Python List of Dictionaries to C++

Question

I have a Python list of dictionaries that I would like to translate to C++.

My list looks more or less like this:

myList = [{"v_A": 5, "v_B": 3}, {"v_A": 1, "v_B":2}, {"v_A":0, "v_B": 0}]

My code is designed such that when I look for e.g. velocities of particle 0, I do:

v = myList[0]["v_"+letter]

where letter would be a string, "A" or "B", depeding on the output of a sorting function I have. I have found this is the most convenient way to achieve my purpose in Python.

I am trying to translate my code to C++ (being a bit of a newbie) and I don't know how I could keep the "dictionary"-like features (that is, accessing values through a key that can be built concatenating strings) and the benefits the list together like in myList.

I read this question without much success, being unable to understand: C++ Equivalent of Python List of Dictionaries?

After some research, I thought of a using a vector of unordered maps, but I am unsure on how to define it and work with it. I thought of:

vector< unordered_map <string, int >> myList;

However, I am getting errors all the time. Besides, I don't know how to introduce the values in the dictionaries or access the elements. Am I approaching this from a completely wrong side?

EDIT: I found that if I define and insert the elements piece-wise, it seems to work:

std::vector <unordered_map <string, int> > myList;
unordered_map <string, int> tempMap;
tempMap.insert(std::make_pair("v_A", 10));
myList.push_back(tempMap);
cout << myList[0]["v_A"];

returns correctly 10. Is there a way to do this more efficient/ in a simpler way?

What errors? With what code? Please provide a [mcve] to show what you have tried doing so far — UnholySheep, Dec 27 '19 at 10:17
Please show what you tried and what these errors are. Although it is unclear whether this is the best approach to the problem in C++, the definition you showed should work and can be used almost the same way as in Python. — walnut, Dec 27 '19 at 10:18

walnut · Answer 1 · 2019-12-28T09:27:53.160

Inserting in the list can be done much simpler.

At initialization you can do

std::vector<std::unordered_map<std::string, int>> myList {
    {{"v_A", 5}, {"v_B", 3}},
    {{"v_A", 1}, {"v_B", 2}},
    {{"v_A", 0}, {"v_B", 0}}
};

and you can later increase the size of the vector with push_back:

myList.push_back({{"v_A", 0}, {"v_B", 0}});

or you can add to a dictionary in one of the existing elements the same way as in python:

myList[0]["v_C"] = 12;

You cannot add elements to the vector this way though. (Accessing a vector out-of-bounds causes undefined behavior.)

Elements can be read as in Python with myList[0]["v_C"].

You cannot concatenate a character or C style string to a string literal directly with + as in v = myList[0]["v_"+letter] (depending on the type of letter it may compile but will probably not do what you expect it to do). But if you use

using namespace std::string_literals;

after the includes in your .cpp file, then you can write

v = myList[0]["v_"s+letter];

The s suffix then causes the string literal to be converted to a std::string, which provides a + operator that actually does concatenation.

Thank you for the advice on how to add the strings! Very helpful! — ma7642, Dec 28 '19 at 09:25

score 1 · Answer 2 · answered Dec 27 '19 at 13:13

With a std::unordered_map you can use operator[] and if the element does not exist it will get inserted into the map.

This is not true for a vector. A vector is a dynamic array and you need to explicitly add elements to it before you can acces the already added elements with operator[].

std::vector<std::unordered_map<std::string, int>> myList;
myList[0]; // Invalid since the vector currently is empty.

myList.push_back({}); // Insert a default-constructed element in the vector

myList[0]; // Refers to the first element in the vector, which is a std::unordered_map<std::string, int>

myList[0]["string"] = 1; // This will create the elements if it does not already exist at mentioned above so this works fine

If you know the size you want you vector to have you can use std::vector::resize or a call to the constructor to set the size.

std::vector<std::unordered_map<std::string, int>> myList(10);
// Create a vector with 10 elements, so [0-9] would be in-bounds.

std::vector<std::unordered_map<std::string, int>> myList;
myList.resize(10); // This will do the same thing but in two steps.

I will keep this in mind for the future, thank you for the advice! — ma7642, Dec 28 '19 at 09:29

score 0 · Accepted Answer · answered Dec 27 '19 at 10:20

0

std::vector<std::unordered_map<std::string, int>> dictList; // This is a list of dicts

std::unordered_map<std::string, int> dict; // Here we create a specific dict we want to add to our list of dicts

dict["key"] = 10; // This is how you add (key, value) pair into dict in c++

dictList.push_back(dict); // This is how you add your dict to the list of dicts

answered Dec 27 '19 at 10:20

Duško Mirković

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2
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Best to use `push_back(std::move(dict))`—or else use `dictList.emplace_back()` to create a new, empty map *in* the vector and then populate it there. – Davis Herring Jan 03 '20 at 18:18

Python List of Dictionaries to C++

3 Answers3