CODEWITHSUNDEEP
c++stringc++11dictionaryboost
peter55555
peter55555

Reputation: 1475

std::map<string, string> to string (first values)

I'd like to create std::string that contains first elements of std::map<std::string, std::string> separate by some delimiter (it can be from STL or Boost). Is there any better solution (one line) than loop? Like boost::algorithm::join for std::vector<std::string>.

Upvotes: 4

Views: 11324

Answers (5)

Mezo
Mezo

Reputation: 49

if you are using a json library such as the header only nlohmann::json

you can simply try casting it to a json in a single line. note: be careful of exceptions.

std::cout << "My map or vector: " << ((json)myMap)) << "\n";

Edit: this might lazy i suppose but it gets the job done lol.

Upvotes: 0

Edgar Rokjān
Edgar Rokjān

Reputation: 17483

If you don't want to use boost, then try std::accumulate:

const std::string delimiter = "#";
const std::string result = std::accumulate(M.begin(), M.end(), std::string(),
[delimiter](const std::string& s, const std::pair<const std::string, std::string>& p) {
    return s + (s.empty() ? std::string() : delimiter) + p.first;
});

In the code above M is a std::map<std::string, std::string>.

Upvotes: 5

Sebastian Redl
Sebastian Redl

Reputation: 72009

This can be done elegantly using Boost.Range's map_keys and Boost.StringAlgo's join:

std::string get_keys(const std::map<std::string, std::string>& map) {
  return boost::algorithm::join(
    map | boost::adaptors::map_keys,
    ", ");
}

http://www.boost.org/doc/libs/1_61_0/boost/algorithm/string/join.hpp

http://www.boost.org/doc/libs/1_61_0/libs/range/doc/html/range/reference/adaptors/reference/map_keys.html

Upvotes: 7

Ami Tavory
Ami Tavory

Reputation: 76316

As M.M. correctly pointed out, you can use boost::range for this (to which I added boost::string).

If your map is m, then the last line of

std::vector<std::string> o;
boost::copy(m | boost::adaptors::map_keys, std::back_inserter(o));
boost::algorithm::join(o, ", ");

is results. (Unfortunately, this requires a huge number of header files.)

Example

#include <boost/range/adaptor/map.hpp>
#include <boost/range/algorithm/copy.hpp>
#include <boost/assign.hpp>
#include <boost/algorithm/string/join.hpp>
#include <algorithm>
#include <iostream>
#include <map>
#include <vector>

int main()
{
    std::map<std::string, std::string> m;
    m["hello"] = "world";
    m["goodbye"] = "now";

    std::vector<std::string> o;
    boost::copy(m | boost::adaptors::map_keys, std::back_inserter(o));
    std::cout << boost::algorithm::join(o, ", ") << std::endl;
}

This outputs

$ ./a.out 
goodbye, hello
$

Upvotes: 1

BiagioF
BiagioF

Reputation: 9715

Your algorithm should be something like this:

std::string get_keys(const std::map<std::string, std::string>& map) {
  std::string result;
  std::for_each(map.cbegin(),
                map.cend(),
                [&result](const decltype(map)::value_type& p) {
                  result += p.first;
                  result += ", ";
                });

  // Erase the last ", " from the string, if present
  if (result.size() > 0) {
    result.erase(result.size() - 2, 2);
  }

  return result;
}

Essentially you have to cycle for each element in the map and add it into the string. The complexity is O(N) with N the number of element in the map.

You can improve the performance of the algorithm applying reserve on the string result.

If you know the average length of the string key, the you can initialize the variable with:

std::string result;
result.reserve(map.size() * AVG_LENGTH_STR_KEY);

This will improve a lot the operator+= operation in the cycle.

Upvotes: 2

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