0 < negative loop condition bug or misunderstanding on my part

[Ali Çehreli]

On 03/06/2012 09:11 PM, ixid wrote:
 > I'm writing my first basic algorithms, this one is merge sort. This
 > version throws an exception when array.length - setSize is negative
 > (which should be fine, the rest of my function would deal with it):
 >
 > template mergeSort(T)
 > {
 > void mergeSort(ref T[] array, const T setSize = 100)
 > {
 > T[][] merge;
 > merge.length = array.length / setSize;
 > T ii, jj;
 > for(ii = 0, jj = 0;ii < array.length - setSize;ii += setSize, ++jj)

We don't know what T is, but I assume a signed type like int.

array.length is size_t, i.e. an unsigned type. Unsigned types have this 
nasty habit of converting the entire expression to unsigned (that is a 
rule since C). So array.length - setSize above is size_t.

In other words, it is never negative.

 > merge[jj] = array[ii..ii + setSize];
 >
 > ...
 >
 > If I make the seemingly pointless change to this:
 >
 > template mergeSort(T)
 > {
 > void mergeSort(ref T[] array, const T setSize = 100)
 > {
 > T[][] merge;
 > merge.length = array.length / setSize;
 > T ii, jj;
 > T temp2 = array.length - setSize;

There, you are setting temp2's size to be T (e.g. int), so temp2 can be 
negative.

 > for(ii = 0, jj = 0;ii < temp2;ii += setSize, ++jj)
 > merge[jj] = array[ii..ii + setSize];
 >
 > Where it's a temporary variable then it works perfectly well.

Ali