Read integers from file - line by line

Question

How can I read integers from a file to the array of integers in c++? So that, for example, this file's content:

would become:

int *arr = {23, 31, 41, 23};

?

I actually have two problems with this. First is that I don't really know how can I read them line by line. For one integer it would be pretty easy, just file_handler >> number syntax would do the thing. How can I do this line by line?

The second problem which seems more difficult to overcome for me is - how should I allocate the memory for this thing? :U

Use std::vector instead of the array and push_back new integers, the vector will grow allocating memory automatically — piokuc, Apr 20 '13 at 17:15

score 3 · Accepted Answer · edited Mar 20 '17 at 13:13

3

std::ifstream file_handler(file_name);

// use a std::vector to store your items.  It handles memory allocation automatically.
std::vector<int> arr;
int number;

while (file_handler>>number) {
  arr.push_back(number);

  // ignore anything else on the line
  file_handler.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}

edited Mar 20 '17 at 13:13

michael-slx

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answered Apr 20 '13 at 17:15

Vaughn Cato

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Uhh, vectors...I'd have to pass this one by parameter to the other function - is this o.k.? I've heard it's pretty slow. – user2252786 Apr 20 '13 at 17:17
2

@user2252786: if you pass a reference it isn't slow. And it probably isn't that slow anyway. – Vaughn Cato Apr 20 '13 at 17:17
1

@user2252786: It's not slow. Don't listen to everything you hear (except it's here on SO). – Kerrek SB Apr 20 '13 at 17:24
@user2252786: Vectors are just as fast as arrays. http://stackoverflow.com/q/3664272/14065 – Martin York Apr 20 '13 at 17:36
Doing `for(int number; file_handler>>number;)` is cleaner than using while IMO – EmilioAK Sep 30 '22 at 00:17

Martin York · Answer 2 · 2013-04-20T18:10:20.660

2

don't use array use vector.

#include <vector>
#include <iterator>
#include <fstream>

int main()
{
    std::ifstream      file("FileName");
    std::vector<int>   arr(std::istream_iterator<int>(file), 
                           (std::istream_iterator<int>()));
                       // ^^^ Note extra paren needed here.
}

edited Apr 20 '13 at 18:10

answered Apr 20 '13 at 17:33

Martin York

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score 1 · Answer 3 · answered Apr 20 '13 at 17:17

You can just use file >> number for this. It just knows what to do with spaces and linebreaks.

For variable-length array, consider using std::vector.

This code will populate a vector with all numbers from a file.

int number;
vector<int> numbers;
while (file >> number)
    numbers.push_back(number);

Peter R · Answer 4 · 2013-04-20T17:29:42.767

1

Here's one way to do it:

#include <fstream>
#include <iostream>
#include <iterator>

int main()
{
    std::ifstream file("c:\\temp\\testinput.txt");
    std::vector<int> list;

    std::istream_iterator<int> eos, it(file);

    std::copy(it, eos, std::back_inserter(list));

    std::for_each(std::begin(list), std::end(list), [](int i)
    {
        std::cout << "val: " << i << "\n";
    });
    system("PAUSE");
    return 0;
}

edited Apr 20 '13 at 17:29

answered Apr 20 '13 at 17:24

Peter R

2,865
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You can use std::copy to print as well: `std::copy(list.begin(), list.end(), std::ostream_iterator(std::cout, "\n"));` – Martin York Apr 20 '13 at 17:38
Yes but I'm limited to a delimiter and for the clarity of the output I wanted to put the "val:" prefix. – Peter R Apr 20 '13 at 18:05

Read integers from file - line by line

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