expression

问题 Is there a way to construct an XPath that evaluates whether an element's value is in a predefined list of values? Something akin to this: /Location/Addr[State='TX or AL or MA'] Which would match nodes whith State elements for Texas, Alabama, or Massachusetts? I know that I can unpack the expression: /Location/Addr[State='TX] or /Location/Addr[State='AL'], etc... But this is a bit cumbersome since the xpaths are quite long, as is the list of values. My google-fu isn't turning up much on the

问题 I currently use C for numerical computations. I've heard that using C++ Expression Templates is better for scientific computing. What are C++ Expression Templates in simple terms? Are there books around that discuss numerical methods/computations using C++ Expression Templates? In what way, C++ Expression Templates are better than using pure C? 回答1: What are C++ Expression Templates in simple terms? Expression templates are a category of C++ template meta programming which delays evaluation

问题 I'm wondering if it's guaranteed that in a Java program, the boolean expression on the right of a conjunction (exp2 above) will NOT be evaluated as long as the expression on the left (exp1) evaluated to false. I'm wondering because I have an expression like the following: if (var != null && var.somePredicate()) // do something If Java is not guaranteed to stop evaluating (var != null && var.somePredicate()) after it sees that var is null, then it may try to evaluate var.somePredicate() which

问题 I'm looking for a relatively simpler (when compared with writing a parser) way to evaluate boolean expressions in Java, and I do not want to use the JEP library. I have a String expression like: (x > 4 || x < 8 && p > 6) and my aim is to replace the variables with values. Is there a way by which I can evaluate this expression? Bear in mind that this can be any level deep so writing a parser would be very complex. 回答1: You could use the scripting engine in Java6 and the choose any of the

问题 Here is the code: mf = function(..., expr) { expr = substitute(expr) print(class(expr)) print(str(expr)) expr } mf(a = 1, b = 2, expr = {matrix(NA, 4, 4)}) Output: [1] "{" length 2 { matrix(NA, 4, 4) } - attr(*, "srcref")=List of 2 ..$ :Class 'srcref' atomic [1:8] 1 25 1 25 25 25 1 1 .. .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860> ..$ :Class 'srcref' atomic [1:8] 1 26 1 41 26 41 1 1 .. .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile'

问题 Introduction The C11 standard (ISO/IEC 9899:2011) has introduced a new definition of side effect sequencing within an expression (see related question). The sequence point concept has been complemented with sequenced before and sequenced after relations which are now the basis for all definitions. Section 6.5 "Expressions", point 2 says: If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the

问题 Description I have a expression to point on a property of my type. But it does not work for every property type. "Does not mean" means it result in different expression types. I thought it will ever result in a MemberExpression but this is not the case. For int and Guid it results in a UnaryExpression and for string in a MemberExpression . I am a little confused ;) Some sample code My class public class Person { public string Name { get; set; } public int Age { get; set; } } Test Code Person

问题 Is there a reason why the following piece of code doesn't work in IE? While with FF and other sane browsers it splits the string by the given expression, in IE it simply doesn't work. var str = "abc<font id=\"something\">def</font>gh"; alert(str.split(/(\<.*?\>|.)/).length); Thank you. 回答1: you could add the code below to you program and it will work. var split; // Avoid running twice; that would break the `nativeSplit` reference split = split || function (undef) { var nativeSplit = String

问题 I came across the following program, which compiles without errors or even warnings: int main(){ <:]{%>; // smile! } Live example. What does the program do, and what is that smiley-expression? 回答1: That's an empty lambda using a digraph disguise. Normal lambdas don't have beards. 回答2: The program uses digraphs to represent the following: [] {}; This is a lambda expression that does nothing. The corresponding symbols have these equivalents: <: = [ %> = } Though they are generally unneeded

问题 I was wondering if there is any difference in the way SQL performs on these join statements: SELECT * FROM a,b WHERE a.ID = b.ID SELECT * FROM a JOIN b ON a.ID = b.ID SELECT * FROM a JOIN b USING(ID) Is there a performance difference? Or algorithmic difference? Or is it just syntactic sugar? 回答1: There is no difference in performance. However, the first style is ANSI-89 and will get your legs broken in some shops. Including mine. The second style is ANSI-92 and is much clearer. Examples: