'Logic of for loop using with array to access elements of the array in java

My code to access elements of array using for loop. The output of the program is [19,17,15] which are the elements of array int a[] = { 12, 15, 16, 17, 19, 23 }. Output after following code is written:

public class Tester {
    public static void main(String s[]) {
        int a[] = { 12, 15, 16, 17, 19, 23 };
        for (int i = a.length - 1; i > 0; i--) {
            if (i % 3 != 0) {
                --i;
            }
            System.out.println(a[i]);
        }
    }
}

Algorithm:

  1. Iteration 1: i=5 so a[5]=23. "if statement" gets true and --i execute. so i=4. Hence, a[4]=19 will get print as first element in output.

  2. Iteration 2: i=3 so a[3]=17. "if statement" gets true again and --i should execute but it skips that condition as I tried using debugging tool to see how it is working. And output is a[3]=17 which is wrong I think.

Could you help me in understanding why it gives 17 as output? As in my opinion it should skip that part.

javaarraysdebuggingdata-structures


Solution 1:[1]

Here is a step by step explanation. (i % 3 != 0) checks to see if i is not divisible by 3. Also note that in this context, your post and pre-decrements of i are not relevant as the outcome would be the same no matter how they are decremented.

 i = 5;
 i not divisible by 3 so i-- = 4
 print a[4] = 19
 i-- at end of loop = 3
 i is divisible by 3 so i remains 3
 print a[3] = 17
 i-- at end of loop = 2
 i not divisible by 3 so i-- = 1
 print a[1] = 15
 i-- = 0 so loop terminates.

Solution 2:[2]

In iteration 2, where i=3 the condition is false, since 3 % 3 = 0

Sources

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Source: Stack Overflow

Solution Source
Solution 1
Solution 2 Boaz Lev

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