Why does my array print more than I want?

Question

I'm trying to write an array in c++, which consist of 10 integers that are all 1, unless changed through keyboard input. For some reason my code prints out way more than 10 integers and I'm not sure why.

int main()
{
    int index, value;
    int count = 0;
    int myData [10];
    for (int i = 0; i < sizeof(myData); i++)
    {
        myData[i] = 1;
    }

    do
    {
        for (int i = 0; i < sizeof(myData); i++)
        {
            std::cout << myData[i] << " ";
        }

        std::cout << "Input index: ";
        std::cin >> index;
        std::cout << "Input value: ";
        std::cin >> value;

        myData[index] = value;

    } while ((0 <= index) && (index <= sizeof(myData)));

    std::cout << "Index out of range. Exit." << std::endl;
    
    return 0;
}

EDIT I got it to work thank you :D but for some reason now I'm getting a buffer overflow. Anyway to fix this?

`sizeof` gives you the memory space in _bytes_, not the number of array entries. — Fred Larson, Feb 07 '22 at 13:59
`sizeof` [doesn't mean what you think it means](https://en.cppreference.com/w/cpp/language/sizeof) — Drew Dormann, Feb 07 '22 at 13:59
[How do I find the length of an array?](https://stackoverflow.com/questions/4108313/how-do-i-find-the-length-of-an-array) — Drew Dormann, Feb 07 '22 at 14:01
@LilyBujo: Best advice in C++ is to avoid using C-style arrays. Use standard library containers, like `std::vector` or `std::array`. — Fred Larson, Feb 07 '22 at 14:02
You may want to use a range based for loop. `for (auto val : myData) { std::cout << val << " "; }` — drescherjm, Feb 07 '22 at 14:14

Marek R · Answer 1 · 2022-02-07T14:33:42.263

Replacing sizeof(myData) with std::size(myData) or with sizeof(myData) / sizeof(int) is fine, but IMO it would be better if you just use range-based for loop or/and use constant to define size of that array:

int main()
{
    static const DataSize = 10;
    int index, value;
    int count = 0;
    int myData [DataSize];
    for (auto &x : myData)
    {
        x = 1;
    }

    do
    {
        for (auto x : myData)
        {
            std::cout << x << " ";
        }

        std::cout << "Input index: ";
        std::cin >> index;
        std::cout << "Input value: ";
        std::cin >> value;

        myData[index] = value;

    } while (0 <= index && index < DataSize);

    std::cout << "Index out of range. Exit." << std::endl;
    
    return 0;
}

Disclaimer: your code is still invalid, but this is another issue.

score 2 · Answer 2 · edited Feb 09 '22 at 16:40

2

As Silvio said, it stores number of bytes. You can also use a pointer as well

So your condition in for loop can look like

for (size_t i = 0; i < (sizeof(myData)/sizeof(*myData); ++i)

edited Feb 09 '22 at 16:40

Attis

573
1
7
19

answered Feb 07 '22 at 14:06

Faekr

128
7

score 1 · Answer 3 · answered Feb 07 '22 at 13:59

sizeof returns the number of bytes, not the number of ints. Consider

for (int i = 0; i < sizeof(myData) / sizeof(int); i++) {
  std::cout << myData[i] << " ";
}

This will only work if the array is statically-sized like yours is. If the array is dynamically-allocated, as in new int[10], then C++ doesn't keep track of its size so that's your job. Of course, you should never be using new int[10] anyway, since std::vector is a thing and does keep track of size.

Why does my array print more than I want?

3 Answers3