#include <iostream.h>
#include <string.h>
#include <vector.h>
#include <fstream.h>
using std::string;
using std::vector;
void function(vector<string>&fn,vector<string>&fnum,vector<string>&ln,vector<string>&lnum);
void main()
{
	  ifstream fin,f2,f3,f4;
	  //First name input
	  vector <string> fname(25);
	fin.open("firstname.dat", ios :: in); 
	for( int i= 0;i<25;i++)
		fin>>fname[i];
	fin.close();
	//Second name input
	  vector <string> fnum(25);
	f2.open("firstnum.dat", ios :: in); 
	for(int i= 0;i<25;i++)
		f2>>fnum[i];
	f2.close();
	//Third Name
	  vector <string> lname(25);
	f3.open("lastname.dat", ios :: in); 
	for( int i= 0;i<25;i++)
		f3>>lname[i];
	f3.close();
	//Forth Name
	  vector <string> lnum(25);
	f4.open("lastnum.dat", ios :: in); 
	for( int i= 0;i<25;i++)
		f4>>lnum[i];
	f4.close();
	function(fname,fnum,lname,lnum);		
	
}
void function(vector<string>&fn,vector<string>&fnum,vector<string>&ln,vector<string>&lnum)
{
	for(int i=0;i<25;i++)
	{

		cout<<fnum[i]<<'\t'<<fn[i]<<'\t'<<ln[i]<<'\t'<<lnum[i]<<endl;
	}
}

I need to make so the First Numbers and names display along with their last name counterparts in the functions
Files attached below

Your read loops are incorrect. There is a couple extra steps you have to take. The vectors are initially empty, so one way to do it is to append each string like this:

string ln;
for( int i= 0;i<25;i++)
{
    fin >> ln;
    fname.push_back(ln);
}
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