[英]check map keys are in set using std::equal or std::all_of
I'm new to C++ and predicates but I'm running into a problem. 我是C ++和谓语的新手,但遇到了问题。 I'm trying to check if all keys in an
unordered_map
exist in a set
or even keys in another map with different value type for that matter. 我试图检查
unordered_map
中的所有键是否存在于一个set
或者甚至存在于另一个具有不同值类型的映射中的键。 Essentially, [key in set_ for key in map_.keys()]
OR [key in map2_ for key in map1_.keys()]
in python. 本质上,
[key in set_ for key in map_.keys()]
[key in map2_ for key in map1_.keys()]
[key in set_ for key in map_.keys()]
或[key in map2_ for key in map1_.keys()]
。
one approach is the following: 一种方法如下:
for( auto itr = map_.begin(); itr != map_.end(); ++itr ) {
if( set_.find( itr->first ) == set_.end() ) return false;
}
But I want to use std::all_of
or std::equal
to achieve the same. 但是我想使用
std::all_of
或std::equal
实现相同的目的。 Here's what I have so far: 这是我到目前为止的内容:
some_function() {
//set_ and map_ in scope
areEqual = std::equal( map_.begin(), map_.end(), set_,
[]( auto itr ){ return set_.find( itr->first ) != set_.end(); } );
}
compiler complains that set_
is out of scope in return set_.find(...)...
编译器抱怨
set_
超出范围, return set_.find(...)...
I also tried 我也试过
class checker {
public:
bool operator()( map_itr itr )
{ return set_.find( itr->first ) != set_.end(); }
};
but ran into the same problem. 但是遇到了同样的问题。
Any help or pointers is greatly appreciated :) 任何帮助或指针,不胜感激:)
Lambdas can't access variables declared in its scope by default. 默认情况下,Lambda无法访问在其作用域中声明的变量。 In order to access your
set_
variable from the lambda function, you should capture it in the lambda function. 为了从lambda函数访问
set_
变量,应在lambda函数中捕获它。
The capture list is defined between the []
characters at the beginning of the lambda function. 捕获列表是在lambda函数开头的
[]
字符之间定义的。
In this case, I would use [&]
which captures all objects from the scope by reference, including your set_
variable. 在这种情况下,我将使用
[&]
通过引用从作用域中捕获所有对象,包括set_
变量。
some_function() {
areEqual = std::equal( map_.begin(), map_.end(), set_,
[&]( auto itr ){ return set_.find( itr->first ) != set_.end(); } );
}
You should read the Lambda expressions page on cppreference for further information. 您应该阅读cppreference上的Lambda表达式页面以获取更多信息。
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