ternary-operator

问题 While developing an application, I had the following problem. I wanted to return an empty std::list<string> when a given function pointer was null, or the result of that function otherwise. This is a simplified version of my code: typedef std::list<std::string> (*ParamGenerator)(); std::list<std::string> foo() { /* ... */ ParamGenerator generator = ...; if(generator) return generator(); else return {}; } However, I usually like to use the ternary ( ?: ) operator in these cases, so I tried

问题 While developing an application, I had the following problem. I wanted to return an empty std::list<string> when a given function pointer was null, or the result of that function otherwise. This is a simplified version of my code: typedef std::list<std::string> (*ParamGenerator)(); std::list<std::string> foo() { /* ... */ ParamGenerator generator = ...; if(generator) return generator(); else return {}; } However, I usually like to use the ternary ( ?: ) operator in these cases, so I tried

问题 I have the following simple code piece: List<XXXXBean> queryPeriodData() { if (CollectionUtils.isEmpty(res)) { return Collections.emptyList(); } return res; } It works. but if I change to this, there is a compile error... return CollectionUtils.isEmpty(res) ? Collections.emptyList() : res; error message is "Type mismatch: cannot convert from List< capture#1-of ? extends Object> to List< XXXXBean>" I don't know the difference between the two way. 回答1: try Collections.<XXXXBean>emptyList() in

问题 this code doesn't work int main(){ cout << 5 ? (5 ? 0 : 2) : 5; system("pause"); return 0; } this code works int main(){ cout << (5 ? (5 ? 0 : 2) : 5); system("pause"); return 0; } can't understand why? 回答1: cout << 5 ? (5 ? 0 : 2) : 5; is parsed as (cout << 5) ? (5 ? 0 : 2) : 5; 回答2: This is due to operator precedence rules. << has higher precedence than ? , so your first expression is parsed as: (cout << 5) ? (5 ? 0 : 2) : 5; Brackets are necessary in this case to get the parse you want. 来源

问题 What's the standard line to add to the ternary operator in order to do nothing if the condition is not met? Example: int a = 0; a > 10 ? a = 5 : /*do nothing*/; Using a seems to do the trick, but I am wondering if there is a more generally accepted way. 回答1: That will do it: a = a > 10 ? 5 : a; or simply: if (a > 10) a = 5; 回答2: Another option: a ? void(a = 0) : void(); What's good about this one is that it works even if you can't construct an instance of decltype(a = 0) to put into the 'do

问题 Can anyone please explain to me what happens behind the scenes when you use ternary operator? does this line of code: string str = 1 == 1 ? "abc" : "def"; is generated as a simple if / else statement? Consider the following: class A { } class B : A { } class C : A { } Now using ternary expression as follows: A a1 = 1 == 1 ? new B() : new C(); this doesn't even compile with this error: Type of conditional expression cannot be determined because there is no implicit conversion between

问题 Can anyone please explain to me what happens behind the scenes when you use ternary operator? does this line of code: string str = 1 == 1 ? "abc" : "def"; is generated as a simple if / else statement? Consider the following: class A { } class B : A { } class C : A { } Now using ternary expression as follows: A a1 = 1 == 1 ? new B() : new C(); this doesn't even compile with this error: Type of conditional expression cannot be determined because there is no implicit conversion between

问题 I have a piece of code like below: var selected = “A”; bool isSelected = selected == "A" || selected == "C"; var codeLists = new { displayProperty1 = isSelected ? "property1" : null, displayProperty2 = isSelected ? "property2" : null, displayProperty3 = selected == "C" ? "property3" : null }; So, my goal is to eliminate a property if it does not satisfy a condition. In the above code, selected is "A" . So, displayProperty3 would have a value of null . But I want to eliminate displayProperty3

问题 Note: It's true that this question has been answered for many other languages. However, I could not find an answer for Python, so do not mark as duplicate. Is there a difference in performance between the if-else statement and the ternary operator in Python? 回答1: I doubt there is a performance difference. They compile to equivalent sequences of bytecodes: >>> def f(): ... return a if b else c ... >>> dis.dis(f) 2 0 LOAD_GLOBAL 0 (b) 2 POP_JUMP_IF_FALSE 8 4 LOAD_GLOBAL 1 (a) 6 RETURN_VALUE >>

问题 Here's what I have. Map data = new HashMap<>(); // assume this has been populated public int getLastestVersion() { // data.get("PATH_TO_DESIRED_POINT") would be an integer return data.get("PATH_TO_DESIRED_POINT") == null ? 0 : (int)data.get("PATH_TO_DESIRED_POINT"); } I'm trying to avoid violating DRY, but I want to be able to keep the simplicity of the ternary. Is there something I can do? 回答1: If you are using Java8, you can use getOrDefault method: return data.getOrDefault("PATH_TO_DESIRED