Question

If I have code like:

int pop()
{
    return stack[--stp];
}

I know that it is doing two things. It is returning the value contained in the one-dimensional array ‘stack’ in element ‘stp’. It is also decrementing ‘stp’.

But which order does this happen in?

Does it return the value of element stp, then decrement stp?

Or does it decrement stp, then return the value of the element now referred to by the decremented stp?

If the code is:

int top()
{
    return stack[stp-1];
}

Does it work any differently?

My apologies, I know this is very common coding style – I still have some trouble making sense of concise, uncommented code – even basics like this. Sorry.

Answer 1

It will decrement stp, and then return the value from the array at the location of the new stp value.

--stp is a “prefix decrement”, and it is defined to decrement the argument (stp), and then return the new value. It has a counterpart called the “postfix decrement”, stp--, which decrements stp and then returns the old value – so stack[stp--] will give you the value the current stp offset, but still decrement stp.

Finally, your version with stack[stp-1] will return the value from the same place as stack[--stp], but stp itself will be unchanged.