We often need to go through elements in a collection, but only operating on elements that fulfill a certain condition. E.g. only if the number is odd:

for i in 1...10 {
    if i % 2 == 0 {
        print(i)
    }
}

Here's another way to do it, using filter():

for i in (1...10).filter({$0 % 2 == 0}) {
    print(i)
}

But this is probably the more idiomatic way to do it in Swift:

for i in 1...10 where i % 2 == 0 {
    print(i)
}

Note that you can use where clauses in a similar manner in other places, such as in switch-case statements.

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