evaluation

问题 let's say I want to select the behaviour of a certain preprocessor directive evaluating at compile time the concatenation of a constant string and the result of another macro. #define CASE1 text1 #define CASE2 text2 #define CASE3 text3 #define SCENARIO 3 /** the following won't work - for examplification purposes only**/ #define FUNCTION CASE##SCENARIO /** whenever I write FUNCTION, I expect to see text3 **/ I am having an hard time thinking of a viable solution, as the preprocessor is a one

问题 I am looking for a way to evaluate a Swift Bool concisely in a single if statement, when the Bool is the property of an optional object: var objectWithBool: ClassWithBool? // ... if let obj = objectWithBool { if obj.bool { // bool == true } else { // bool == false } } else { // objectWithBool == nil } Is there are way to combine these if statements? In Objective-C this could easily be done, as a nil object can be evaluated in the same expression: if (objectWithBool.bool) { // bool == true }

问题 Guys I am up with evaluating a string mathematical expression. First I imported the library using System.Linq.Expressions; Then in my codes I did, Expression e = new Expression("(450*5)+((3.14*7)/50)*100"); double result = e.Evaluate(); however I get the error as Cannot create an instance of the abstract class or interface 'System.Linq.Expressions.Expression' Why the above is not working? How can I evaluate this ? 回答1: In order to evaluate expressions like this in c#, you have to use Roslyn.

问题 I want to know if these two definitions of nth are equal: I. is defined as macro: (defmacro -nth (n lst) (defun f (n1 lst1) (cond ((eql n1 0) lst1) (t `(cdr ,(f (- n1 1) lst1))))) `(car ,(f n lst))) II. is defined as a bunch of functions: (defun f (n lst) (cond ((eql n 0) lst) (t `(cdr ,(f (- n 1) lst))))) (defun f1 (n lst) `(car ,(f n `',lst))) (defun --nth (n lst) (eval (f1 n lst))) Am i get the right idea? Is macro definition is evaluating of expression, constructed in its body? 回答1: OK,

问题 Dealing with another SO question, I was wondering if the code below has an undefined behavior: if (str.equals(str = getAnotherString())) { // [...] } I tend to think the str reference from which the equals() call is made is evaluated before the further str assignment passed as argument. Is there a source about it? 回答1: This is clearly specified in the JLS Section 15.12.4: At run time, method invocation requires five steps. First, a target reference may be computed. Second, the argument

问题 Dealing with another SO question, I was wondering if the code below has an undefined behavior: if (str.equals(str = getAnotherString())) { // [...] } I tend to think the str reference from which the equals() call is made is evaluated before the further str assignment passed as argument. Is there a source about it? 回答1: This is clearly specified in the JLS Section 15.12.4: At run time, method invocation requires five steps. First, a target reference may be computed. Second, the argument

问题 I can not use the subset argument of any function with mapply . The following calls fail with the subset argument, but they work without: mapply(ftable, formula = list(wool ~ breaks, wool + tension ~ breaks), subset = list(breaks < 15, breaks < 20), MoreArgs = list(data = warpbreaks)) # Error in mapply(ftable, formula = list(wool ~ breaks, wool + tension ~ : # object 'breaks' not found mapply(xtabs, formula = list(~ wool, ~ wool + tension), subset = list(breaks < 15, breaks < 20), MoreArgs =

问题 I can not use the subset argument of any function with mapply . The following calls fail with the subset argument, but they work without: mapply(ftable, formula = list(wool ~ breaks, wool + tension ~ breaks), subset = list(breaks < 15, breaks < 20), MoreArgs = list(data = warpbreaks)) # Error in mapply(ftable, formula = list(wool ~ breaks, wool + tension ~ : # object 'breaks' not found mapply(xtabs, formula = list(~ wool, ~ wool + tension), subset = list(breaks < 15, breaks < 20), MoreArgs =

问题 It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. Closed 7 years ago . I'm developing a game(Tank Game 2D),(eg - link ) AI player. My player will be one of 5 other players(AI also) who playing for obtaining maximum coins appears randomly somewhere in the grid.(take a look at the

问题 When I use Boost.Asio, creating object such as ip::tcp::socket or deadline_timer as std::shared_ptr and copy captured it to the completion handler as lambda expression. I curious that what happens if I use move capture instead of copy capture. I think that it is dangerous. In the following example, I think that tim = std::move(tim) is evaluated before tim->async_wait . So tim no longer has valid pointer. It is my guess. In order to trace std::shared_ptr 's behavior, I created std::shared_ptr