Java For Loop Confusion (iteration) "Don't know why it still went through!"

Question

So I was trying to figure out if the the first iteration in a for loop does not go through the termination condition.

Since when I called it in the main method with an input of 4 IsPrime(4) the for loop still went through. I was expecting it to not go through since i = 2 and n/2 = 4/2 = 2 which will be 2 == 2 which will meet the condition to terminate but it went through and I got the right output but I don't get why it did not fail.

Please help.

public static boolean isPrime(int n){

    if(n <= 1){
        return false;
    }

    for(int i = 2; i <= n/2; i++){
        if(n % i == 0){
            return false;
        }
    }
    return true;
}

`for (int i = 2; i < n/2; i++) { ... }` says start i at 2 and loop while i is less than or equal to n/2. This means the condition evaluates to false (hence terminating the loop) when i is *greater* than n/2. This happens first when i is 3. — Zachary, Feb 08 '18 at 04:00

Matt · Answer 1 · 2018-02-08T04:47:30.533

2

Your for loop condition says continue while i <= n/2.

If you start with n == 4 then n/2 == 2. On the first iteration i == 2 so the for loop condition 2 <= 2 is true and it will iterate once.

Then i++ is executed so now i == 3 and fails the condition so there is no second iteration.

edited Feb 08 '18 at 04:47

answered Feb 08 '18 at 03:59

Matt

3,677
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Thanks I figured it out. – Hartej Lehal Feb 08 '18 at 04:13

score 0 · Answer 2 · edited Feb 08 '18 at 06:11

0

You seem to have a confusion around how for loop works. Let's see the operation on a per-iteration basis :

Iteration 1 :

i = 2
i <= 2 ? --> True
n % i == 0 ? --> 2 % 2 == 0? True
return false
End of function

Now let's assume your for loop looked something like this :

 for(int i = 2; i <= n/2; i++){
        if(n % i == 0){
            System.out.println("Remainder is 0");
        }
    }

Then the following will the iteration sequence :

Iteration 1

i = 2
i <= 2 ? --> True
n % i == 0 ? --> 2 % 2 == 0? True
print --> Remainder is 0
i++ --> i = 3 now

Iteration 2

i = 3
i <= 3 ? --> False
Cannot proceed further. For loop condition failed.
End of function

edited Feb 08 '18 at 06:11

Sai Kishore

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answered Feb 08 '18 at 04:12

LeoNeo

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Yea I was thinking of the opposite. I was thinking that when the condition was met it will stop the loop. I just realized it was the other way around thanks for the detailed explanation – Hartej Lehal Feb 08 '18 at 04:20
glad it helped. It is confusing sometimes :) – LeoNeo Feb 08 '18 at 13:30

Java For Loop Confusion (iteration) "Don't know why it still went through!"

2 Answers2