Accessing a std::map of std::map like an array

Question

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}}}};

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

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

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.

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

Karthik T · Accepted Answer · 2014-04-22 08:54:07Z

4

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.

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

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

Should become

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

answered Apr 22, 2014 at 8:54

Karthik T

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4 Comments

soupso Over a year ago

Thank you so much for the reply. I missed that basic thingy. What I need is to have the mapping of {"mHz", {{"Hz", 1e-3}}} and {"mHz", {{"mHz", 1e-3}}}... How could I achieve this without mentioning "mHz" for the second time...I mean how do I correctly initialize without repetition of the key ?

soupso Over a year ago

Ohh awesome...Let me try that. Thank you so very much for your quick reply !!

Karthik T Over a year ago

If the brackets are confusing, I suggest initializing it in 2 steps, first the inner map, and then use it to init the outer map

soupso Over a year ago

Thank you so much !! It worked like a charm...:) :) I dont know how could I missed that basic stuffs ...

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