C++ Check if Key Exists using Map within a Map - c++

Question

I have a map defined as map<string, map<string,int> > grandMap; and I need to check if the internal map has a key term. If so, count it. Here's what I tried already:
auto countOccurrences(string term) -> int
{
int count = 0;
for(auto entry : grandMap)
{
if(entry->second.find(term)!=entry->second.end())
{
count++;
cout << "counted" << endl;
}
}
return count;
}
But I get the following error(s):
415.cpp:50:11: error: base operand of '->' has non-pointer type 'std::pair<const std::basic_string<char>, std::map<std::basic_string<char>, int> >'
415.cpp:50:37: error: base operand of '->' has non-pointer type 'std::pair<const std::basic_string<char>, std::map<std::basic_string<char>, int> >'
...which clearly points to my attempt to get the second of entry, which I thought would be an element of the grandMap, but it doesn't seem to work quite how I'd like...
So what is the right way to go about this?

Answer 1

The problem is that you are using operator -> instead of operator .:
if (entry.second.find(term) != entry.second.end())
// ^ ^
Also, to avoid premature pessimization (see this GoTW by Herb Sutter for a definition), you should accept the term argument by reference to const, and you should also use auto const& in the range-based for loop.
Moreover, the count_if standard algorithm seems to be what you are looking for:
// Using C++14's return type deduction for regular functions...
auto count(std::string const& term)
{
return std::count_if(std::begin(grandMap), std::end(grandMap),
[&] (decltype(grandMap)::value_type const& entry)
{
return (entry.second.find("hello") != std::end(entry.second));
});
}
Here is a complete program:
#include <map>
#include <string>
#include <algorithm>
#include <iostream>
std::map<std::string, std::map<std::string, int>> grandMap =
{ { "1", { { "hello", 42 }, { "hi", 1337 } } },
{ "2", { { "hello", 42 }, { "hello", 1729 } } }};
auto count(std::string const& term)
{
return std::count_if(std::begin(grandMap), std::end(grandMap),
[&] (decltype(grandMap)::value_type const& entry)
{
return (entry.second.find("hello") != std::end(entry.second));
});
}
int main()
{
std::cout << count("hello");
}
And the corresponding live example.