CS 441/539 Frequently Asked Questions (FAQs)
The following list of FAQs is provided for your convenience. If your question isn't answered below, don't despair! We want to be sure that you get an answer! To seek out the answer you need, you can: And, of course, you can always ask questions in class. Don't be shy.
We want to help you!

Homework 4
I'm having trouble understanding the problem for #5. Could you clarify it?

Let me give an example. Suppose you have an shop that customizes automobiles that offers the following services (or operations): install alarm, install sunroof, install child safety seat, install sound system, replace tires, re-paint. You have one machine which does each of the operations but it can only handle one car at a time. The input would consist of a set of customers, each of whom brings in a car with a list of operations (i.e. services) it would like. So an example input would be: 
car 1: {re-paint, sunroof, sound system}
car 2: {sound system, replace tires, alarm, safety seat}
car 3: {sound system}
car 4: {safety seat, alarm}
For each car, you can do these operations in any order. But only one operation per car can be done at a time. So you could not re-paint car 1 at the same time a sound system is being installed. Also, remember that you could not be installing a sound system on car 2 and car 3 at the same time because they both require the same machine.

The goal is to schedule all the operations to finish up everything as soon as possible (so you can go home).

For problem 3, in the TSP definition are the weights restricted to be non-negative?

Yes. In TSP for all u,v, w(u,v) >= 0.

In Problem 7, are the jobs unit length?

No. The first paragraph should read: "Consider the following scheduling problem. You are given n jobs where job i is specified by an
earliest start time si and a processing time pi. In homework 1, problem 7 we consider a preemptive version of this problem and gave
a greedy algorithm to give an optimal preemptive schedule."

So the jobs are NOT unit length but rather job i has length pi.

Homework 3
What does "an integer n-vector x" mean in problem 4? Can you give an example of the integer-programming problem?

An integer n-vector x is simply integers x1, x2, ..., xn. An example of an integer programming (IP) input is to find integer values for x1, x2, and x3 to

minimize x1 + 3x2 - 2x3

Subject to: x1 - 2x2 + 10 x3 >= 20 and -4x1 + x2 <= 5

For the decision version of IP (which is what you are to look at in problem 4) the objective function does not matter.
The question is whether or not there is a solution x1, x2, x3 where each of these variables takes on an
integer value (i.e. an integer 3-vector) for which the two constraints are both satisfied.

For this example, the answer is "yes" since using x1 = 10, x2 = 0, x3 = 1 satisfies the constraints.

Homework 1
For problem number 2 does song 2 have to be on the first CD or can it start another CD?

Another way of viewing the problem is you need to decide how to partition the songs into the CDs. The solution you output must be of the form: 
CD 1: song 1, song 2, ..., song i
CD 2: song i, song i+1, ... song j
.
.
.
You can just decide which song is "song i", which is "song j" and so on.

So to specifically answer the question, if song 2 is the first song on CD 2 then song 1 must be the only song on CD 1.

For problem 7, can we assume that the start and processing times are integer? Does a job i have to start at time si or can it start later? Does a job have to be suspended?

Each job here is specified by a real-valued earliest start time si >=0 and real-valued processing time pi >0. A job can be processed beginning at si or any time after si. That is why it is called an earliest start time. A job can be suspended and resumed any number of times (including none at all). The sum of the times when job i is processed must add to pi.

Adminstrative

I wanted to register for CS 539T but WUCRSL said it does not exist. How can I correct this?

Send an email to Dr. Goldman (sg@cs.wustl.edu) indicating this and she will have you switched to CS 539T.

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