Vector subscript out of range while read vector

Question

I get crash when I try to run this program and error is Expression: vector subscriptout of range. What's the problem?

std::vector<DepositCustomers> Deposits; //depositcustomers is a class
std::ifstream fin("in.txt");
int contor = 1;
while (!fin.eof())
{
    Deposits.resize(contor);
    fin >> Deposits[contor];
    contor++;
}

I've tried without resize and is the same thing.

Have you tried using the `push_back(...)` method to add elements? — c-a, May 30 '18 at 12:10
Look what [`resize()`](http://en.cppreference.com/w/cpp/container/vector/resize) does. — Gaurav Sehgal, May 30 '18 at 12:11
Possible duplicate of [Why does the indexing start with zero in 'C'?](https://stackoverflow.com/questions/7320686/why-does-the-indexing-start-with-zero-in-c) — user1810087, May 30 '18 at 12:18

Sander De Dycker · Answer 1 · 2018-05-30T12:16:17.350

4

If you resize the vector to size 1, then there is only room for 1 item in the vector. Since vectors use 0-based indexes, this item is at index 0. So, when you try to place something at index 1, it fails with the observed error.

To make the code work, you can make the following change :

fin >> Deposits[contor - 1];

Or probably better yet, use push_back to save yourself the hassle of having to resize.

edited May 30 '18 at 12:16

answered May 30 '18 at 12:11

Sander De Dycker

15,403
1
31
37

score 1 · Answer 2 · answered May 30 '18 at 12:32

The problem is that vectors, like arrays, are zero-indexed - if you have k elements, they are indexed from 0 to k - 1.
Thus, Deposits[contor] is out of range for the vector, since it has contor elements.
(You're lucky that you built a debug version that checks the indexing and found out quickly.)

You could fix this by writing Deposits[contor-1], but you have another issue.

That issue is that you're using .eof() as a loop condition.
eof() does not mean what you think it means, and you will store one element too many.
(See this Q&A for details.)

The "default" C++ input loop looks like

std::vector<DepositCustomers> Deposits;
std::ifstream fin("in.txt");
DepositCustomers customer;
while (fin >> customer)
{
    Deposits.push_back(customer);
}

And a loop-free version is

std::vector<DepositCustomers> Deposits;
std::ifstream fin("in.txt");
using iterator = std::istream_iterator<DepositCustomers>;
std::copy(iterator(fin), iterator(), std::back_inserter(Deposits));

Vector subscript out of range while read vector

2 Answers2