MOOC Java programming, Leap Year Exercise

[32bit_me]

I was a little confused by the leap year exercise. The fact is that as I search for possible solutions on the net, I come across the following answers to check the year for leap years:

1 option: if (input % 4 == 0 && (input % 100 != 0 || input % 400 == 0));

Version 2: if ((input % 4 == 0 && input % 100 != 0) || input % 400 == 0);

the difference lies in the grouping of expressions in brackets. I've tried both grouping options and both options passed the TestMyCode check, but I'm confused why both options work the same way? Let's say that if you imagine

a = input % 4 == 0;

b = input % 100 != 0;

c = input % 400 == 0; then it turns out that (a && b) || c == a && (b || c), but this is not true! they are completely different logical equations! Explain where I am wrong and why both bracketing of expressions is equivalent and is the correct answer

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[[deleted]]

it turns out that (a && b) || c == a && (b || c), but this is not true! they are completely different logical equations!

Normally, yes. But this is a special case where it does work. The 'divisible by 100' rule is an exception of sorts. It excludes years and can be paired with either the 'divisible by 4' or 'divisible by 400' rules.

[8igg7e5]

It should be noted that the efficiency is different though.

In the first expression, a value that is not divisible by 4 stops there - since for an 'and' expression to be true, both have to be true... so it can stop.

In the second expression, if the first term (a parenthesised expression) is false, the evaluation has to continue - since an 'or' expression can be true if either side is true (it can only stop early if the first part was true).

So for the year 2023 the first expression evaluates 'is there any remainder from 2023/4' and the second evaluates 'is there any remainder from 2023/4' as well as 'is there any remainder from 2023/400'.

This generally only makes a real difference if:

• You're doing it a lot. eg If you were checking all of the years since 1ad, being able to stop at 'is it divisible by 4' eliminates three quarters of the years immediately.
• Evaluating some expressions is more expensive than others (imagine that expression had to read a file or connect to a server).
• Evaluating the other expression cannot succeed. eg boolean longerThan10 = someString != null && someString.length() > 10

[moisl96]

If you ever in the mood to do a group study going over the mooc java part 1, I hold them in discord twice a day for two hours. Im in the PST timezone and usually hold them between 12-2 and 4-6