When should we use Radix sort?

Question

It seems Radix sort has a very good average case performance, i.e. O(kN): http://en.wikipedia.org/wiki/Radix_sort

Yet it seems like most people are still using

Answer 1

k = "length of the longest value in Array to be sorted"

n = "length of the array"

O(k*n) = "worst case running"

k * n = n^2 (if k = n)

so when using Radix sort make sure "the longest integer is shorter than the array size" or vice versa. Then you going to beat Quicksort!

The drawback is: Most of the time you cannot assure how big integers become, but if you have a fixed range of numbers radix sort should be the way to go.