C++ the dynamic array and file reading don't work together

Question

Such a code:

int _tmain(int argc, _TCHAR* argv[])
{
    int *test = new int[];
    ifstream f("ofile.txt");
    for (int i=0,v; i<10; i++)
    {
        f>>v;
        test[i]=1; 
        cout<<"\nv = "<<v<<", i = "<<i<<endl;
    }

    return 0;
}

caused of this message after compiling:

enter image description here

I guess (correct me if I'm wrong) here is some error about memory, but details are unknown for me. If I remove one of them (file reading or array) it works. So it will be great to hear an explanation of the problem.

You didn't specify a size for you allocation!

πάντα ῥεῖ
– πάντα ῥεῖ

2013-12-29 11:57:07 +00:00
Commented Dec 29, 2013 at 11:57 — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Dec 29, 2013 at 11:57
i assume you also meant test[i]=v; not test[i]=1;

RichardPlunkett
– RichardPlunkett

2013-12-29 12:32:00 +00:00
Commented Dec 29, 2013 at 12:32 — RichardPlunkett
– RichardPlunkett, Commented Dec 29, 2013 at 12:32

Michael J · Accepted Answer · 2013-12-29 18:25:08Z

5

You're thinking java. To allocate an array like that you need to give a size. e.g.

    int *test = new int[20];

However a better scheme would be to use a vector of integers.

    #include <iostream>
    #include <fstream>
    #include <vector>
    #include <algorithm>  // for sort()

    int main(int argc, char *argv[])
    {
        std::vector<int> data;
        std::ifstream fsin("ofile.txt");

        int count = 0;
        while (fsin.good())
        {
            int v;
            fsin >> v;
            if (fsin.good())
            {
                data.push_back(v);
                std::cout << "\nv = " << v << ", i = " << count++ << std::endl;
            }
        }

        std::sort(data.begin(), data.end());

        for (size_t i=0; i<data.size(); i++)
            std::cout << i << '\t' << data[i] << std::endl;

        return 0;
    }

edited Dec 29, 2013 at 18:25

answered Dec 29, 2013 at 12:02

Michael J

8,0292 gold badges26 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

srgg6701 Over a year ago

The goal is to save the file (it contains numbers only in every string) content in array which may be re-sorted later. I guess your example will be convinient after a little changing. But I still can't assemble all this together. Plsss, point out how to do that.

Michael J Over a year ago

@srgg6701 - hich part is unclear? Each time around the loop you read a value from the file into 'v', then append that value to the vector 'test'. You can then do whatever is required with the vector data.

srgg6701 Over a year ago

Anybody, tell me - why there the condition if (fsin.good()) is repeated inside the loop while (it is already set)? Is it a mistake or it is really necessary?! I'm confused :(

Michael J Over a year ago

The inner test confirms that "fin >> v" succeeded.

OneOfOne · Accepted Answer · 2013-12-29 11:57:51Z

3

You have to allocate a fixed size array, int *test = new int[]; shouldn't even work.

If the size is known, use int *test = new int[size];, otherwise try using a std::vector or std:array.

answered Dec 29, 2013 at 11:57

OneOfOne

100k22 gold badges195 silver badges187 bronze badges

Collectives™ on Stack Overflow

C++ the dynamic array and file reading don't work together

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related