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I happen to like to write separate subroutines for sorting, especially if I am going to do several different sorts that are need to be identical ... in that case, it’s only one thing to maintain and nothing to keep in-sync.

Multi-field comparison functions are made very easy by the <=> operator, (edit, see below) and ‘cmp’, both of which (as per perldoc perlop), “returns -1, 0, or 1 depending on whether the left argument is numerically or string-wise less than, equal to, or greater than the right argument.”   Combine this either of these with ||, which “performs a short-circuit logical-OR operation.   (“Short-circuit,” in the case of logical-OR, mans that if the left-hand part is determined to be “True,” evaluation of the right-hand part is omitted since “True OR anything == True.”)   Since any non-zero value is True, a series of <=> comparisons can be chained using || to produce the intended result.   It is clear at a glance what the code is doing.

{ $$a{'last_name'} cmp $$b{'last_name'} || $$a{'first_name'} cmp $$b{'first_name'} // evaluated only if 'last_ +name's are equal }

Very Important Edit!   It has very graciously been pointed out to me that the <=> operator is numeric whereas the cmp operator is the string operator that, in the foregoing example, I of course should have used.   (The above example has been edited to include it.)   One of the serious “gotchas” of Perl, which I confess that I have never quite understood, is that there are two entirely-separate sets of comparison and relational operators:   one for strings, and another for non-strings.   The original code, which used the <=> operator, would have compiled and ran, but would not have produced the intended result.   Thank-you to whoever-it-was for pointing this error out to me.   It is a critical error indeed.   But, with the change as noted, the original premise once again holds.   (We can defer the question of “why does Perl do this” ... indefinitely.)


In reply to Re: Multiple Sort on selected column by sundialsvc4
in thread Multiple Sort on selected column by huchister

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