Very basic radix sort

Question

I just wrote a simple iterative radix sort and I'm wondering if I have the right idea.
Recursive implementations seem to be much more common.

I am sorting 4-byte integers (unsigned to keep it simple).
I am using 1-byte as the 'digit'. So I have 2^8=256 buckets.
I am sorting the most significant digit (MSD) first.
After each sort I put them back into array in the order they exist in buckets and then perform the next sort.
So I end up doing 4 bucket sorts.
It seems to work for a small set of data. Since I am doing it MSD I'm guessing that's not stable and may fail with different data.

Did I miss anything major?

#include <iostream>
#include <vector>
#include <list>

using namespace std;

void radix(vector<unsigned>&);
void print(const vector<list<unsigned> >& listBuckets);
unsigned getMaxForBytes(unsigned bytes);
void merge(vector<unsigned>& data, vector<list<unsigned> >& listBuckets);

int main()
{
    unsigned d[] = {5,3,6,9,2,11,9, 65534, 4,10,17,13, 268435455, 4294967294,4294967293, 268435454,65537};
    vector<unsigned> v(d,d+17);

    radix(v);
    return 0;
}

void radix(vector<unsigned>& data)
{
    int bytes = 1;                                  //  How many bytes to compare at a time
    unsigned numOfBuckets = getMaxForBytes(bytes) + 1;
    cout << "Numbuckets" << numOfBuckets << endl;
    int chunks = sizeof(unsigned) / bytes;

    for(int i = chunks - 1; i >= 0; --i) 
    {
        vector<list<unsigned> > buckets;            // lazy, wasteful allocation
        buckets.resize(numOfBuckets);

        unsigned mask = getMaxForBytes(bytes);
        unsigned shift = i * bytes * 8;
        mask = mask << shift;

        for(unsigned j = 0; j < data.size(); ++j)
        {
            unsigned bucket = data[j] & mask;       //  isolate bits of current chunk
            bucket = bucket >> shift;               //  bring bits down to least significant

            buckets[bucket].push_back(data[j]); 
        }

        print(buckets);

        merge(data,buckets);
    }
}

unsigned getMaxForBytes(unsigned bytes)
{
    unsigned max = 0;
    for(unsigned i = 1; i <= bytes; ++i)
    {
        max = max << 8;
        max |= 0xFF;
    }

    return max;
}

void merge(vector<unsigned>& data, vector<list<unsigned> >& listBuckets)
{
    int index = 0;
    for(unsigned i = 0; i < listBuckets.size(); ++i)
    {
        list<unsigned>& list = listBuckets[i];
        std::list<unsigned>::const_iterator it = list.begin();

        for(; it != list.end(); ++it)
        {
            data[index] = *it;
            ++index;
        }
    }
}

void print(const vector<list<unsigned> >& listBuckets)
{
    cout << "Printing listBuckets: " << endl;
    for(unsigned i = 0; i < listBuckets.size(); ++i)
    {
        const list<unsigned>& list = listBuckets[i];

        if(list.size() == 0) continue;

        std::list<unsigned>::const_iterator it = list.begin();  //  Why do I need std here!?
        for(; it != list.end(); ++it)
        {
            cout << *it << ", ";
        }

        cout << endl;
    }
}

Update:
Seems to work well in LSD form which it can be modified by changing the the chunk loop in radix as follows:

for(int i = chunks - 1; i >= 0; --i)

Answer 1

Let's look at en example with two-digit decimal numbers:

49, 25, 19, 27, 87, 67, 22, 90, 47, 91

Sorting by the first digit yields

19, 25, 27, 22, 49, 47, 67, 87, 90, 91

Next, you sort by the second digit, yielding

90, 91, 22, 25, 27, 47, 67, 87, 19, 49

Seems wrong, doesn't it? Or isn't this what you are doing? Maybe you can show us the code if I got you wrong.

If you are doing the second bucket sort on all groups with the same first digit(s), your algorithm would be equivalent to the recursive version. It would be stable as well. The only difference is that you'd do the bucket sorts breadth-first instead of depth-first.

Answer 2

You also need to make sure you Sort every bucket from MSD to LSD before reassembling. Example: 19,76,90,34,84,12,72,38 Sort into 10 buckets [0-9] on MSD B0=[];B1=[19,12];B2=[];B3=[34,38];B4=[];B5=[];B6=[];B7=[76,72];B8=[84];B9=[90]; if you were to reassemble and then sort again it would not work. Instead recursively sort each bucket. B1 is sorted into B1B2=[12];B1B9=[19] Once all have been sorted you can reassemble correctly.