How do I check if loop output is a digit and only print the output with digits to the console.

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C++

Every other output in this for loop is a series of digits. I would like to check if the output is digits and if they are. Print them to the console screen.

What I have tried:

#include<iostream> //i.o
#include<cctype> //for isdigit
#include<sstream> //to phrase string
#include<string>
#include <vector>
#include <fstream>

using namespace std;
int main( void )
{
    std::string word;
    std::vector<std::string> file;

    std::ifstream in( "Files.txt" );

    while ( in >> word )
        file.push_back( word );

    for ( size_t i=0; i<file.size(); i++ )

        std::cout<< file.at(i) << '\n';
    in.close();

    return 0;
}