Java Notes

Reverse an array

This reverse method uses two subscripts: one that starts at the left (beginning, top) of the array, and one that starts at the right (end, bottom) of the array.

//========================================================= reverse1
public static void reverse1(int[] b) {
    int left  = 0;          // index of leftmost element
    int right = b.length-1; // index of rightmost element
    
    while (left < right) {
        //... Exchange the left and right elements
        int temp = b[left]; 
        b[left]  = b[right]; 
        b[right] = temp;
        
        //... Move the bounds toward the center
        left++;
        right--;
    }
}

A for loop managing two indexes is shown below. Both loops are the same speed, so the choice should be for the one which is more readable to you. I personally find for loops with several variables difficult to read, so usually avoid them.

//========================================================= reverse2
public static void reverse2(int[] b) {
	for (int left=0, right=b.length-1; left < right; left++, right--) {
		//... Exchange the left and right elements
		int temp = b[left]; 
		b[left]  = b[right]; 
		b[right] = temp;
	}
}