Sleep Sort – The King of Laziness / Sorting while Sleeping

In this algorithm we create different threads for each of the elements in the input array and then each thread sleeps for an amount of time which is proportional to the value of corresponding array element.

Hence, the thread having the least amount of sleeping time wakes up first and the number gets printed and then the second least element and so on. The largest element wakes up after a long time and then the element gets printed at the last. Thus the output is a sorted one.

All this multithreading process happens in background and at the core of the OS. We do not get to know anything about what’s happening in the background, hence this is a “mysterious” sorting algorithm.

Example : Let’s assume (for convenience) we have a computer that’s so slow it takes 3 seconds to work through each element:

INPUT: 8 2 9 

3s: sleep 8
6s: sleep 2
8s: "2" (2 wakes up so print it)
9s: sleep 9
11s: "8" (8 wakes up so print it)
18s: "9" (9 wakes up so print it)

OUTPUT: 2 8 9

To implement sleep sort, we need multithreading functions, such as _beginthread() and WaitForMultipleObjects(). Hence we need to include windows.h to use these functions. This won’t compile on Online IDE. We must run it in your PC (Note this code is for WINDOWS and not for LINUX).

To perform a sleep sort we need to create threads for each of the value in the input array. We do this using the function _beginthread().

In each of the threads we assign two instructions:

1) Sleep : Sleep this thread till arr[i] milliseconds (where arr[i] is the array element which this thread is associated to). We do this using Sleep() function. The Sleep(n) function suspends the activity associated with this thread till ‘n’ milliseconds. Hence if we write Sleep(1000), then it means that the thread will sleep for 1 second (1000 milliseconds = 1 second)

2) Print : When the thread ‘wakes’ up after the sleep then print the array element – arr[i] which this thread is associated to.

After creating the threads, we process these threads. We do this using WaitForMultipleObjects().

// C implementation of Sleep Sort
#include <stdio.h>
#include <windows.h>
#include <process.h>

// This is the instruction set of a thread
// So in these threads, we "sleep" for a particular
// amount of time and then when it wakes up
// the number is printed out
void routine(void *a)
    int n = *(int *) a; // typecasting from void to int

    // Sleeping time is proportional to the number
    // More precisely this thread sleep for 'n' milliseconds

    // After the sleep, print the number
    printf("%d ", n);

/* A function that performs sleep sort
_beginthread() is a C run-time library call that creates a new
'thread' for all the integers in the array and returns that

Each of the 'thread' sleeps for a time proportional to that
integer and print it after waking.

We pass three parameters to _beginthread :-
1) start_address --> start address of the routine/function
					 which creates a new thread
2) stack_size --> Stack Size of the new thread (which is 0)
3) arglist --> Address of the argument to be passed

The return value of _beginthread() function is a handle to the
thread which is created. So we must accept is using the datatype-
'HANDLE' which is included in windows.h header
'HANDLE' datatype is used to represent an event/thread/process etc
So 'HANDLE' datatype is used to define a thread
We store the threads in an array - threads[] which is declared
using 'HANDLE' datatype.

WaitForMultipleObjects() is a function that processes the threads
and has four arguments-
1) no_of_threads --> Number of threads to be processed
2) array_of_threads --> This is the array of threads which should be
						processed. This array must be of the type
3) TRUE or FALSE --> We pass TRUE if we want all the threads in the
                     array to be processed
4) time_limit --> The threads will be processed until this time limit
				  is crossed. So if we pass a 0 then no threads will
				  be processed, otherwise if we pass an INFINITE, then
				  the program will stop only when all the threads
				  are processed. We can put a cap on the execution
				  time of the program by passing the desired time
				  limit */
void sleepSort(int arr[], int n)
    // An array of threads, one for each of the elements
    // in the input array
    HANDLE threads[n];

    // Create the threads for each of the input array elements
    for (int i = 0; i < n; i++)
        threads[i] = (HANDLE)_beginthread(&routine, 0,  &arr[i]);

    // Process these threads
    WaitForMultipleObjects(n, threads, TRUE, INFINITE);

// Driver program to test above functions
int main()
    // Doesn't work for negative numbers
    int arr[] = {34, 23, 122, 9};
    int n = sizeof(arr) / sizeof(arr[0]);

    sleepSort (arr, n);



1) This algorithm won’t work for negative numbers as a thread cannot sleep for a negative amount of time.

2) Since this algorithm depends on the input elements, so a huge number in the input array causes this algorithm to slow down drastically (as the thread associated with that number has to sleep for a long time). So even if the input array element contains only 2 elements, like- {1, 100000000}, then also we have to wait for a much longer duration to sort.

3) This algorithm doesn’t produce a correct sorted output every time. This generally happens when there is a very small number to the left of a very large number in the input array.
For example – {34, 23, 1, 12253, 9}.
The output after sleep sorting is {9, 1, 23, 34, 1223}

A wrong output also occurs when the input array is reverse sorted initially, like- {10, 9, 8, 7, 6, 5}.

The reason for such an unexpected output is because some time is taken between scanning through each element as well as some other OS operations (like inserting each threads in a priority queue for scheduling). We cannot simply ignore the time taken by all these things.

We describe this using the below example-

Let's assume (for convenience) we have a computer that's
so slow it takes 3 seconds to work through each element: 
INPUT: 10 9 8 7 6 5

3s: sleep 10
6s: sleep 9
9s: sleep 8
12s: sleep 7
13s: "10" (10 wakes up so print it)
15s: sleep 6
15s: "9" (9 wakes up so print it)
17s: "8" (8 wakes up so print it)
18s: sleep 5
19s: "7" (7 wakes up so print it)
21s: "6" (6 wakes up so print it)
23s: "5" (5 wakes up so print it)

OUTPUT: 10 9 8 7 6 5 

The above output is just an example.
Obviously, modern-day computers computer are not so slow (to take 3 seconds to scan through each element).
In reality running sleep sort on a modern computer on the above array gives the output – {9, 5, 7, 10, 8, 6}

How to fix this ?
1) We can fix this by repeatedly sleep sorting on the new output until the output becomes sorted. Every time it will sort the elements more accurately.

2) The wrong output as discussed earlier happens due to the time taken by other OS works and scanning through each element.

In our program we have used the function Sleep(arr[i]), which means that each thread associated with the array elements sleep for ‘arr[i]’ milliseconds. Since milliseconds is a very small quantity and other OS tasks can take more time than ‘arr[i]’ milliseconds which ultimately can cause an error in sleep sorting. Increasing the sleeping time by even 10 times can give a sorted output as the OS tasks will finish all its task in between this much sleep, hence not producing any errors.

Had we used Sleep(10*arr[i]) instead of just Sleep(arr[i]) then we will certainly get a more precise output than the latter one. For example the input array – {10, 9, 8, 7, 6, 5} will give the correct sorted output – {5, 6, 7, 8, 9, 10} if we use Sleep(10*arr[i]) instead of just Sleep(arr[i]) .

However it is still possible that Sleep(10*arr[i]) will give wrong results for some test cases. To make it more precise increase the sleep time more , say something like – Sleep(20*arr[i]).

Hence the bottom line is that more the sleep time, more accurate the results are. (Sounds interesting, eh?) . But again that would increase the runtime of this algorithm.

Exercise to the readers-
1) The above algorithm tries to sort it in ascending order. Can you sort an input array in descending order using sleep sort. Think upon it.

2) Is it a comparison based sorting algorithm ? How many comparisons this algorithm makes ?
[ Answer : No, it makes zero comparisons ]

3) Can we do sleeping sort without using windows.h header and without using Sleep() function?
[One idea can be to create a priority queue where the elements are arranged according to the time left before waking up and getting printed. The element at the front of the priority queue will be the first one to get waked up. However the implementation doesn’t looks easy. Think on it.]

Time Complexity
Although there are many conflicting opinions about the time complexity of sleep sort, but we can approximate the time complexity using the below reasoning-

Since Sleep() function and creating multiple threads is done internally by the OS using a priority queue (used for scheduling purposes). Hence inserting all the array elements in the priority queue takes O(Nlog N) time. Also the output is obtained only when all the threads are processed, i.e- when all the elements ‘wakes’ up. Since it takes O(arr[i]) time to wake the ith array element’s thread. So it will take a maximum of O(max(input)) for the largest element of the array to wake up. Thus the overall time complexity can be assumed as O(NlogN + max(input)),
where, N = number of elements in the input array, and input = input array elements

Auxiliary Space
All the things are done by the internal priority queue of the OS. Hence auxiliary space can be ignored.

Sleep Sort is related to Operating System more than any other sorting algorithm. This sorting algorithm is a perfect demonstration of multi-threading and scheduling done by OS.

The phrase “Sorting while Sleeping” itself sounds very unique. Overall it is a fun, lazy, weird algorithm. But as rightly said by someone- “If it works then it is not lazy”.

This article is contributed by Rachit Belwariar. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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