你可以向谓词传递一个附加参数吗? [英] Can you pass an additional parameter to a predicate?
问题描述
我尝试过滤一个向量,使其只包含一个特定的值。
I'm trying to filter a vector so it contains only a specific value.
例如。确保向量只包含值abc的元素。
e.g. Make sure the vector only contains elements of the value "abc."
现在,我试图用 remove_copy_if
。
使用std算法时,是否有任何方法可以向谓词传递一个附加参数?
Is there any way to pass an additional parameter to a predicate when using one of std's algorithms?
std::vector<std::string> first, second;
first.push_back("abc");
first.push_back("abc");
first.push_back("def");
first.push_back("abd");
first.push_back("cde");
first.push_back("def");
std::remove_copy_if(first.begin(), first.end(), second.begin(), is_invalid);
我希望将以下函数作为谓词传递,但似乎更有可能结束比较 remove_copy_if
和下一个检查的当前值。
I'm hoping to pass the following function as a predicate but it seems more likely that this would just end up comparing the current value being examined by remove_copy_if
and the next.
bool is_invalid(const std::string &str, const std::string &wanted)
{
return str.compare(wanted) != 0;
}
我有一种感觉,我可能接近这个错误,所以任何建议
I have a feeling I'm probably approaching this wrong so any suggestions would be appreciated!
感谢
推荐答案
p>
Define a functor instead:
struct is_invalid
{
is_invalid(const std::string& a_wanted) : wanted(a_wanted) {}
std::string wanted;
bool operator()(const std::string& str)
{
return str.compare(wanted) != 0;
}
};
std::remove_copy_if(first.begin(),
first.end(),
second.begin(),
is_invalid("abc"));
或如果C ++ 11使用lambda:
or if C++11 use a lambda:
std::string wanted("abc");
std::remove_copy_if(first.begin(), first.end(), second.begin(),
[&wanted](const std::string& str)
{
return str.compare(wanted) != 0;
});
注意输出向量 second
remove_copy_if()之前具有元素
:
Note that the output vector, second
, must have elements before the call to remove_copy_if()
:
// Create 'second' after population of 'first'.
//
std::vector<std::string> second(first.size());
std::string wanted = "abc";
int copied_items = 0;
std::remove_copy_if( first.begin(), first.end(), second.begin(),
[&wanted, &copied_items](const std::string& str) -> bool
{
if (str.compare(wanted) != 0) return true;
copied_items++;
return false;
});
second.resize(copied_items);
复制函数谓词需要更多的努力来保留 copied_items
信息。请参阅在C ++中通过引用传递std algos谓词了解建议的解决方案。
As functor predicates are copied more effort is required to retain the copied_items
information. See Pass std algos predicates by reference in C++ for suggested solutions.
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