Revere elements

[Amit Chohan]

hi all
the following code reverses elements in an array. Is there a more efficient way of writing it.

public static void reverse(int[]a){
int right = a.length-1;
for(int i = 0; i<right; i++){
int old = a[right];
a[right] = a[i];
a[i]=old;
right--;
}
}  

thanks
Amit Chohan

[Maulin Vasavada]

hi amit,
it seems the method u use is fine in int[] case.
here is the code i tired,

class reverselist {
        public static void reverse1(int[] a) {
                int right = a.length-1;
                for(int i = 0; i<=right; i++){
                        int old = a[right];
                        a[right] = a[i];
                        a[i]=old;
                        right--;
                }
        }
        public static int[] reverse2(int[] a) {
                int[] b = new int[a.length];
                int j=0;
                for(int i=a.length-1; i>=0 ;i--)
                        b[j++] = a[i];
                return b;
        }
        public static void reverse3(int[] a) {
                int j=0;
                for(int i = a.length-1; i >=0 ; i--){
                        int old = a[i];
                        a[j] = a[i];
                        a[i]=old;
                        j++;
                }
        }
        public static void reverse4(int[] a) {
                int j=0;
                for(int i = a.length-1; i >=0 ; i--){
                        a[i] ^= a[j];
                        a[j] ^= a[i];
                        a[i] ^= a[j];
                        j++;
                }
        }
        public static void reverse5(int[] a) {
                int len = a.length-1;
                int j= len;
                for(int i = 0; i <= len ; i++){
                        a[i] ^= a[j];
                        a[j] ^= a[i];
                        a[i] ^= a[j];
                        j--;
                }
        }
        public static void main(String[] args) {
        
                int[] a = new int[Integer.parseInt(args[0])];
                int[] b= new int[a.length];
                
                for(int i=0; i < a.length; i++) {
                        a[i] = i;
                        b[i] = i;
                }
                long startTime = 0;
                long endTime = 0;
                
                long times[] = new long[5];
                int j=0;
                startTime = System.currentTimeMillis();
                reverse1(a);
                endTime = System.currentTimeMillis();
                //System.out.println("Total time in reverse1():"+(endTime-startTime));
                times[j++] = endTime-startTime;
                a = b;
                startTime = System.currentTimeMillis();
                reverse2(a);
                endTime = System.currentTimeMillis();
                //System.out.println("Total time in reverse2():"+(endTime-startTime));
                times[j++] = endTime-startTime;
                a = b;
                startTime = System.currentTimeMillis();
                reverse3(a);
                endTime = System.currentTimeMillis();
                //System.out.println("Total time in reverse3():"+(endTime-startTime));
                times[j++] = endTime-startTime;
                a = b;
                
                startTime = System.currentTimeMillis();
                reverse4(a);
                endTime = System.currentTimeMillis();
                //System.out.println("Total time in reverse4():"+(endTime-startTime));
                times[j++] = endTime-startTime;
                a = b;
                startTime = System.currentTimeMillis();
                reverse5(a);
                endTime = System.currentTimeMillis();
                //System.out.println("Total time in reverse5():"+(endTime-startTime));
                times[j++] = endTime-startTime;
                a = b;
                System.out.println(times[0]+"\t"+times[1]+"\t"+times[2]+"\t"+times[3]+"\t"+times[4]);
        }
}

and a script to generate multiple results to get average,

#!/bin/bash
i=10142952
echo "Starting reverse program..."
while [ $i -le 10143092 ]
do
        java -Xms50M -Xmx80M reverselist $i
        i=`expr $i + 1`
done
echo "DONE:reverse program."

the average was,
Reverse-1: 275.65
Reverse-2: 747.40
Reverse-3: 474.40
Reverse-4: 808.24
Reverse-5: 802.33

may be if we post this to Performance forum it would get more responses...
regards
maulin.

[Maulin Vasavada]

btw,
the average is in milliseconds to perform reverse operation...
thanks
maulin