像数组一样访问std :: map的std :: map [英] Accessing a std::map of std::map like an array

问题描述

我已经初始化了std :: map的std :: map，如下所示:

I have initialize a std::map of a std::map like as below:

static std::map<std::string, std::map<std::string, float>> _ScalingMapFrequency = {
            {"mHz",    {{"mHz",     1.0}}},
            {"mHz",    {{"Hz",      1e-3}}},

            {"Hz",     {{"mHz",     1e+3}}},
            {"Hz",     {{"Hz",      1.0}}}};

现在我正尝试通过以下方式访问浮点值:

And now I am trying to access the float values in the following way:

std::cout<<"  the scaling factor is     :"<<_ScalingMapFrequency["mHz"]["Hz"];

编译和运行代码没有问题，但是我希望得到"1e-3"，而我总是得到"0".我需要以数组形式访问std :: map"_ScalingMapFrequency"，这是设计决策.

There is no problem when I compile and run the code but I am expecting to get "1e-3" instead I am always getting a "0". I need to access the std::map "_ScalingMapFrequency" as an array, that being the design decision.

我犯了什么错误?请给我一些指导，我将不胜感激.

What mistake I am making ? Please give me some pointer and I would greatly appreciate.

推荐答案

地图不能有重复的键，因此当您执行 {"mHz"，{{"Hz"，1e-3}}}，，它覆盖了第一个，而不是合并它们.

A map cannot have duplicate keys, thus when you do {"mHz", {{"Hz", 1e-3}}}, for the second time, it overwrites the first one, as opposed to merge them.

您应该更改构造函数，以便将它们合并为开始.

You should change the constructor so that they are merged to begin with.

 {"mHz",    {{"mHz",     1.0}},
 {"mHz",    {{"Hz",      1e-3}},

应该成为

 {"mHz",    {{"mHz",     1.0},
            {"Hz",      1e-3}}},

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