Skipping numbers in a for loop in Java. How can I make this more clean?

Question

I need to create a for loop starting at 305 and ending at 397.

for (int i = 305; i < 397; i++) {
    //do stuff
}

The problem is, I need to skip 310, 321, 332, 343, 354, 365, 376, and 387.

I know I can do this by placing the following in the loop:

int[] skip = { 310, 321, 332, 343, 354, 365, 376, 387 };
for (int i2 : skip) {
    if (i != i2) {
        //do stuff
    }
}

But I feel like there is a much more efficient, clean way to achieve this.

Solution 1

You can also simply add if (i % 11 == 2) continue; at the start of the loop:

for (int i = 305; i < 397; i++) {
  if (i % 11 == 2) continue;
  //do stuff
}

This is because you want to skip a number every 11 steps, starting at 310. So, you want to skip numbers for which i - 310 is a multiple of 11, so for which i - 310 + 11*X is a multiple of 11 whatever X is. So, I've chosen 28 for X, because i - 310 + 11 * 28 equals i - 2. This is easier to read. Finally, the test is simply (i - 2) % 11 == 0, and is equivalent to i % 11 == 2. This method of calculus is named basic modular arithmetic.

Solution 2

If you're on java 8, You can use the new streams api with IntStream. You should also use a Set to check for membership quickly. Sets.newHashSet is a helper from guava, but you can also just create a HashSet by hand.

Set<int> intsToSkip = Sets.newHashSet( 310, 321, 332, 343, 354, 365, 376, 387 );
IntStream.range(305, 397)
    .filter(i => ! intsToSkip.contains(i))
    .forEach(i => {
        // do stuff
    })

Solution 3

The Java8 solution could look like this:

List<Intger> valuesToProcess = IntStream.range(305,397).collect(Collectors.toList());
valuesToProcess.removeAll(Arrays.asList(310, 321, 332, 343, 354, 365, 376, 387 ));

for(Integer value : valuesToProcess) {
  // do stuff here
}

Solution 4

The numbers to be skipped are those with i % 11 == 2.

Using this, the Java 8 way is:

 IntStream.range(305, 397)
     .filter(i -> i % 11 != 2)
     .forEach(i -> {
         // do stuff
     });

And for older Java versions do:

for (int i = 305; i < 397; i++) {
    if (i % 11 != 2) {
        // do stuff
    }
}

Solution 5

There seems to be a pattern of the numbers you need to ignore.

When the 2nd digit is exactly one higher than the 3rd digit, you ignore the number.

for (int i = 305; i < 397; i++) {
    String s = String.valueOf(i);
    if (s.charAt(1) == s.charAt(2) + 1) {
        continue; // ignore
    }
    //do stuff
}

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