Increment/Decrement Confusion

Question

What is happening when we decrement the code here:

temp[--countArray[getDigit(position, input[tempIndex], radix)]]

If temp is 1 in this case: are we decrementing first so that we are assigning to 0? How immediate is this decrement? It always seems to confuse me within array brackets.

Answer 1

Try unpacking the brackets on different levels of indentation:

temp[                                                // get this index in temp
    --                                               // decrement by 1
    countArray[                                      // get this index in countArray
        getDigit(position, input[tempIndex], radix)  // call getDigit()
    ]
]

In human-readable terms, it calls getDigit() to index into countArray, then decrements that value and uses it to index into temp.

---

The decrement operator --x is different from x-- because of what it returns. By the end of the operation, x always ends up as one less than it was, but --x returns the new value of x, while x-- returns the old value of x from before it was decremented. The same applies for ++x and x++.

Answer 2

Let me break this down some. Here's some code that's equivalent to above:

int digit = getDigit(position, input[tempIndex], radix);
countArray[digit]--;
int count = countArray[digit];
temp[count] // Do something with the value

Incidentally, this is a classic illustration of why you shouldn't sacrifice clarity to brevity.