如何在C ++中对ifstream使用两次迭代器? [英] How do I use an iterator on an ifstream twice in C++?

问题描述

我是C ++的新手，我对将迭代器与ifstream一起使用感到困惑.在下面的代码中，我有一个名为dataFile的ifstream变量.

I'm new to C++ and I'm confused about using iterators with ifstream. In this following code I have an ifstream variable called dataFile.

在代码中，我首先对文件进行一次遍历，以计算文件中有多少个字符(是否有更有效的方法?).然后，我创建一个具有该大小的矩阵，然后再次进行迭代以填充该矩阵.

In the code I first iterate through the file once to count how many characters it has (is there a more efficient way to do this?). Then I create a matrix of that size, and iterate through again to fill the matrix.

问题在于迭代器拒绝第二次迭代，并且不会做任何事情.我尝试使用dataFile.clear()从一开始就重置了ifstream，但这没有用，可能是因为我对迭代器有一些深刻的误解.有人可以帮我吗?

The problem is that the iterator refuses to iterate the second time around, and will not do anything. I tried resetting the ifstream from the beginning by using dataFile.clear(), but this didn't work, probably because I have some deep misunderstanding about iterators. Could someone help me please?

typedef istreambuf_iterator<char> dataIterator;
for (dataIterator counter(dataFile), end; counter != end; ++counter, ++numCodons) {} // Finds file size in characters.
MatrixXd YMatrix = MatrixXd::Constant(3, numCodons, 0);
dataFile.clear(); // Resets the ifstream to be used again.
for (dataIterator counter(dataFile), end; counter != end; ++counter) {...}

推荐答案

istreambuf_iterator是一个输入迭代器，它一旦被递增，其先前值的所有副本都可能无效，而不是正向迭代器，它在用于以下情况时可以保证有效性多遍算法.有关迭代器类别的更多信息，请参见此处.

istreambuf_iterator is an input iterator which once has been incremented, all copies of its previous value may be invalidated, not a forward iterator which guarantees validity when used in multipass algorithms. More about the category of iterators, see here.

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