C++ 使用带有谓词的 std::find
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using std::find with a predicate
提问by mahmood
I want to use std::findfunction along with a predicate (not sure if I use the correct word). Here is the code
我想将std::find函数与谓词一起使用（不确定我是否使用了正确的词）。这是代码
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
class foo {
public:
typedef pair< int, vector<int> > way;
typedef pair< int, int > index;
typedef pair< index, vector<way> > entry;
vector< entry > table;
void bar()
{
vector<int> v1;
v1.push_back(1);
v1.push_back(2);
way w = make_pair( 1, v1 );
vector<way> v2;
v2.push_back(w);
index id = make_pair( 10, 20 );
entry en = make_pair( id, v2 );
table.push_back( en );
}
void insert()
{
index new_id = make_pair( 10, 20 );
if ( find(table.begin(), table.end(), new_id) != table.end() ) {
// index matched in the table
// then I will push back a new pair (way)
// to the second part of the entry
}
}
};
int main()
{
foo f;
f.bar();
f.insert();
return 0;
}
As you can see, find()should search the tablebased on the first element in each entry. Right now, it says that ==is not overloaded to compare a pair.
如您所见，find()应该table根据每个条目中的第一个元素进行搜索。现在，它说==比较 a 没有过载pair。
回答by Christian Rau
You want std::find_if:
你想要std::find_if：
...
if(find_if(table.begin(), table.end(), [&new_id](const entry &arg) {
return arg.first == new_id; }) != ...)
EDIT:If you don't have C++11 (and therefore no lambdas), you have to create a custom functor (function or function object) to do the comparison of entry::firstwith the searched index:
编辑：如果您没有 C++11（因此没有 lambdas），您必须创建一个自定义函子（函数或函数对象）来entry::first与 searched进行比较index：
struct index_equal : std::unary_function<entry,bool>
{
index_equal(const index &idx) : idx_(idx) {}
bool operator()(const entry &arg) const { return arg.first == idx_; }
const index &idx_;
};
...
if(find_if(table.begin(), table.end(), index_equal(new_id)) != ...)
EDIT:Since an indexis just a pair of ints, you may also just capture it by value than const reference, to keep the code clearer and more concise, but it doesn't really matter either.
编辑：由于 anindex只是一对ints，您也可以只按值捕获它而不是 const 引用，以使代码更清晰和更简洁，但这也无关紧要。
回答by Daniel Laügt
In C++11, you can use also std::any_of
在 C++11 中，你也可以使用 std::any_of
if (std::any_of(table.cbegin(), table.cend(),
[&new_id](const entry &arg) { return arg.first == new_id; }))
