如何在从字符串到向量的映射中访问向量中的索引

Question

If I have the following code, which stores 3 vectors as values in an unordered_map and maps to them via the string in their first index as a key:

unordered_map<string, vector <vector<string>>> my_map;
vector<string> vec1 = {"banana", "apple"};
vector<string> vec2 = {"banana", "banana"};
vector<string> vec3 = {"banana", "pear"};

my_map["banana"].push_back(vec1);
my_map["banana"].push_back(vec2);
my_map["banana"].push_back(vec3);

cout << my_map["banana"][0] << my_map["banana"][1] << my_map["banana"][2];

How can I access specific indices of the unordered_map's vector? I tried to do this in the last line but it does not work. I have also tried using .at(0), etc instead of the double brackets but that also did not work.

Answer 1

my_map["banana"][0] is a std::vector<std::string> , so you can't output that directly. You could overload the << operator, or just specify another index.

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You have an std::unordered_map<std::string, std::vector<std::vector<std::string>>> , where each key has a vector of vectors.

So my_map["banana"].push_back(vec1) pushes vec1 to the main vector. If you want to access the values from vec1 , you have to specify 2 indices, because:

 my_map["banana"][0] [0] ^^^^^^ ^ ^ key vector 0 element 0 in the vector (in main vector) at position 0 in the main vector

Note: This will return "banana"

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If you want get "pear" , you would use:

 my_map["banana"] [2] [1] ^^^^^^^ ^ ^ key third vector second element (vec3) ("pear")