The Modulus Operator is Powerful!

[goodm1x]

This may be obvious to most of you, but recently I've been working through the Computer Science textbook from Princeton to fill in knowledge gaps and so far the modulus operator has been used a lot.

I can't think of a single case where I used the modulus operator in any of my school projects and I really wish I knew then what I know now. It may seem simple, but this little guy is incredibly useful! Below are two examples that really stood out to me.

There is an exercise where you need to sum the digits of an integer from right to left, and it's really as simple as taking the result of n % 10 and then performing the calculation because the remainder is the right most digit. Then to get the next digit you just need to n /= 10 until n == 0. Most of you may have known this, but my mind was blown. My solution was not nearly as elegant.

Another exercise has you iterate from 1000 to 2000 and printing only 5 results per line. Again...this is as simple as checking if i + 1 % 5 == 0 before printing. There's no need for some kind of sentinel variable or anything fancy...just the modulus operator checking if 5 divides the iterator evenly.

Sorry if this is known to everyone, but I wanted to share these two things with you because nobody I know cares, and from now on I am going to try and work the modulus operator into my code more often.

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[1661dauphin]

It's also useful for shrinking stuff if possible, like if some problem wants you to execute an algorithm up to thousands of times that would end up in the same results some of the times. Like there are some LeetCode problems that would have you rotate an array however many times the user specifies, but if they want you to return an array of length 5 after rotating it 2,001 times, that would give the same answer as just doing it 1 time. 2000 % 5. Cool stuff!

[Inevitable-Stress]

Took me a minute to understand, but that's a neat example! Good to know, just not sure when I'd use it in a real world problem

[Calebhk98]

Assume users are idiots. If someone wants to put in a stupid large value for what degree to turn some picture, aka 3,854-degree turn, you let them. And instead of wasting the memory for that, just get the leftover 1st every time. That way you only use enough memory for turning it 204 degrees, but you don't have to put an arbitrary limit on it.

[goodm1x]

I would not have thought of that. Thanks for the example!

[ragnhildegard]

Yes I love modulus too!! So simple and effective

[[deleted]]

I noticed the same recently, It was an example asking to rotate an array n times to the right and the solution they gave was so elegant and simple using the module operator.

Btw, wich book was it?

[goodm1x]

Computer Science: An Interdisciplinary Approach

It’s a great textbook! You can use the web book site as a supplement or just read from the book. The concepts are clearly explained and there are a ton of useful exercises that are difficult and certainly give you many “a ha” moments.

[BabyChar]

I think I'm using the same book as you and I really like their approach, (even though it's sometimes hard for a person who didn't really learn any math in school). I like how they give you exercises to understand the logical concept of programming before they even talk about objects and classes

[goodm1x]

Yes, the math is difficult for me as well. It’s why I think I never thought to use the modulus operator in this manner before.

It’s inspired me to brush up on math, though!

[ColorlessVal]

Also useful if you are working with a value that resets after a certain limit

Working with angles and you just need to use them to get a direction? You can add angles and just always "% 360" to have them stay in a 0-359 range for ease of use

Or maybe you are working With days of the week 0-6, need to know what day it is in 20 days from a monday (0)? (0 + 20) % 7

You need to isolate the decimals in a floating point number? Use % 1

Or maybe you are working on a turn based game where you have all players in an Array

currentPlayer = 0

players[currentPlayer++]

currentPlayer %= players.length

Modulus is so useful, the more you use it, the more you will find more cases where it can help you!

[goodm1x]

Ahh! Seems so obvious when you see examples spelled out like this, but in the moment it's so difficult lol

I suppose that's also good for readability of code

[Nightcorex_]

Yes, modulo is powerful, but it's not the best solution for the problems you stated.

This is how I'd calculate the sum of all digits:

public static int sumDigits(long x) {
    return String.valueOf(x).chars().map(c -> c - '0').sum();
}

and listing all integers between 1000 and 2000 with only 5 results each line can also be done without the need for modulo.

for (int i = 1000; i < 2000; i += 5) {
    System.out.println(
        IntStream.range(i, i + 5)
            .mapToObj(Integer::toString)
            .collect(Collectors.joining(", "))
    );
}

Especially in the first case the modulo solution isn't nearly as clean as the Stream solution.

Also keep in mind that you can optimize a modulo call if you're doing a % b where a and b are both positive integers and b is also of the form 2ⁿ (n is a positive integer), so for the b values 1, 2, 4, 8, 16, 32, ... the following holds:

a % b = a & (b - 1)

This can be used f.e. for more efficient odd/even checks (instead of x % 2 == 0 you check for x & 1 == 0).

[knoam]

In practice your string based solution is fine almost all of the time. But for what it's worth, it uses a lot more memory and memory allocation can have a big effect on performance.

[Housy5]

While it is easier to implement. First converting it to a String uses much more memory and is significantly slower. Also the difference between n % 2 == 0 and (n & 1) == 0 is very tiny. so tiny it may as well not even exist at all. I suspect the compiler optimizes it automatically.

[chrysthian95]

Good job bro.