It is said that array element access is quick in a programming language knowing a numerical index of the element. I assume this is because computer memory is kind of like an array with each memory slot having a numerical address. So, knowing the memory address we can go directly to that address and access a value stored there.
How is that memory location accessed quickly? Is this done at the level of Microcode?
The local variable actually references the memory location of the first element in the array. The elements in the array are of a fixed size (such as 4 bytes each for an integer) and stored in order in a contiguous block of memory. The calculation for the memory location of an element is
memory location of first element + (size in bits of single element * index of element)for a single dimensional array. Note that the index used in the calculation is zero-based, which is why most languages use zero-based indices for arrays. The elements of an array might actually be references to objects to keep the element sizes fixed, rather than the object itself which might be of a variable size.Extra explanation for multidimensional array: Element locations in a multidimensional array can be calculated using
location of element [0][0] + (size of single element * (index for 1st dimension + (index for 2nd dimension * length of 1st dimension) + ... + (index for nth dimension * length of (n-1)th dimension * ... * length of 1st dimension)))in the column major system. There are two systems, column major and row major. Look at this page for a visualization This math is still rather simple for a computer even though it looks complicated here. Multidimensional arrays could also be implemented as an array of arrays, which would be slightly slower and less memory efficient.Out of bounds checking: The calculation for the element position does not account for the indices being larger than the length of the array, which can lead to accessing memory locations outside the array. The C language does not prevent this. Java will throw an OutOfBoundsException, but accessing the array becomes slightly more expensive due a check being performed. Out of bounds checking is language specific, if it's implemented