如何使用地图C ++中的值获取匹配键 [英] how to get matching key using the value in a map C++

问题描述

我有一个带有struct作为值类型的映射

I have a map with a struct as a value type

map<int id, struct_t*> table

struct_t
{
int prev;
int wt;
string name;
}

仅使用上一个，我需要找到相应的ID.提前非常感谢！

Using only prev, I need to find the corresponding id. Thanks so much in advance!

int key=0;
for(auto it = table.begin(); it != table.end(); ++it)
{
     if(table[(*it).first].prev == ?)
}

这是我的地图数据的样子:

This is how my map data looks like:

id    prev abundance  thing
1573  -1      0       book
1864  1573    39      beds
2075  1864    41      tray
1760  2075    46      cups

对于每个ID，我需要找到下一个匹配ID.因此，对于prev列中的1573，我需要找到一个匹配的'id'，即1864.而且，std :: next不起作用，因为数据集不一定在下一个元素中具有匹配的ID.希望这会有所帮助！

For each id, I need to find the NEXT matching id. So, for 1573 from the prev column I need to find a matching 'id' which is 1864. Also, std::next doesn't work because the data set can have the matching ids not necessarily in the next element.Hope this helps!

请帮助我！！！我的老板已经很失望，因为我花了太多时间学习C ++(已经3个星期了！)

PLEASE PLEASE help me!!! MY boss is already disappointed that I'm taking so much time to learn C++ (its been 3 weeks already!)

推荐答案

如果您拥有现代的编译器(支持lambda)，则可以执行以下操作:

If you've got a modern compiler (supports lambdas), you can do the following:

const int prevToFind = 10;
auto findResult = std::find_if(std::begin(table), std::end(table), [&](const std::pair<int, struct_t*> &pair)
{
    return pair.second->prev == prevToFind;
});

int foundKey = 0; // You might want to initialise this to a value you know is invalid in your map
struct_t *foundValue = nullptr
if (findResult != std::end(table))
{
    foundKey = findResult->first;
    foundValue = findResult->second;

    // Now do something with the key or value!
}

如果您使用的是较旧的编译器，请告诉我，我可以将示例更新为使用谓词类.

Let me know if you have an older compiler, and I can update the example to use a predicate class instead.

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