variable-assignment

问题 Given the following example: declare @i int select @i = 1, @i = 2 select @i Will @i always be 2? This is about the most trivial example I can think of, but I am considering using this for swapping values in variables. I also believe this method of assignment (select) is not ANSI compliant (however useful), but don't really care to have portable code in this case. UPDATE Thanks to @MichaelFredrickson, we have @MartinSmith's answer and reference to MSDN on this. I am now struggling with what

问题 This instruction works: SELECT INTO unsolvedNodes array_agg(DISTINCT idDestination) FROM road WHERE idOrigin = ANY(solvedNodes) AND NOT (idDestination = ANY(solvedNodes)); But I would like to use something this way: SELECT INTO unsolvedNodes array_agg(DISTINCT idDestination), lengths array_agg(length) FROM road WHERE idOrigin = ANY(solvedNodes) AND NOT (idDestination = ANY(solvedNodes)); How to use only one "SELECT INTO" instruction to set multiple variables? 回答1: In PL/pgSQL you can SELECT

问题 This instruction works: SELECT INTO unsolvedNodes array_agg(DISTINCT idDestination) FROM road WHERE idOrigin = ANY(solvedNodes) AND NOT (idDestination = ANY(solvedNodes)); But I would like to use something this way: SELECT INTO unsolvedNodes array_agg(DISTINCT idDestination), lengths array_agg(length) FROM road WHERE idOrigin = ANY(solvedNodes) AND NOT (idDestination = ANY(solvedNodes)); How to use only one "SELECT INTO" instruction to set multiple variables? 回答1: In PL/pgSQL you can SELECT

问题 Using the complex class and library, how do I assign a complex number to a variable? I understand that I can set the value when I first instantiate the complex number. I also understand that I can assign one instantiated complex number to another. How can I directly assign a complex number to a variable? Reference: http://www.cplusplus.com/reference/complex/complex/operators/ Example: #include <iostream> #include <complex> int main() { complex<double> a(1.2,3.4), b; cout << a; //-> (1.2,3.4)

问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: R: How to convert string to variable name? In R, I'm writing a for-loop that will iteratively create variable names and then assign values to each variable. Here is a simplified version. The intention is to create the variable's name based on the value of iterating variable i, then fill the new variable with NA values. (I'm only iterating 1:1 below since the problem occurs isn't related to the looping itself,

问题 int matrix[9][9],*p; p=matrix[0]; this works and gives first row of matrix , but how to get first column of matrix I've tried p=matrix[][0]; ? Also I don't understand why below code gets compiler error ? int matrix[9][9],p[9]; // it looks really ugly, byt why it doesn't work ? p=matrix[0]; // compiler gives "invalid array assigment" is it because multidimensional arrays are arrays of arrays - and we should interpret matrix[i][j] as j-th element of i-th nested array ? 回答1: In C/C++,

问题 I have read the question here: Is it problematic to assign a new value to a method parameter?. However it is not clear to me if doing something like: public void myMethod(Object obj) { doSomething(obj); obj = getNewObj(); } or: public void anotherMethod(Object obj) { obj = doSomething(obj): } This is basically just to avoid declaring a new local variable, is this worth it?, is this seen as a bad practice?. 回答1: This is a bad practice. You will be hard pressed to find a scenario where the

问题 For some types in Python, the is operator seems to be equivalent to the == operator. For example: >>> 1 is 1 True >>> "a spoon" is "a spoon" True >>> (1 == 1) is (2 == 2) True However, this is not always the case: >>> [] == [] True >>> [] is [] False This makes sense for mutable types such as lists. However, immutable types such as tuples seem to display the same behavior: >>> (1, 2) == (1, 2) True >>> (1, 2) is (1, 2) False This raises several questions: Is the == / is equivalence related to

问题 Since PHP 5.3, it is possible to leave out the middle part of the ternary operator. Expression expr1 ?: expr3 returns expr1 if expr1 evaluates to TRUE, and expr3 otherwise. Is there any better or more concise way than following code to set default value of variables? $v = isset($v) ? $v : "default value"; 回答1: Here is a shorter syntax: isset($v) || $v="default value"; 回答2: TL;DR - No, that expression can't be made any shorter. What you want is for the shortened ternary expression to perform

问题 I've actually heard claims both ways. I suspect they are not, but I wanted to get the topic settled. 回答1: C++03 does not know about the existance of threads, therefore the concept of atomicity doesn't make much sense for C++03, meaning that it doesn't say anything about that. C++11 does know about threads, but once again doesn't say anything about the atomicity of assigning pointers. However C++11 does contain std::atomic<T*> , which is guaranteed to be atomic. Note that even if writing to a