inserting int variable in file name [duplicate]

Answer 1

Like this:

char fn [100];
snprintf (fn, sizeof fn, "file_no_%02d.vtk", k);
myfile.open(fn);

Or, if you don't want the leading zero (which your example shows):

snprintf (fn, sizeof fn, "file_no_%d.vtk", k);

Answer 2

In C++11:

while(k<50){
  ifstream myfile("file_no_" + std::to_string(k) + ".vtk");
  // myfile << "data to write\n";
  k++;
}

Answer 3

Use a stringstream (include <sstream>):

while(k < 50){
    std::ostringstream fileNameStream("file_no_");
    fileNameStream << k << ".vtk";

    std::string fileName = fileNameStream.str();

    myfile.open(fileName.c_str());

   // things

    myfile.close();

    k++;
}