iterator

问题 I am having issues with my nested iterator class. The end() sentinel is returning 0, and I am very lost! Any help would be appreciated. I want this program to be able to take any type of input, e.g. an int, create the vector, and be able to fill the array until the end() sentinel has been reached, such as typing Q, or any non-integer. I took out a few lines of unnecessary code to shorten it. template <class T> class Set { private: vector<T> m_element; int size; public: Set() : size(0) {}; ...

问题 I have a list products with these attributes : Identifier Color Size Supplier Supplier attribute is an object that has these attributes : Name Phone For each product in my list, i'd like to display the identifier & the supplier name. How can i do this with struts / jstl ? Here is what i'm trying with no success : <s:iterator value="products"> <tr> <td><s:property value="identifiant"/></td> <td><s:property value="supplier.name"</td> </tr> </s:iterator> 回答1: There should be getters for each

问题 From 24.2.3 Input iterators [input.iterators] 3) [...] Algorithms on input iterators should never attempt to pass through the same iterator twice. They should be single pass algorithms. [...] This IMO restricts some fairly straight-forward optimizations (such as passing through the container once to see how many elements it has) - alas, the motivation is outside the scope of the question. Why this requirement? 回答1: Input iterators are used to iterate over ranges that don't have a material

问题 From 24.2.3 Input iterators [input.iterators] 3) [...] Algorithms on input iterators should never attempt to pass through the same iterator twice. They should be single pass algorithms. [...] This IMO restricts some fairly straight-forward optimizations (such as passing through the container once to see how many elements it has) - alas, the motivation is outside the scope of the question. Why this requirement? 回答1: Input iterators are used to iterate over ranges that don't have a material

问题 So iterating over a string and using operator[] or insert to change characters can invalidate the iterator. Is that also the case for an iteration like this? std::string str = "ABCD"; for(auto&& c : str){ for(int i = 0; i < 3; ++i){ switch(c) { case 'A': c = 'B'; break; case 'B': c = 'C'; break; /*...*/ } //do something } } This code works on gcc and msvc but I don't know if I can trust it. I'm using C++14. 回答1: You are modifying existing characters of string so it is completely safe.

问题 So iterating over a string and using operator[] or insert to change characters can invalidate the iterator. Is that also the case for an iteration like this? std::string str = "ABCD"; for(auto&& c : str){ for(int i = 0; i < 3; ++i){ switch(c) { case 'A': c = 'B'; break; case 'B': c = 'C'; break; /*...*/ } //do something } } This code works on gcc and msvc but I don't know if I can trust it. I'm using C++14. 回答1: You are modifying existing characters of string so it is completely safe.

问题 So iterating over a string and using operator[] or insert to change characters can invalidate the iterator. Is that also the case for an iteration like this? std::string str = "ABCD"; for(auto&& c : str){ for(int i = 0; i < 3; ++i){ switch(c) { case 'A': c = 'B'; break; case 'B': c = 'C'; break; /*...*/ } //do something } } This code works on gcc and msvc but I don't know if I can trust it. I'm using C++14. 回答1: You are modifying existing characters of string so it is completely safe.

问题 I'm trying to implement a simple interpreter in Rust, for which I have created a Tokens struct, which takes source characters and produces either a Token or a ScanError inside a Result : pub struct Tokens<'src> { chars: Chars<'src>, } impl<'src> Iterator for Tokens<'src> { type Item = Result<Token, ScanError>; fn next(&mut self) -> Option<Result<Token, ScanError>> { // ... } } Since Result implements FromIterator , it is simple to collect the result to either the first ScanError or a vector

问题 It is quite clear to me that iterating over a vector shouldn't let the loop body mutate the vector arbitrarily. This prevents iterator invalidation, which is prone to bugs. However, not all kinds of mutation lead to iterator invalidation. See the following example: let mut my_vec: Vec<Vec<i32>> = vec![vec![1,2], vec![3,4], vec![5,6]]; for inner in my_vec.iter_mut() { // <- or .iter() // ... my_vec[some_index].push(inner[0]); // <-- ERROR } Such a mutation does not invalidate the iterator of

问题 In PHP, this evaluates to true : $a = 1 $b = 2 var_dump($a && $b); // true In ruby, this evaluates to 2 : a = 1 b = 2 p a && b # 2 Why does ruby return the value of the last statement (when the first is true and the second one is also true) and does not return a boolean? I have two arrays and I iterate them with an external iterator: a = [1,2,3].to_enum b = [5,6,7].to_enum c = [] begin while a_next = a.next && b_next = b.next result = a_next + b_next p "a[x] + b[x] = c[x] = #{a_next} + #{b