Prerequisite : Pointers in C/C++
Consider int arr. The answer lies in the fact how the compiler interprets arr[i] ( 0<=i<100).
arr[i] is interpreted as *(arr + i). Now, arr is the address of array or address of 0th index element of array. So, address of next element in array is arr + 1 (because elements in array are stored in consecutive memory locations), further address of next location is arr + 2 and so on . Going with above arguments, arr + i means address at i distance away from starting element of array. Therefore, going by this definition, i will be zero for starting element of array because starting element is at 0 distance away from starting element of array. To fit this definition of arr[i], indexing of array starts from 0.
The conclusion is, we need random access in array. To provide random access, compilers use pointer arithmetic to reach i-th element.
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