对于数组，为什么会出现a [5] == 5 [a]? [英] With arrays, why is it the case that a[5] == 5[a]?

问题描述

正如Joel在 Stack Overflow播客#34 中在解决方案

C标准对[]运算符的定义如下:

a[b] == *(a + b)

因此，a[5]的计算结果为:

*(a + 5)

和5[a]的计算结果为:

*(5 + a)

a是指向数组第一个元素的指针. a[5]是距离a的5个元素的值，它与*(a + 5)相同，并且从小学数学我们知道它们是相等的(加法是Stack Overflow podcast #34, in C Programming Language (aka: K & R), there is mention of this property of arrays in C: a[5] == 5[a]

Joel says that it's because of pointer arithmetic but I still don't understand. Why does a[5] == 5[a]?

解决方案

The C standard defines the [] operator as follows:

a[b] == *(a + b)

Therefore a[5] will evaluate to:

*(a + 5)

and 5[a] will evaluate to:

*(5 + a)

a is a pointer to the first element of the array. a[5] is the value that's 5 elements further from a, which is the same as *(a + 5), and from elementary school math we know those are equal (addition is commutative).

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