Help for speed optimization , compare large files C++

[r00ty]

Hi , i have 2 txt files full of names , read line by line and compare. If name from first file exist in second , i don't write it to result file. My code work excelent with small files , but when i try big files (100 MB) , the program is very slow. My friend advise me to use array, but i have problem with coding. Any other ideas is also welcome. Thank you in advance for your help and sorry for bad english , not native for me ! This is code i use:

#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <string>
#include <vector>

using namespace std;

int main()
{

ifstream ifs1("data1.txt");
vector<string> data1;
if (ifs1.is_open()){
string line;
while(getline(ifs1, line)){
data1.push_back(line);
}
}
else{
cout << "Unable to open file data1.txt";
return 1;
}

ifstream ifs2("data2.txt");
if (ifs2.is_open()){
string line;
while(getline(ifs2, line)){
for(size_t i = 0; i < data1.size(); ++i){
if (line == data1)
data1.erase(data1.begin()+i);
}
}
}
else{
cout << "Unable to open file data2.txt";
return 1;
}
ofstream ofs("result.txt");
if (ofs.is_open()){
for(size_t i = 0; i < data1.size(); ++i){
ofs << data1 << endl;
}
}
else{
cout << "Unable to open file result.txt";
return 1;
}
return 0;
}

[mrnutty]

What you are doing is reading the first file and then reading the seconds file. Read both the files at the same time.

[Ancient Dragon]

Read both files at the same time as firstPerson mentioned, then compare the lines as they are read; no need to put them in a vector unless you intend to do something else with them.

[r00ty]

sorry for stupid question , but how to read two files at same time ?

[mrnutty]

sorry for stupid question , but how to read two files at same time ?

You already know how to.

//open file1
//open fil2

while( file1 >> string1 && file2 >> string2 ){
   //compare string and do logic here
}

Now if they are different length then you have to do something a
little extra.

[r00ty]

i try this , but code only read and write first line of first line , don't compare

int main()
{

ifstream ifs1("data1.txt"); //file1
ifstream ifs2("data2.txt"); //file2
ofstream ofs("result.txt");
string line1;
string line2;
while( ifs1 >> line1 && ifs2 >> line2){
    if (line1.compare(line2)!= 0)
        ofs <<line1<< endl;


}