On Wed, 2010-04-21 at 19:20 -0700, Matthew Van Gundy wrote:
> On 4/21/10 3:26 PM, Jeff Newmiller wrote:
> >> There are many ways to skin a cat, here's one:
> >>
> >> void reverse(int forward[], int backward[], unsigned int n) {
> >> unsigned i = n;
> >> while(i--> 0) {
> >> backward[n-i-1] = forward[i];
> >> }
> >> }
> >
> > This reverses and then re-reverses it.
>
> Nope, just reverses it once. I'll admit, it isn't an idiomatic
> construction, but it uses an unsigned index that counts down to reverse
> an array since that's what Brian seemed to be after.
The most idiomatic construction in C for writing an iterator is to use
pointer arithmetic. i.e.
int* thearray;
size_t itssize;
for(int* iterator=thearray; iterator<thearray+itssize;++iterator){
/* do something with the iterator */
}
The loop terminates when the address of iterator is one past the end of
the array. The C standard guarantees that you can *form* the address one
past the end of the array (i.e. on a 32-bit machine with 4GB of address
space, the last element of the array can't abut the end of RAM, or some
compiler-level hack is required to guarantee that you can form the
address). However, the C standard makes no guarantees about being able
to form the address *before* the array, so the following is not
guaranteed to work:
for(int* iterator=thearray+itssize-1; iterator>=thearray;--iterator){
/* do something with the iterator */
}
The following idiom must be used instead:
int* iterator=thearray+itssize;
while( iterator-- > thearray){
/* do something with the iterator. */
}
The asymmetry you're noticing with unsigned int indices is a similar
problem.
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