Test if a number is within 2 of a multiple of 10

Question

I have solved this exercise, but it looks really redundant. Is there a built-in function in Java or a cleaner way to caluclate mod of a value + deviation without multiple OR statements? If I had to calculate this for a deviation of 5, this would then require a lot of repetition.

Given a non-negative number "num", return true if num is within 2 of a multiple of 10. Note: (a % b) is the remainder of dividing a by b, so (7 % 5) is 2.

    public boolean nearTen(int num) {
        return isMod10(num) || isMod10(num - 1) || isMod10(num + 1) || isMod10(num - 2) || isMod10(num + 2);
    }

    private boolean isMod10(int num) {
        return num % 10 == 0;
    }

Answer 1

You should read about congruent integers and modular arithmic. The distance of a number to a base is the minimum distance of its congruent value and its inverted congruent value.

 public boolean nearBase(int num, int base, int deviation) {

        // ..TODO check guards, normalize input or throw exceptions 
        // - base and deviation are expected strict positive integers
        // - deviation is expected smaller than base
        // - num is expected a positive integer

        var congruent = num % base;
        var inverted = base - congruent;
        var distance = Math.min(congruent, inverted);
        return distance <= deviation;
    }

 // your method rewritten ->
 public boolean nearTen(int num) {
        return nearBase(num, 10, 2);
    }

Answer 2

Let's consider the results of numbers around 10 as an example :

7 % 10 = 7
8 % 10 = 8
9 % 10 = 9
10 % 10 = 0
11 % 10 = 1
12 % 10 = 2
13 % 10 = 3

We know that 8 to 12 should be included.

For 10 to 12 it's easy, we just need to check if their modulo is smaller than 2.

public boolean nearTen(int num) {
    int modulo = num % 10;
    return modulo <= 2;
}

For 8 and 9, if we look at the above results, we can see that the difference between their modulo and the original number will give us a number under 2. So we'll add that to the algorithm:

public boolean nearTen(int num) {
    int modulo = num % 10;
    return modulo <= 2 || num - modulo <= 2;
}

With that, we covered all cases. When facing a problem like this one, don't be afraid to write down the results and try to find a match.