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D2 6th June 2013

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If we get given a dynamic programming problem from a table, will they always hint as to what the state etc is? If not how do we work it out?
Original post by Knoyle quiah
What do u mean 2 tables per iteration? so for example, 2 tables of the exact same iteration (theoretically same values) to double check values??


What I do (which is what I think Arsey also meant when he said two tables) is first you divide the pivot row, and then rewrite the table in the same way as the old one, just with the new pivot row. Then you do the other row operations. This way you don't have to look between two different tables and so you are less likely to make mistakes. If you have the textbook, I think it's how they do the first few examples, and then they show you how to do it with one table per iteration.
Original post by brittanna
What I do (which is what I think Arsey also meant when he said two tables) is first you divide the pivot row, and then rewrite the table in the same way as the old one, just with the new pivot row. Then you do the other row operations. This way you don't have to look between two different tables and so you are less likely to make mistakes. If you have the textbook, I think it's how they do the first few examples, and then they show you how to do it with one table per iteration.


i have been doing it from 2 tables all this time :angry::angry: i havnt looked in the text book for over a month but i remember seeing what u said in the text book, this sounds like a much less complicated idea, doing from 2 tables kills me every time
Original post by Knoyle quiah
i have been doing it from 2 tables all this time :angry::angry: i havnt looked in the text book for over a month but i remember seeing what u said in the text book, this sounds like a much less complicated idea, doing from 2 tables kills me every time


I find that using two tables helps me, but if you find it easier with one table, then I would use that :tongue:.
Am I the only one who HATES tables of least distances?

I always get paranoid that there's a quicker route...
Guess who just got their first ever 100% in a decision maths paper :cool:.
Original post by Hamburglar
Am I the only one who HATES tables of least distances?

I always get paranoid that there's a quicker route...


mhmm, only problem with me is, my paranoia is correct, because I ALWAYS miss one :mad:
Reply 227
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smith50
6
Original post by Knoyle quiah
yeah correct :smile: mark scheme wrong


That's another markscheme wrong :biggrin:
Smith
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smith50
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Original post by brittanna
Guess who just got their first ever 100% in a decision maths paper :cool:.


Well done :smile: which paper may i ask
Smith
Original post by smith50
Well done :smile: which paper may i ask
Smith


Thanks, now I just need to do that in the real thing :tongue:. It was the June 2011 Paper.
Reply 230
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smith50
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Original post by brittanna
Thanks, now I just need to do that in the real thing :tongue:. It was the June 2011 Paper.


I reckon for the flows question we may be have to use super sources/sinks never come up so we may be the first
Smith
Reply 231
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Lilmzbest
Original post by brittanna
What I do (which is what I think Arsey also meant when he said two tables) is first you divide the pivot row, and then rewrite the table in the same way as the old one, just with the new pivot row. Then you do the other row operations. This way you don't have to look between two different tables and so you are less likely to make mistakes. If you have the textbook, I think it's how they do the first few examples, and then they show you how to do it with one table per iteration.


Yeah i always get confused because of all the comparing tables, but i just put pencils under the 2 rows i'm using so i can focus on them, and then go back on my calculator to check i've used all the right values :smile:
Reply 232
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Mallika
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Original post by Hamburglar
Classical = Visiting each vertex only once. Practical = Visiting each vertex at least once. If you have a classical problem, then you also have a practical problem.

The reason it gets converted to a classical problem is because the algorithms used visit each vertex only once. Nearest neighbour starts at a vertex, visits every other vertex once only and returns to the start.

However, finding a lower bound by using RMST + 2 smallest arcs doesn't give you a classical problem - that is why they say it is not a viable solution. This is also why when dealing with inequalities, it is strict with respect to the lower bound, but inclusive of the upper bound.

Hope this helps :smile:


Thank you! Just one thing though- isn't it a non-viable solution because normally it doesn't give a tour? And if it does then it's an optimal solution? ( I though that was why it had a strict inequality for the lower bound)
Original post by Mallika
Thank you! Just one thing though- isn't it a non-viable solution because normally it doesn't give a tour? And if it does then it's an optimal solution? ( I though that was why it had a strict inequality for the lower bound)


Yeah, that's exactly it. If it does give a tour, then the lower bound is viable, and since it's therefore the lowest possible value, it is the optimal solution.
Hi, Can anyone tell me why they have chosen to use r as their pivot ROW, I always divide the value by the column numbers and use the smallest one but for this problem they would all be equal?!
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Original post by ChelseaSam
Hi, Can anyone tell me why they have chosen to use r as their pivot ROW, I always divide the value by the column numbers and use the smallest one but for this problem they would all be equal?!


In that case you can choose any of the rows. It makes no difference.
Also, Can anyone tell me if doing row operations like this would be credited all the marks? The answer is correct.
2013-06-04 10.01.32[1].jpg74.2KB
Original post by knowledgecorruptz
In that case you can choose any of the rows. It makes no difference.

I thought that was the case but wanted to check. Thanks :biggrin:
Original post by Mallika
Thank you! Just one thing though- isn't it a non-viable solution because normally it doesn't give a tour? And if it does then it's an optimal solution? ( I though that was why it had a strict inequality for the lower bound)


That's alright. :smile: And yeah you're correct, which is why normally it would be a loose inequality with respect to the lower bound.

If it does give a tour then you have an optimal solution. The thing is, they'd never ask you to write this as an inequality in the exam then, because you have actually found the optimal solution.

But yes if you were to write it as an inequality, then it would be strict with respect to the lower bound if you found a tour. Like I said though, writing it as an inequality makes no sense because you don't need a range of values, you have the optimal solution. (Very rare) :smile:
Reply 239
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Mallika
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Original post by Hamburglar
That's alright. :smile: And yeah you're correct, which is why normally it would be a loose inequality with respect to the lower bound.

If it does give a tour then you have an optimal solution. The thing is, they'd never ask you to write this as an inequality in the exam then, because you have actually found the optimal solution.

But yes if you were to write it as an inequality, then it would be strict with respect to the lower bound if you found a tour. Like I said though, writing it as an inequality makes no sense because you don't need a range of values, you have the optimal solution. (Very rare) :smile:


Thank you again, I think I understand :P
Another question if you don't mind :colondollar:

I was doing practice paper B and saw the LP question was unbalanced- this means if you were asked to solve it using simplex you would need to add slack variables right?
If the supply=demand and the question was balanced, would you still use inequalities or use an equals sign? In the textbook they use inequality signs (so in theory you would need to add slack variables if you were to solve it) but wouldn't they all be 0?

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