访问作为映射中的值的向量以使用另一个向量对其运行相等运算符

Question

I have a vector that is made up of maps which have the following type definition:

std::map< int, std::vector<int> >

Thus, the definition of the vector itself is:

std::vector< std::map< int, std::vector<int> > >

Now I am creating a vector of integers and inserting it in a map with an integer key and adding the map itself to the outer vector:

    std::map<int, std::vector<int> > vDescriptorAtom;
    vDescriptorAtom.insert( std::make_pair(498, vListOfIds) );
    messageDescriptorVector.push_back( vDescriptorAtom );

Where vListOfIds itself is a vector of integers.

At a later stage I want to extract the inner vector and compare with another vector that I possess. I was under the impression that I can easily use the == operator between the two vectors to do the comparison. But I am having some trouble doing this.

I tried:

               for ( int i = 0 ; i <= messageDescriptorVector.size(); i++ )
                {    
                    if ( current_tag_stack == (messageDescriptorVector.at(i))->second )
                    {
                        vFoundMatchingDescriptor = true;
                        break;
                    }
                } 

Note that current_tag_stack has the following definition:

std::vector<int> current_tag_stack;

But I am getting this error:

base operand of â->â has non-pointer type âstd::map<int, std::vector<int, std::allocator<int> >, std::less<int>, std::allocator<std::pair<const int, std::vector<int, std::allocator<int> > > > >â

What am I doing wrong ?

EDIT

As suggested by a comment, I tried to access the vector that I have stored inside the map as: messageDescriptorVector.at(i).second but it is giving me this error: âclass std::map<int, std::vector<int, std::allocator<int> >, std::less<int>, std::allocator<std::pair<const int, std::vector<int, std::allocator<int> > > > >â has no member named âsecondâ

Answer 1

messageDescriptorVector.at(i) is a map. You have to obtain an element of that map, in order to get it's second and compare it with your reference vector. So you need one more level of iteration.

for ( int i = 0 ; i < messageDescriptorVector.size(); i++ )
{
  auto& m = messageDescriptorVector.at(i); // m is a map<int, vector<int>>

  for (auto& p : m) // p is a ref to pair<int, vector<int>>
  {
    if ( current_tag_stack == p.second )
    {
      // do something ....

or just

for (auto& m : messageDescriptorVector)
{
  for (auto& p : m)
  {
    if ( current_tag_stack == p.second )
    {
      // do something ....

Answer 2

Aside of issue that you are trying to accessing member second on a map, I would recommend to use typedefs in your code. It will not only simplify your code, but will help to change data structure if necessary. In your case all of this definitions all around your code:

std::map< int, std::vector<int> >
std::vector< std::map< int, std::vector<int> > >
std::map<int, std::vector<int> > vDescriptorAtom;
std::vector<int> current_tag_stack;

Compare it with:

typedef std::vector<int> tag_stack;
typedef std::map<int,tag_stack> tag_stack_map;

tag_stack_map vDescriptorAtom;
tag_stack current_tag_stack;

And imagine one day you would need to change your tag_stack from std::vector to something else. Where it will be easier?

Answer 3

function at() returns a reference to the element at specified location pos, so you should write:

(messageDescriptorVector.at(i)).second

I preferred write this code like:

std::map<int, std::vector<int>> vDescriptorAtom;
std::vector<std::map<int, std::vector<int>>> messageDescriptorVector;

vDescriptorAtom[498] = vListOfIds;
messageDescriptorVector.push_back( vDescriptorAtom );

// do something

for(auto& maps : messageDescriptorVector)
    for(auto& pairs : maps)
        if (current_tag_stack == pairs.second)
            // do something