The landscape of the neighborhood of a single random point is
represented in Figure 2.1. The value of the
objective function was 520 million, achieving only 37% satisfaction,
leaving 803 out of 1267 students nowhere they wanted to be, and only
202 students receiving a perfect score. The histogram shows the nearly
symmetric distribution of the cost changes of this neighborhood. The
significantly better moves are below the cloud line (negative change
in cost), while those above the cloud line are worse.
In Figure 2.2 we isolate the most senior student, who
we'll call `' (at position 10), a resident for eleven consecutive
semesters, and the least senior student, `
' (at position
1267), a first time resident, at the single random data point to see
how they would fare in any other available bed. As it happens,
objects to smoke and requested the apartment-style dorm for all three
choices, with one roommate request, and a special request to be in the
graduate area;
is a smoker with no hall preferences or
roommate requests. They were both randomly placed in virtual beds.
The best place for is the apartment-style dorm (near bed 700)
which matches the special living option he requested and switches with
someone who has no desire to be there.
improves her score
pretty much anywhere we place her; the worst places for her are
non-smoking apartment-style rooms with the non-smoking special living
option (near bed 1000).
Using the same constraint values and data as in the random point, after six rounds taking approximately two hours, the global score declined to 45 million, achieving 97% satisfaction with only 32 out of 1267 students nowhere they wanted to be, and 697 students receiving a perfect score. The result of this complete run, as we can see in Figure 2.3, is a local minimum -- there are no better single moves (below zero) for any student.
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Two slices of the neighborhood landscape for our students, and
, at the optimized point can be seen in Figure
2.4.
got his first hall choice in the graduate
area, but didn't get his roommate request because it was not a
reciprocal request and the special grad area wasn't mutually
specified.
, who was so easy to please, got a perfect
score. Her set of possible alternative moves exhibits the end of a
phenomenon that looks like a `hole' in the cloud.
Figure 2.5 pinpoints the start of this obvious `hole'
between the last second semester resident, (at position 718),
and the first new student (at position 721)
.
got her
second hall choice, but not her requested roommate, a more senior
resident who wanted and got into the apartment-style dorm.
,
assigned to his first hall choice with the scholar's wing special
option and no requested roommates, has a perfect score. Attempting to
move him into any of the coveted apartment-style dorms (beds 673
through 1161) severely hurts his grade with increased costs -- this
begins the vacant region.
The Objective Function has terms that include dependencies between students, not just students and beds independently. As we've
seen with 's assignment, a student requests other students as
roommates, and depending upon the locations of the requested roommates
there may be a `mismatch' or `error'.