Store data from the file into array

Question

I have a file which contain these data :

6
1231234213 12
2312354234 23
3254132515 43

I want to store all these data into an array :

int main() {
    ifstream infile("data.txt");
    ofstream outfile("final.txt");
    int sn = 0;
        infile >> sn;
    int* arr;
    arr = new int[sn];
    for (int i = 0; i < sn; i++)
    {
        infile >> arr[i];
    }
    for (int i = 0; i < sn; i++)
    {
        cout << arr[i] << endl;
    }
}

but I'm getting this instead of those data :

Always test the stream state after an IO transaction. Error conditions that you spend time don't trapping and handling generally waste orders of magnitude more time debugging. — user4581301
– user4581301, Commented Jan 6, 2023 at 19:28
note that 2312354234 is bigger than integer max value 2147483647 (0x7FFFFFFF) — Gian Paolo
– Gian Paolo, Commented Jan 6, 2023 at 19:35
Side note: -842150451 is a magic debug value. In hex it is CDCDCDCD, a pattern used to help you trap uninitialized heap-allocated values. When you see a nice repetition like this or a huge weird number, convert it to hex and see if it is more recognizable. Here's a list of well-known debug values. — user4581301
– user4581301, Commented Jan 6, 2023 at 19:39
The output of 2147483647 is also a helpful number. It is 7FFFFFFF, signifying exactly what Gian is warning you of. — user4581301
– user4581301, Commented Jan 6, 2023 at 19:41
You could switch to use int64_t instead of int if your data requires 64 bit integers. — drescherjm
– drescherjm, Commented Jan 6, 2023 at 19:42

Chris · Accepted Answer · 2023-01-06 20:16:02Z

2

When testing with an array of long long int your problem disappears:

int main() {
    std::ifstream infile("data.txt");
    int n = 0;
    
    infile >> n;

    long long int *arr = new long long int[n];

    for (int i = 0; i < n; i++) {
        infile >> arr[i];
    }

    for (int i = 0; i < n; i++) {
        std::cout << arr[i] << std::endl;
    }

    return 0;
}

Output:

You might make things easier on yourself by using a std::vector.

int main() {
    std::ifstream infile("data.txt");
    int n = 0;
    
    infile >> n;

    std::vector<long long int> vec(n);

    for (auto &i : vec) {
        infile >> i;
    }

    for (auto i : vec) {
        std::cout << i << std::endl;
    }

    return 0;
}

If we use just int, we can check if the read succeeded. How you choose to handle that error is up to you and the requirements of your program.

E.g.

    for (auto &i : vec) {
        if (infile >> i) continue;
        std::cerr << "A problem occurred." << std::endl;
        i = -2;
    }

Now output is:

A problem occurred.
A problem occurred.
A problem occurred.
A problem occurred.
1231234213
12
-2
-2
-2
-2

edited Jan 6, 2023 at 20:16

answered Jan 6, 2023 at 19:54

Chris

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1 Comment

Gian Paolo Over a year ago

this solve the problem (for that input), but doesn't address the main problem in original code: not checking for error while reading from the stream. error will happen again, unhandled, for something such "AB" or "12345678901234567890" in input file

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