can someone explain me why ? Exemple for reference : var a = 1; var b = 2; var c = (a,b); console.log(c); //output : as expected var c = a,b; console.log(c); //output : 1 [EDIT] The title might be a bit confusing. My question is about a misconception between the coma operator and var attribution as somone explained further down Therefore this

问题 I have two boolean lists, e.g., x=[True,True,False,False] y=[True,False,True,False] I want to AND these lists together, with the expected output: xy=[True,False,False,False] I thought that expression x and y would work, but came to discover that it does not: in fact, (x and y) != (y and x) Output of x and y : [True,False,True,False] Output of y and x : [True,True,False,False] Using list comprehension does have correct output. Whew! xy = [x[i] and y[i] for i in range(len(x)] Mind you I could

问题 So, I've got a custom class that has a __mul__ function which works with ints. However, in my program (in libraries), it's getting called the other way around, i.e., 2 * x where x is of my class. Is there a way I can have it use my __mul__ function for this? 回答1: Just add the following to the class definition and you should be good to go: __rmul__ = __mul__ 回答2: Implement __rmul__ as well. class Foo(object): def __mul__(self, other): print '__mul__' return other def __rmul__(self, other):

问题 Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 6 years ago . Just moved my program over from windows to linux and the same code that worked now gives me a segmentation fault when calling the operator<< function from main. (overview) My programs is a Vector class that takes input and returns what the input is, but when reach << endl it crashes, if I remove endl from main()

问题 It seems we cannot call type conversion operator easily in C# generic class. Here is the code. Why? T006 finally archive our target. using System; using System.Collections.Generic; using System.Text; using System.Diagnostics; using System.Reflection; using System.Linq; namespace ConsoleApplication1 { class vec<T, T2> : List<T> where T : class { public vec(IEnumerable<T2> other) { //Converter<T2, T> cvt = (v) => (T)v; // T004 failed, try defined function dynamicly, cannot compile, too. // T006

问题 I had questions about incremental and decremental operators.I couldn't understand why java gave these outputs. x = 5; y = 10; System.out.println(z = y *= x++); // output is 50 x = 2; y = 3; z = 4; System.out.println("Result = "+ z + y++ * x); // output is Result = 46 x = 5; System.out.println( x++*x); // output is 30 x = 5; System.out.println( x*x++); // output is 25 For example, in 2nd println function y is multiplicated without increasing 1 and in 3rd function x is multiplicated with x+1.

问题 When I run this script: #!/bin/bash if [[ "abcd" =~ ^.*$ ]]; then echo "something" fi I get: ./tmp2.sh: line 3: conditional binary operator expected ./tmp2.sh: line 3: syntax error near `=~' ./tmp2.sh: line 3: `if [[ "abcd" =~ ^.*$ ]]; then' I've tried all suggestions I've found, but still the same:/ Help me, please! 回答1: Given that you're seeing a bash-specific error message , we can rule out that something other than bash is running the script (if it were a POSIX-features-only shell, such