You're right ysth, it does seem to work. :)

Advice to the OP: It is a good idea for this type of problem to devise your own test suite before you start coding a solution. To demonstrate, here is a test script that I created for ysth's solution. It's not exactly comprehensive, but I attempt to take care of the most obvious of boundary conditions.

The way it's coded, it would be easy for you to come up with a list of your own conditions that you want to test for. Or maybe rules that you would like to change. Such as is a point of inflection truly a local min or max? Your test suite can spell out what you expect or desire. 
use Test::More;

use strict;

#################################
# Test Data

# Each element contains [ [Test Data], [Expected Min], [Expected Max] 
+]
# Note: The reverse of any data, should simply reverse the expected re
+sults.
my @testdata = (
    [# Empty
        [], [], [],
        ],
    [ # One Entry
        [3], [3], [3],
        ],
    [ # Two (Up slope)
        [2, 4], [2], [4],
        ],
    [ # Two (No slope)
        [3, 3], [3], [3],
        ],
    [ # Three (No slope)
        [4, 4, 4], [4], [4],
        ],
    [ # Three (Up slope, beginning point of inflection)
        [4, 4, 6], [4], [6],
        ],
    [ # Three (Up slope, ending point of inflection)
        [4, 6, 6], [4], [6],
        ],
    [ # Three (negative inflection)
        [3,6,2], [3, 2], [6],
        ],
    [ # Three (positive infection)
        [10, 5, 10], [5], [10, 10],
        ],
    [ # Mixed Data
        #    mi      Mi   m         i     M   i   m      Mi
        [qw(1 1 2 3 4 4 3 2 3 4 5 6 6 6 7 8 7 7 7 3 5 6 9 9)],
        [qw(1 2 3)],
        [qw(4 8 9)],
        ],
);

# One test each for min and max, and then times two for reverse.
plan tests => 2 * (2 * @testdata);


#################################
# Tests

foreach (@testdata) {
    my ($data, $min, $max) = @$_;

    my ($rmin, $rmax) = local_min_max(@$data);
    is_deeply($rmin, $min, "min of [@$data]");
    is_deeply($rmax, $max, "max of [@$data]");

    my @reversed = reverse @$data;
    ($rmin, $rmax) = local_min_max(@reversed);
    is_deeply($rmin, [reverse @$min], "min of [@reversed]");
    is_deeply($rmax, [reverse @$max], "max of [@reversed]");
}

    
#################################
# Functions

sub local_min_max {
    # Boundary Conditions
    return ([], []) if @_ == 0;
    return ([@_], [@_]) if @_ == 1;
    
    my @array = @_;
    
    my @minima;
    my @maxima;
    my $prev_cmp = 0;

    for (0 .. $#array - 1) {
        my $cmp = $array[$_] <=> $array[$_+1];
        if ($cmp && $cmp != $prev_cmp) {
            push @minima, $array[$_] if $cmp < 0;
            push @maxima, $array[$_] if $cmp > 0;
            $prev_cmp = $cmp;
        }
    }
    
    push @minima, $array[-1] if $prev_cmp >= 0;
    push @maxima, $array[-1] if $prev_cmp <= 0;
    
    return (\@minima, \@maxima);
}

[download]
Ok, now back to less amusing problems.

- Miller

In reply to Re^2: finding local minima/maxima for a discrete array by wind
in thread finding local minima/maxima for a discrete array by Anonymous Monk

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