Thread: unordered_map of vectors

you have to create enough space for a value at index 7 in your vector. the easiest way to do this is by calling the resize() member function on an entry in your map.

auto & vec = x["mumb 5"]; // Get vector
vec.resize(8); // Resize to make index 7 valid
vec.at(7) = 65; // Fill element

Best is to use push_back. Avoid index operator since it can cause undefined behaviour if you access out-of-bounds elements. .at() function helps you catch these programming errors earlier.

In C++11 the easiest way is to use an std::array:

Code:

unordered_map<string, array<int,8>> x;
x["mumb 5"][7] = 65;

Originally Posted by Elysia

Avoid index operator since it can cause undefined behaviour if you access out-of-bounds elements.

I suspect you're referring to the vector's index operator. both the map and its vector elements have an index operator in this case, so it was a little unclear to me at first which one you were referring to.

just to clarify for the OP, there's no risk of "out of bounds" access with the map itself. using an "index" that doesn't exist in an stl map will simply create that element and return its associated value.

Originally Posted by King Mir

In C++11 the easiest way is to use an std::array

but only if you know you need fixed size elements, whose size is known at compile time.

Originally Posted by Elkvis

but only if you know you need fixed size elements, whose size is known at compile time.

Which is exactly what the OP asked for:

Originally Posted by baxy

hi,

a simple one : how to create an unordered map of fixed size vectors?

Code:

unordered_map<string, vector<int>> x;

x["mumb 5"][7] = 65; // this does not work since my vector size is not set.

thnx

Originally Posted by King Mir

Which is exactly what the OP asked for:

but we don't know if the size is guaranteed to be a compile-time constant. it could be dependent on user input or some other run-time parameter. perhaps the OP can weigh in on this.

Originally Posted by Elkvis

I suspect you're referring to the vector's index operator. both the map and its vector elements have an index operator in this case, so it was a little unclear to me at first which one you were referring to.

I can confirm that I did refer to the vector.