C++：奇怪的除法输出

Question

我有以下代码：

    int x = arr[arr.size() -1] - arr[0];
    int n = arr.size();
    int dx = x/n;
    int dy = (arr[arr.size() -1] - arr[0])/arr.size();
    std::cout << dx << "  " << dy << std::endl;

当我的 arr = [15, 13, 12] 时，我得到了 dx = -1，但是 dy = 1431655764 (arr is vector<int>)

为什么 dx 和 dy 不同？谢谢！

Answer 1

Why dx and dy are different?

对于dy，注意std::vector::size is std::vector::size_type, which is an unsigned integer type. Then the result of (arr[arr.size() -1] - arr[0])/arr.size() is unsigned too. The result is overflowed的return类型，然后赋值给dy类型int。

Unsigned integer arithmetic is always performed modulo 2ⁿ where n is the number of bits in that particular integer. E.g. for unsigned int, adding one to UINT_MAX gives 0, and subtracting one from 0 gives UINT_MAX.

对于dx，先将arr.size()赋值给n（其类型为int），然后计算为x/n，结果为int 也是。然后把结果赋值给dx就可以了

C++：奇怪的除法输出

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