I am wondering why my quickSort is so slow. It takes 10-20 seconds to sort the following array. Bubblesort (shown below) does it automatically.
public static void quickSort(int[] tab, int lowIndex, int highIndex) {
if (tab == null || tab.length == 0) {
return;
}
int i = lowIndex;
int j = highIndex;
int pivot = tab[lowIndex + (highIndex - lowIndex) / 2];
while (i <= j) {
while (tab[i] < pivot) {
i++;
}
while (tab[j] > pivot) {
j--;
}
if (i <= j) {
int c = tab[i];
tab[i] = tab[j];
tab[j] = c;
i++;
j--;
}
if (lowIndex < j) {
quickSort(tab, lowIndex, j);
}
if (i < highIndex) {
quickSort(tab, i, highIndex);
}
}
}
public static void bubbleSort(int[] arr) {
int n = arr.length - 1;
while (n >= 1) {
for (int i = 0; i < n; i++) {
if (arr[i] > arr[i + 1]) {
int c = arr[i];
arr[i] = arr[i + 1];
arr[i + 1] = c;
}
}
n--;
}
}
public static void main(String[] args) {
int[] t = new int[] { 5, 1, 33, 13, 21, 2, 12, 4,
2, 3, 53, 2, 125, 23, 53, 523, 5, 235, 235, 235, 23, 523, 1,
2, 41, 2412, 412, 4, 124, 1, 241, 24, 1, 43, 6, 346, 457, 56,
74, 5, 3, 5, 1, 33, 13, 21, 2, 12, 4,
2, 3, 53, 2, 125, 23, 53, 523, 5, 235, 235, 235, 23, 523, 1,
2, 41, 2412, 412, 4, 124, 1, 241, 24, 1, 43, 6, 346, 457, 56,
74, 5, 3, 74, 5, 3, 5, 1, 33, 13, 21, 2, 12, 4,
2, 3, 53, 2, 125, 23, 53, 523, 5, 235, 235, 235, 23, 523, 1,
2, 41, 2412, 412, 4, 124, 1, 241, 24, 1, 43, 6, 346, 457, 56,
74, 5, 3 };
Basically, your bubble sort is broken in some way - using the implementation Darren Engwirda linked to in his comment at http://www.mycstutorials.com/articles/sorting/quicksort, and running the sorts with a newly created array each time, I get the following times:
bash-3.1$ time java -cp bin Sort none
real 0m0.079s
user 0m0.000s
sys 0m0.015s
bash-3.1$ time java -cp bin Sort quick
real 0m0.084s
user 0m0.015s
sys 0m0.015s
bash-3.1$ time java -cp bin Sort bubble
real 0m0.115s
user 0m0.000s
sys 0m0.000s
where the infrastructure to run the three tests is:
private static int[] data () {
return new int[] { ... the same data as in the OP ... };
}
public static void main(String[] args) {
if ( args.length == 0 || args [ 0 ].equals ( "bubble" ) ) {
for ( int i = 0; i < 1000; ++i ) {
int[] data = data();
bubbleSort ( data );
}
} else if ( args [ 0 ].equals ( "none" ) ) {
for ( int i = 0; i < 1000; ++i ) {
int[] data = data();
}
} else if ( args [ 0 ].equals ( "quick" ) ) {
for ( int i = 0; i < 1000; ++i ) {
int[] data = data();
quickSort( data, 0, data.length-1 );
}
} else if ( args [ 0 ].equals ( "quick_op" ) ) {
for ( int i = 0; i < 1000; ++i ) {
int[] data = data();
quickSort_op( data, 0, data.length-1 );
}
}
}
So a correctly implemented quick sort takes ~5μs to sort the data, and your bubble sort takes ~36μs to sort the data. The quick sort algorithm performs better than bubble sort for this data.
Moving the recursive call outside the loop means your code sorts the array ( though I haven't checked if there is any other flaw in it ), with the following result:
bash-3.1$ time java -cp bin Sort op_quick
real 0m0.108s
user 0m0.015s
sys 0m0.015s
Which is still faster than the bubble sort, but not as quick as the other implementation - I think you may be overlapping j and i and visiting parts of the array more than once.