//open a file using fstream objects say file and functions like file.open() in input mode
// Create a string variable which can act as a buffer for holding the values
while(getline( <file descriptor value>, <buffer object>)!=NULL)
//extract string value of key into skey and convert it into integer using istringstream
//extract string value of key into value
//With the help of a new fstream object write into the file you want it to.
//erase the buffer for providing space for next input using file.erase() function