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Re^5: The error says the value is uninitialized, but it works anyway

by choroba (Archbishop)
on Aug 18, 2019 at 19:58 UTC ( #11104658=note: print w/replies, xml ) Need Help??


in reply to Re^4: The error says the value is uninitialized, but it works anyway
in thread The error says the value is uninitialized, but it works anyway

Note that each in scalar context returns the next key.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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Re^6: The error says the value is uninitialized, but it works anyway
by afoken (Canon) on Aug 18, 2019 at 22:08 UTC
    Note that each in scalar context returns the next key.

    One of the things I rarely use.

    each iterates over the keys of a hash, returning different values when called repeatedly.

    >perl -E '%h=map{ $_ => 1 } qw( a b c d e f ); while ($k=each %h) { sa +y $k }' a c d b f e >

    But the MUMPS $ORDER function is different. Keys in MUMPS are always sorted, unlike Perl's hash keys. And keys must not be empty. $ORDER returns the next key for the key you pass into. It does not iterate. Passing -1 as a second argument to $ORDER returns the previous key instead of the next one. If there is no previous / next key, an empty string is returned.

    KILL ^H SET ^H("D")=1 SET ^H("E")=1,^H("F")=1 S ^H("A")=1,^H("B")=1,^H("C")=1 ; ^-- ^H now contains the keys "A", "B", "C", "D", "E", "F" ; WRITE $ORDER(^H("")),! ; ^-- writes "A", the first key (in order) of ^H ; W $O(^H("C")),! ; ^-- writes "D", the next key after "C" in ^H ; W $O(^H("F")),! ; ^-- writes "", because "F" is the last key in ^H ; W $O(^H("D"),-1),! ; ^-- writes "C", the key before "D" in ^H ; W $O(^H("A"),-1),! ; ^-- writes "", because "A" is the first key in ^H ; SET K="" FOR SET K=$ORDER(^H(K)) QUIT:K="" DO .WRITE K WRITE ! ; ^-- writes "ABCDEF", all keys of ^H in order ; S K="" F S K=$O(^H(K,-1)) Q:K="" D .W K W ! ; ^-- writes "FEDCBA", all keys of ^H in reverse order (-1 passed as + second argument to $ORDER) ; S K="B" F S K=$O(^H(K)) Q:K="" Q:K="E" D .W K W ! ; ^-- writes "CD", all keys of ^H following "B" and stopping at the +end of keys or at "E", in order ; S K="B" F S K=$O(^H(K)) Q:K="" Q:K]]"E" D .W K W ! ; ^-- writes "CDE", all keys of ^H following "B" and stopping at the + end of keys or at the first key sorting after "E", in order

    Actually, $ORDER is a "new" (*cough*) function, it can handle negative and non-numeric keys. The original function used to iterate over keys was $NEXT, and it was intended to be used only with positive integers, i.e. plain arrays or sparse arrays. It returns -1 if no more keys can be found. Unfortunately, -1 is a legal key. So if $NEXT returns -1, you can not know if you found the last key or you just happen to have -1 as a key. $ORDER fixes that problem, and $NEXT is provided for legacy code from the dark ages.

    Alexander

    --
    Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

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