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# D1 (Decision 1) 17 May 2013 Official Thread Watch

1. (Original post by Dilzo999)
I got 2.6989898 something not 2.01. THIS IS CONFUSING XD.
No I mean in general , if you had 2.01 would you round to 3 or 2
2. (Original post by otrivine)
No I mean in general , if you had 2.01 would you round to 3 or 2
It's always rounding up in D1
3. (Original post by Fortitude)
It's always rounding up in D1
thanks, so you round up in everything! right

Define: Digraph

4. (Original post by otrivine)
thanks, so you round up in everything! right

Define: Digraph

Ok digraph = graph in which its edges have a direction associated with them (directed edges)

For you, starting off nice ...Degree/Valency = ...
5. (Original post by Fortitude)
Ok digraph = graph in which its edges have a direction associated with them (directed edges)

For you, starting off nice ...Degree/Valency = ...

yes exactly right

degree/valency of a vertex is the number of arcs(edges)incident to it.
6. (Original post by otrivine)
yes exactly right

degree/valency of a vertex is the number of arcs(edges)incident to it.
That is correct
7. (Original post by Fortitude)
That is correct

Define:loop
8. (Original post by otrivine)

Define:loop
We need to know that?!
9. (Original post by Fortitude)
We need to know that?!

Well I guess, cause it was in the definition?

Loop is an edge where it starts and finishes at the same vertex/
10. (Original post by otrivine)
Well I guess, cause it was in the definition?

Loop is an edge where it starts and finishes at the same vertex/
So that was in the textbook? I just learnt the ones which were given in the spec
11. (Original post by Fortitude)
So that was in the textbook? I just learnt the ones which were given in the spec
OMG, you are right, I spent so much time as well learning these definitions not in our spec

ok ill give you a different one.

Define:tree
12. (Original post by otrivine)
OMG, you are right, I spent so much time as well learning these definitions not in our spec

ok ill give you a different one.

Define:tree
Tree = connected graph with no cycles

Define Matching
13. (Original post by Fortitude)
Tree = connected graph with no cycles

Define Matching
Perfect!

Matching is the 1-1 pairing of some or all of the vertices such that the elements of X joins to the elements of Y.
14. (Original post by otrivine)
Perfect!

Matching is the 1-1 pairing of some or all of the vertices such that the elements of X joins to the elements of Y.
Correct
15. (Original post by Fortitude)
Correct
Define:Minimum spanning tree

Give 3 differences between Kruskals and prims alogrithm
16. (Original post by Fortitude)
So that was in the textbook? I just learnt the ones which were given in the spec
Your right ! What on earth am I doing
17. (Original post by posthumus)
Your right ! What on earth am I doing
Good Morning
18. (Original post by otrivine)
Define:Minimum spanning tree

Give 3 differences between Kruskals and prims alogrithm
MST = is a subgraph which includes all vertices & is also a tree in which the total length of all its edges is as small as possible.

• Kruskal's starts from the smallest edge while in Prim's the starting vertex is given.
• The spanning tree in Prim's 'grows' while in Kruskals it chaotic (there's probably a much better way of explaining this)
• Can apply prim's from a matrix, you can't with kruskals

And while we're on this topic, how do you answer questions which ask that: a route has to be included in the MST which algorithm would you use & how would you adapt it ??
19. (Original post by posthumus)
Your right ! What on earth am I doing
I'm sure they will only ask the definitions that they've mentioned in the spec
20. (Original post by Fortitude)
MST = is a subgraph which includes all vertices & is also a tree in which the total length of all its edges is as small as possible.

• Kruskal's starts from the smallest edge while in Prim's the starting vertex is given.
• The spanning tree in Prim's 'grows' while in Kruskals it chaotic (there's probably a much better way of explaining this)
• Can apply prim's from a matrix, you can't with kruskals

And while we're on this topic, how do you answer questions which ask that: a route has to be included in the MST which algorithm would you use & how would you adapt it ??

all correct

The answer is always Krusckals and you say you start with AB and DE (example). This is because with Krusckals alogrithm can be built with non connected sub trees while prims only the tree grows and has to be connected all the time

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