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# Decision 1: One Way System Nearest Neighbour watch

1. I don't understand how a one way system works from a matrix table? How do you do it? Thank you!

QP:
http://img443.imageshack.us/img443/7...6159cc31f4.png

MS:
http://img163.imageshack.us/img163/4...d869976093.png
2. (Original post by Konnichiwa)
I don't understand how a one way system works from a matrix table?

How do you do it? Thank you!
Best guess. If you have an entry in row i, column j, then you can go from i to j, but if there is no entry in row j, column i, then you can't go from j to i, directly.

If that doesn't cover it, can you elaborate, preferably with an example.
3. I just don't understand how one does this:
http://img443.imageshack.us/img443/7...6159cc31f4.png

Mark Scheme:
http://img163.imageshack.us/img163/4...d869976093.png

I know how to do the nearest neighbour algorithm, but the one way system just puzzles me.
4. (Original post by Konnichiwa)
I know how to do the nearest neighbour algorithm, but the one way system just puzzles me.
To go from B to A is 3.1 km and from A to B is 1.7 km.

Since the entire system is one way, if the one way system is reversed, then

To go from B to A is 1.7 km and from A to B is 3.1 km.

So, you can

either transpose the entire matrix,

or, where you were previously working with rows, now work with columns. Switch round the two labels in the top left: "FROM" and "TO"

Sorry, I hadn't noticed that you'd replied.
5. The mark scheme 5)b) shows the nearest neighbour algorithm but working with rows, how do I know to work in columns or rows?

And for 5)a), how do I know whether to get the length/weight from the columns or rows?

Is it always rows? Because if I do columns, I will get a completely different answer to the mark scheme. However, rows is correct.
6. (Original post by Konnichiwa)
The mark scheme 5)b) shows the nearest neighbour algorithm but working with rows, how do I know to work in columns or rows?

And for 5)a), how do I know whether to get the length/weight from the columns or rows?

Is it always rows? Because if I do columns, I will get a completely different answer to the mark scheme. However, rows is correct.
For parts a,b, it's rows, and for part d, it's columns.

It's a question of where you're going FROM and TO, as labelled on the table.

Because FROM in down the left hand side, you're working in rows.

It's only when the direction of the one-way system is reversed that FROM becomes the top row, and you're then working in columns.
7. Ahhh, thanks so much, I understand it now. Now I won't need to lose 10 marks.

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Updated: January 3, 2013
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