CODEWITHSUNDEEP
c++
smilingbuddha
smilingbuddha

Reputation: 14670

Reading only SPECIFIC range of lines in a text file C++

Hi I have a text file which contains some numerical data. Of that text file ONLY the lines 14 to 100 have to be read into my C++ program. Each of these lines contain three numbers corresponding to x,y,z coordinates of a point. Thus, coordinates are given for 87 points in all.

I want to put these numbers into the arrays xp[87] yp[87] and zp[87].

How do I perform this?

Uptil now I have been used to the following 

ifstream readin(argv[1])//Name of the text file 

for (int i=0; i<=86; ++i)
{
readin>>xp[i]>>yp[i]>>zp[i];
}

But this technique works only for those files which contain 87 lines and the data to be read starts from the first line itself.

In the present case I want to ignore ALL lines before line 14 and ALL lines after line 100

Upvotes: 2

Views: 4984

Answers (3)

Enlico
Enlico

Reputation: 28480

After several years, Range-v3 allows one to write this:

#include <fstream>
#include <iostream>
#include <range/v3/view/drop.hpp>
#include <range/v3/view/getlines.hpp>
#include <range/v3/view/take.hpp>

using namespace ranges;
using namespace ranges::views;

int main() {

    std::ifstream ifs{"inputFile"};

    auto lines = getlines(ifs) | take(100) | drop(15/* which is 14 - 1 */);

    for (auto i : lines) {
        std::cout << i << std::endl;
    }
}

Requires C++17.

Upvotes: 0

K-ballo
K-ballo

Reputation: 81379

In the present case I want to ignore ALL lines before line 14

Since you have to actually read the file to know where a line ends and a new one begins, you will have to then read 13 lines. Use getline() and a dummy string to hold the results from it.

and ALL lines after line 100

Just close the stream and be done with it.

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477398

Read line by line, for most flexibility in your format:

#include <fstream>
#include <sstream>
#include <string>

std::ifstream infile("thefile.txt");
std::string line;

unsigned int count = 0;

while (std::getline(infile, line))
{
  ++count;
  if (count > 100) { break; }    // done
  if (count < 14)  { continue; } // too early

  std::istringstream iss(line);
  if (!(iss >> x[count - 14] >> y[count - 14] >> z[count - 14]))
  {
    // error
  }
}

// all done

Upvotes: 5

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