I'm doing an assignment for my Data Structures class and I can't figure out why my Insertion Sort is much faster than my merge sort. I have posted 2 version one with he fast insertion sort code and one with the slow version I discovered later. My professor was really surprised and said I understood it well. I believe its because I avoided a time consuming task, but I did this unintentionally and I'm still trying to learn time complexity so it's confusing me.
(Slow version)
package sorting;
import java.util.*;
public class Sort {
public static int[] insertion_sort (int[] array) {
int i;
for (int j = 1; j < array.length; j++) {
int key = array[j-1]; // assigns the key to the first value of the array
i = j-1; // i is always one less than j
while (i >0 && array[i] > key) {
array[i+1] = array[i];
i = i -1;
}
array[i+1] = key;
}
return array;
}
public static int[] merge_sort (int[] array, int p, int r) {
if (p < r) {
int q = (p + r) /2;
Sort.merge_sort(array, p, q);
Sort.merge_sort(array, q+1, r);
Sort.merge(array, p, q, r);
}
return array;
}
public static int[] merge (int[] array, int p, int q, int r) {
int n1 = q - p +1;
int n2 = r - q;
int[] L = new int[n1+1];
int[] R = new int [n2+1];
for (int i = 0; i < n1; i++) {
L[i] = array[p+i];
}
for (int j = 0; j <n2; j++) {
R[j] = array[q+j+1];
}
L[n1] = Integer.MAX_VALUE;
R[n2] = Integer.MAX_VALUE;
int i = 0;
int j = 0;
for (int k = p; k<= r; k++) {
if (L[i] <= R[j]) {
array[k] = L[i];
i++;
}
else {
array[k] = R[j];
}
}
return array;
}
/*
* n: the size of the output array
* k: the maximum value in the array
*/
public static int[] generate_random_array (int n, int k) {
List<Integer> list;
int[] array;
Random rnd;
rnd = new Random(System.currentTimeMillis());
list = new ArrayList<Integer> ();
for (int i = 1; i <= n; i++)
list.add(new Integer(rnd.nextInt(k+1)));
Collections.shuffle(list, rnd);
array = new int[n];
for (int i = 0; i < n; i++)
array[i] = list.get(i).intValue();
return array;
}
/*
* n: the size of the output array
*/
public static int[] generate_random_array (int n) {
List<Integer> list;
int[] array;
list = new ArrayList<Integer> ();
for (int i = 1; i <= n; i++)
list.add(new Integer(i));
Collections.shuffle(list, new Random(System.currentTimeMillis()));
array = new int[n];
for (int i = 0; i < n; i++)
array[i] = list.get(i).intValue();
return array;
}
/*
* Input: an integer array
* Output: true if the array is acsendingly sorted, otherwise return false
*/
public static boolean check_sorted (int[] array) {
for (int i = 1; i < array.length; i++) {
if (array[i-1] > array[i])
return false;
}
return true;
}
public static void print_array (int[] array) {
for (int i = 0; i < array.length; i++)
System.out.print(array[i] + ", ");
System.out.println();
}
public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println("Insertion sort starts ------------------");
for (int n = 100000; n <= 1000000; n=n+100000) {
int[] array = Sort.generate_random_array(n);
long t1 = System.currentTimeMillis();
array = Sort.insertion_sort(array);
long t2 = System.currentTimeMillis();
long t = t2 - t1;
boolean flag = Sort.check_sorted(array);
System.out.println(n + "," + t + "," + flag);
}
System.out.println("Insertion sort ends ------------------");
System.out.println("Merge sort starts ------------------");
for (int n = 100000; n <= 1000000; n=n+100000) {
int[] array = Sort.generate_random_array(n);
//Sort.print_array(array);
long t1 = System.currentTimeMillis();
array = Sort.merge_sort(array, 0, n-1);
long t2 = System.currentTimeMillis();
long t = t2 - t1;
//Sort.print_array(array);
boolean flag = Sort.check_sorted(array);
System.out.println(n + "," + t + "," + flag);
}
System.out.println("Merge sort ends ------------------");
}
}
(Slow version)
import java.util.*;
public class Sort {
public static int[] insertion_sort (int[] array) {
int i;
for (int j = 1; j < array.length; j++) {
int key = array[j]; // assigns the key to the first value of the array
i = j-1; // i is always one less than j
while (i >=0 && array[i] > key) {
array[i+1] = array[i];
i = i -1;
}
array[i+1] = key;
}
return array;
}
public static int[] merge_sort (int[] array, int p, int r) {
if (p < r) {
int q = (p + r) /2;
Sort.merge_sort(array, p, q);
Sort.merge_sort(array, q+1, r);
Sort.merge(array, p, q, r);
}
return array;
}
public static int[] merge (int[] array, int p, int q, int r) {
int n1 = q - p +1;
int n2 = r - q;
int[] L = new int[n1+1];
int[] R = new int [n2+1];
for (int i = 0; i < n1; i++) {
L[i] = array[p+i];
}
for (int j = 0; j <n2; j++) {
R[j] = array[q+j+1];
}
L[n1] = Integer.MAX_VALUE;
R[n2] = Integer.MAX_VALUE;
int i = 0;
int j = 0;
for (int k = p; k<= r; k++) {
if (L[i] <= R[j]) {
array[k] = L[i];
i++;
}
else {
array[k] = R[j];
}
}
return array;
}
/*
* n: the size of the output array
* k: the maximum value in the array
*/
public static int[] generate_random_array (int n, int k) {
List<Integer> list;
int[] array;
Random rnd;
rnd = new Random(System.currentTimeMillis());
list = new ArrayList<Integer> ();
for (int i = 1; i <= n; i++)
list.add(new Integer(rnd.nextInt(k+1)));
Collections.shuffle(list, rnd);
array = new int[n];
for (int i = 0; i < n; i++)
array[i] = list.get(i).intValue();
return array;
}
/*
* n: the size of the output array
*/
public static int[] generate_random_array (int n) {
List<Integer> list;
int[] array;
list = new ArrayList<Integer> ();
for (int i = 1; i <= n; i++)
list.add(new Integer(i));
Collections.shuffle(list, new Random(System.currentTimeMillis()));
array = new int[n];
for (int i = 0; i < n; i++)
array[i] = list.get(i).intValue();
return array;
}
/*
* Input: an integer array
* Output: true if the array is acsendingly sorted, otherwise return false
*/
public static boolean check_sorted (int[] array) {
for (int i = 1; i < array.length; i++) {
if (array[i-1] > array[i])
return false;
}
return true;
}
public static void print_array (int[] array) {
for (int i = 0; i < array.length; i++)
System.out.print(array[i] + ", ");
System.out.println();
}
public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println("Insertion sort starts ------------------");
for (int n = 100000; n <= 1000000; n=n+100000) {
int[] array = Sort.generate_random_array(n);
long t1 = System.currentTimeMillis();
array = Sort.insertion_sort(array);
long t2 = System.currentTimeMillis();
long t = t2 - t1;
boolean flag = Sort.check_sorted(array);
System.out.println(n + "," + t + "," + flag);
}
System.out.println("Insertion sort ends ------------------");
System.out.println("Merge sort starts ------------------");
for (int n = 100000; n <= 1000000; n=n+100000) {
int[] array = Sort.generate_random_array(n);
//Sort.print_array(array);
long t1 = System.currentTimeMillis();
array = Sort.merge_sort(array, 0, n-1);
long t2 = System.currentTimeMillis();
long t = t2 - t1;
//Sort.print_array(array);
boolean flag = Sort.check_sorted(array);
System.out.println(n + "," + t + "," + flag);
}
System.out.println("Merge sort ends ------------------");
}
}
