Checking whether an element exist or not in the map within map

Question

I have a map of the form map > :

For Example: I am storing the intensity value at 2-D co-ordinate(x,y) i

Answer 1

This is bit ugly but should work too : (using C++11)

std::map<int, std::map<int, double> > intensityValue;
int x,y;
auto it = std::find_if(intensityValue.begin(),
                    intensityValue.end(),
                    [x,y](const std::pair<int, std::map<int, double>>& p){
                      return p.first==x && 
                             p.second.find(y) !=p.second.end();
                    }
                    );

  if(it != intensityValue.end())
  {
      //Got it !
  } 

Answer 2

You can use std::map::find together with short-circuit evaluation:

bool foundXY = instensityValue.find(x) != intensityValue.end() &&
               intensityValue[x].find(y) != intensityValue[x].end();

or std::map::count:

bool foundXY = instensityValue.count(x) && intensityValue[x].count(y)

Answer 3

Write a short function for it to ensure the minimum number of map finding is called.

bool hasIntensity(int x, int y)
{
    map<int, map<int, double> >::const_iterator i = intensityValue.find(x);
    if (i == intensityValue.end()) return false;
    map<int, double>::const_iterator j = i->second.find(y);
    return j != (i->second.end());
}

If you want to get the actual value when the element is found, just utilize j->second.

Answer 4

Use std::map::find

auto outerIt = intensityValue.find(x);
if (outerIt != intensityValue.end()) {
    auto innerIt = outerIt->find(y);
    if (innerIt != outerIt->end()) {
        // Do something with the found value
        return;
    }
}
// Didn't return, so it wasn't found

That said, in my experience, using a single map of pairs for this kind of thing is more efficient and easier to use than a nested map. It fits better into standard algorithms and doesn't involve nearly as much tree navigation.

template <typename T, typename U, typename V>
using map2d = std::map<std::pair<T, U>, V>;

int main() {
    map2d<int, int, double> myMap {
        {{3, 4}, 808.14f},
        {{1, 2}, 333.33f}
    };
    auto it = myMap.find({3, 4});
    if (it != myMap.end()) {
        std::cout << it->second << std::endl;
    }
}

Answer 5

You can use std::map::find and check if the element exists before accessing it. You can read the usage/documentation here: http://en.cppreference.com/w/cpp/container/map/find