How to insert vector of integers into Key, Value of std::map

user_3152377 0
Viewed 2753

Goal: Read numerical text files into vectors and then add the vectors to key,value std::map so that I can reference them by the key name I have specified for them, later.

Thought this would be easy and I am surprised that I can't find an answer for this already on StackOverflow.

Result Expected:

Print1 = {100,200,500,600}

Print2 = {7890,5678,34567,3,56}

Print3["NameA"] = Print1

Print3["NameB"] = Print2

If my process is inefficient or going in the wrong direction, I would appreciate the pointers.

I keep getting Semantic Issue build fails and no viable conversion from pair <const basic_string>

Current Code:

#include <string.h>
#include <iostream>
#include <map>
#include <utility>
#include <vector>

const std::string& key(const std::pair<std::string, std::string>& keyValue)
{
    return keyValue.first;
}

const std::string& value(const std::pair<std::string, std::string>& keyValue)
{
    return keyValue.second;
}

int main() 
{
    std::vector<int> print1;
    std::ifstream inputFile("numbers.txt");

    // test file open
    if (inputFile) 
    {
        double value; 
        // read the elements in the file into a vector
        while ( inputFile >> value ) {
            print1.push_back(value);
        }
    }
    inputFile.close();

    std::vector<int> print2;
    std::ifstream inputFile2("numbers2.txt");

    // test file open
    if (inputFile2) 
    {
        double value;              
        // read the elements in the file into a vector
        while ( inputFile2 >> value ) {
            print2.push_back(value);
        }
    }       
    inputFile2.close();

    std::map<std::string, std::vector<int>> contacts;   
    contacts["alice"] = print1;
    contacts["bob"] = print2;

    std::vector<std::string> keys(contacts.size());
    std::vector<int> values(contacts.size());

    transform(contacts.begin(), contacts.end(), keys.begin(), key);
    transform(contacts.begin(), contacts.end(), values.begin(), value);

    std::cout << "Keys:\n";
    copy(keys.begin(), keys.end(), std::ostream_iterator<std::string>(std::cout, "\n"));

    std::cout << "\n";

    std::cout << "Values:\n";
    copy(values.begin(), values.end(), std::ostream_iterator<std::string>(std::cout, "\n"));
    return 0;
}
ifstream stdmap key-value stdvector c++
2 Answers

First of all, there is no point in arguing that Xcode didn't show any error msgs for your code. Try either turning on all the compiler warnings or try in online compilers. The result will be not disappointing: https://godbolt.org/z/cU54GX

If I have understood correctly, you want to store your information from two files(the integer values) in a std::map, where its key = std::string and value = vector of integer array.

If so,

1. You have your problem starting from reading integers from files. There you are using double for no reason and storing to std::vector<int> (i,e print1 and print2).

2. Secondly, what if your file has not been open?. inputFile.close(); and inputFile2.close(); will close it anyway, without knowing the fact. This is wrong. The proper way would be:

inputFile.open("numbers.txt", std::ios::in); // opening mode
if (inputFile.is_open()) { 
   // do stuff
   inputFile.close(); // you need closing only when file has been opened
}

3. If your intention is to only print keys and values, you don't need to parse them to different vectors. You can do it directly:

for(const std::pair<kType, vType>& mapEntry: contacts)
{
    std::cout << "Key: " << mapEntry.first << "   Values: ";
    for(const int values: mapEntry.second) std::cout << values << " ";
    std::cout << std::endl;
}

In c++17 you can use Structured binding

for(const auto& [Key, Values]: contacts)
{
    std::cout << "Key: " << Key << "   Values: ";
    for(const int value: Values) std::cout << value << " ";
    std::cout << std::endl;
}

4. If you really want to parse them to a different vector; first of all, the data structure for storing keys is wrong:

std::vector<int> values(contacts.size());
          ^^^^^^

which should have been a vector of vector of integers, as your vType = std::vector<int>. That is, 

std::vector<std::vector<int>> values
           ^^^^^^^^^^^^^^^^^^

Secondly, you have the functions key(const std::pair<std::string, std::string>& keyValue) and value(const std::pair<std::string, std::string>& keyValue) wrong, where you are passing a pair of strings as keys and values.

This should have been 

typedef std::string    kType;  // type of your map's key
typedef std::vector<int> vType;// value of your map's value
std::pair<kType, vType>

However, you can simply replace with lambdas, which would be more intuitive, in the sense of having the functions next to the line where you needed. For example,

std::vector<kType> keysVec; 
keysVec.reserve(contacts.size());
auto getOnlyKeys   = [](const std::pair<kType, vType>& mapEntry){  return mapEntry.first; };
std::transform(contacts.begin(), contacts.end(), std::back_inserter(keysVec), getOnlyKeys);

See an example code here

user_96098400

You can reference map element directly which will create an entry if doesn't exist, and just fill it from the file read loop:

#include <iostream>
#include <fstream>
#include <map>
#include <vector>
#include <string>


int main()
{
    std::map<std::string, std::vector<int>> m;
    int num;

    auto &&alice = m["alice"];
    std::ifstream if_alice("numbers1.txt");

    while (if_alice >> num)
        alice.push_back(num);

    if_alice.close();

    auto &&bob = m["bob"];
    std::ifstream if_bob("numbers2.txt");

    while (if_bob >> num)
        bob.push_back(num);

    if_bob.close();

    // test
    for (auto &&data : m)
    {
        std::cout << "Name: " << data.first << "\t";
        for (int num : data.second)
            std::cout << num << " ";
        std::cout << "\n";
    }
}
user_87460070
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