C++ reading txt iterator

Question

I need to deduce length of text file and save the numbers it contains (int) in a vector. I wrote' the code below:

    std::ifstream data(file_in);

    unsigned length=distance(std::istream_iterator<int>(data),std::istream_iterator<int>());

    copy(std::istream_iterator<int>(data),std::istream_iterator<int>(),vec.begin());

However, even if the length is correct, no number is saved into my vector, but if I remove the second line numbers are saved. I think it's a problem of iterator... but I don't know how to solve it, do I need to use the function advance?

And that's what the "single pass" means in "single pass iterator"... — Kerrek SB, Jun 29 '14 at 18:42

score 0 · Accepted Answer · edited May 23 '17 at 10:26

0

I've no idea how you use length in your code but here is a small piece of code that do what you need:

std::ifstream data(file_in);
std::vector<int> vec;
std::copy(std::istream_iterator<int>(data), std::istream_iterator<int>(), 
          std::back_inserter(vec));

All the magic is handled by std::back_inserter. There's no need to compute the length of the file.

You can find here a similar problem.

edited May 23 '17 at 10:26

Community

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answered Jun 29 '14 at 18:44

Hiura

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Even better, initialize the vector using the iterators. – chris Jun 29 '14 at 18:47
I need to calculate the length! – user3758182 Jun 29 '14 at 18:50
@chris: yes, unless `vec` is a member variable. Since the OP is using `copy` I used it too. – Hiura Jun 29 '14 at 19:17
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@user3758182: what's wrong with `vec.size()`? – Hiura Jun 29 '14 at 19:18
@Hiura, If it's a member, initialize it with a constructor initializer list. – chris Jun 29 '14 at 19:26
@chris and then you have to have `data` as a member too which might not make sense, or as a parameter but this might not be the OP's original design. Anyway, in the linked answer I show how to construct a vector from iterators. – Hiura Jun 29 '14 at 19:51

score 0 · Answer 2 · answered Jun 29 '14 at 18:44

There is no point trying to count the number of integers that way. You can only do that by parsing the file, and if you've done that, why parse it again?

std::ifstream data(file_in);
std::vector<int> vec;
int i;
while (data >> i)
{
    vec.push_back(i);
}

C++ reading txt iterator

2 Answers2