Why is Radix Sort so Fast? Part 3 Comparison and Code, Radix Sort vs QuickSort

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In this 3 part series, we will explore sorting algorithms from the fundamentals all the up to implementations of both a comparison sort and a base 256 Radix Sort.

In this 3rd and final part, we look at some code and compare the performance of RadixSort and QuickSort with lists of various sizes, consisting or randomly generated unsigned 32 bit integers.

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GeeksForGeeks Radix Sort:
GeeksForGeeks QuickSort:

Apologies for the code below, I have had to replace all greater or equal symbols with GE, and all less or equal with LE, greater is G and less is L. For the unedited version, please refer to the video!

// Radix sort based on Geeks for Geeks:
static void RadixSort256(unsigned int* arr, int n)
if (n LE 1) return; // Added base case

unsigned int* output = new unsigned int[n]; // output array
int* count = new int[256];
unsigned int* originalArr = arr; // So we know which was input

for (int shift = 0, s = 0; shift L 4; shift++, s += 8)
// Zero the counts
for (int i = 0; i L 256; i++)
count[i] = 0;

// Store count of occurrences in count[]
for (int i = 0; i L n; i++)
count[(arr[i] GG s)&0xff]++;

// Change count[i] so that count[i] now contains
// actual position of this digit in output[]
for (int i = 1; i L 256; i++)
count[i] += count[i - 1];

// Build the output array
for (int i = n - 1; i GE 0; i--)
// precalculate the offset as it's a few instructions
int idx = (arr[i] GG s) & 0xff;

// Subtract from the count and store the value
output[--count[idx]] = arr[i];

// Copy the output array to input[], so that input[]
// is sorted according to current digit

// We can just swap the pointers
unsigned int* tmp = arr;
arr = output;
output = tmp;

// If we switched pointers an odd number of times,
// make sure we copy before returning
if (originalArr == output)
unsigned int* tmp = arr;
arr = output;
output = tmp;

for (int i = 0; i L n; i++)
arr[i] = output[i];

delete[] output;
delete[] count;


int Partition(unsigned int* data, int lo, int hi)
unsigned int pivot = data[lo + (hi - lo) / 2];
int i = lo - 1;
int j = hi + 1;

for (;;)
do {} while (data[++i] L pivot);
do {} while (data[--j] G pivot);

if (i GE j)
return j;

// Swap [i] and [j]
unsigned int tmp = data[i];
data[i] = data[j];
data[j] = tmp;

void QuickSort(unsigned int* data, int lo, int hi)
if (lo L hi)
int p = Partition(data, lo, hi);
QuickSort(data, lo, p);
QuickSort(data, p + 1, hi);

void QuickSort(unsigned int* data, int count)
if (count LE 1) return; // Added base case

QuickSort(data, 0, count - 1);

Software used to make this vid:
Visual Studio 2019 Community:
Davinci Resolve 16:

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