Question

I have numerical text data lines ranging between 1mb – 150 mb in size, i need to write lines of numbers related to heights, for example: heights=4 , new text must include lines: 1,5,9,13,17,21…. consequentially.

i have been trying to find a way to do this for a while now, tried using a list instead of vector which ended up with compilation errors.

I have cleaned up the code as advised. It now writes all lines sample2 text, all done here. Thank you all

I am open to method change as long as it delivers what i need, Thank you for you time and help.

following is what i have so far:

#include <iostream>
#include <fstream>
#include <string>
#include <list>
#include <vector>

using namespace std;

int h,n,m;
int c=1;

int main () {

cout<< "Enter Number Of Heights: ";
cin>>h;

ifstream myfile_in ("C:\\sample.txt");
ofstream myfile_out ("C:\\sample2.txt");
string line;
std::string str;
vector <string> v;
if (myfile_in.is_open()) {
myfile_in >> noskipws;
int i=0;
int j=0;
while (std::getline(myfile_in, line)) {
v.push_back( line );
++n;
if (n-1==i) {
myfile_out<<v[i]<<endl;
i=i+h;
++j;
}
    }
cout<<"Number of lines in text file: "<<n<<endl;
}

else cout << "Unable to open file(s) ";

cout<< "Reaching here, Writing one line"<<endl;

system("PAUSE");
return 0; 
}

Answer 1

You need to use seekg to set the position at the beginning of the file, once you have read it (you have read it once, to count the lines (which I don’t think you actually need, as this size is never used, at least in this piece of code)

And what is the point if the inner while? On each loop, you have

int i=1;
myfile_out<<v[i]; //Not writing to text
i=i+h;

So on each loop, i gets 1, so you output the element with index 1 all the time. Which is not the first element, as indices start from 0. So, once you put seekg or remove the first while, your program will start to crash.

So, make i start from 0. And get it out of the two while loops, right at the beginning of the if-statement.

Ah, the second while is also unnecessary. Leave just the first one.

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EDIT:
Add

myfile_in.clear();

before seekg to clear the flags.

Also, your algorithm is wrong. You’ll get seg fault, if h > 1, because you’ll get out of range (of the vector). I’d advise to do it like this: read the file in the while, that counts the lines. And store each line in the vector. This way you’ll be able to remove the second reading, seekg, clear, etc. Also, as you already store the content of the file into a vector, you’ll NOT lose anything. Then just use for loop with step h.

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Again edit, regarding your edit: no, it has nothing to do with any flags. The if, where you compare i==j is outside the while. Add it inside. Also, increment j outside the if. Or just remove j and use n-1 instead. Like

if ( n-1 == i )