reading variable length input in C++ using stream

Question

I have following requirement which is to be read in to program.

The first line contains two space-separated integers denoting the respective values of p (the number of variable-length arrays) and q(the number of queries). Each line of the subsequent lines contains a space-separated sequence for each element in to array .

Each of the subsequent lines contains two space-separated integers describing the respective values of i (an index in array ) and j (an index in the array referenced by ) for a query.

2 2
3 1 5 4
5 1 2 8 9 3
0 1
1 3

In example above i have 2 arrays and 2 queries. First array is 3,3,5, 4 and second array 5 1 2 8 9 3.

My question is how can I read this data in my container. Note: I cannot enter input from console, here some test program provides input.

I have written as below

int iNoOfVectors = 0;
    int iNoOfQueries = 0;
    cin >> iNoOfVectors >> iNoOfQueries;
    cout << iNoOfVectors ; 
    vector<vector<int>> container;
    container.reserve(iNoOfVectors);
    for(int i = 0; i < iNoOfVectors; i++ ) {
        int temp;
        std::string line;
        while (std::getline(std::cin, line))
        {
            std::cout << line << std::endl;
        }
    }

output of above

2
3 1 5 4
5 1 2 8 9 3
0 1
1 3

How can I get variable length array elements in to my container.

Thanks

Reading line by line is a good start. Then you can put that line into an std::istringstream and use std::istream_iterator to construct a vector. — Some programmer dude
– Some programmer dude, Commented Nov 3, 2019 at 9:01

A M · Accepted Answer · 2019-11-03 09:50:54Z

If you want to read similar data from a string into a vector, you need to do the following 2 steps:

Put the contents of the string into a std::istringstream
Iterate over the elements in the istringstream with the std::istream_iterator

Example:

#include <iostream>
#include <string>
#include <sstream>
#include <algorithm>
#include <iterator>
#include <vector>

int main() {

    // The source string
    std::string stringWithIntegers{ "5 1 2 8 9 3" };

    // Build an istringstream with the above string as data source
    std::istringstream iss{ stringWithIntegers };

    // Define variable 'data'. Use range constructor and stream iterator    
    std::vector<int> data{std::istream_iterator<int>(iss), std::istream_iterator<int>()};

    // Display result
    std::copy(data.begin(), data.end(), std::ostream_iterator<int>(std::cout, "\n"));

    return 0;
}

Also copying the data is possible:

    std::vector<int> data{};

    std::copy(
        std::istream_iterator<int>(iss),
        std::istream_iterator<int>(),
        std::back_inserter(data)
    );

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