Make two separate character iterators so that scanning uppercase letters does not consume the lowercase pass:Each filtering iterator preserves the original order of its selected characters. Concatenation therefore yields all uppercase letters in source order, then all lowercase letters in source order, omitting other characters. The two
def upperLower(s: String): Iterator[Char] = {
val upper = new FilterIterator[Char]((c: Char) => c.isUpper, s.iterator)
val lower = new FilterIterator[Char]((c: Char) => c.isLower, s.iterator)
new ConcatIterator[Char](upper, lower)
}s.iterator calls are essential: reusing a single consuming iterator would let the first pass discard lowercase characters before the second pass could return them. Construction remains lazy; an empty string yields an exhausted iterator without a special case. Articles by others on the same topic
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