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

1. Guys when your doing quicksort is it better to keep the pivots and write them out each time or just do the line thing till the bottom l??

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2. (Original post by Flounder1)
Guys when your doing quicksort is it better to keep the pivots and write them out each time or just do the line thing till the bottom l??

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The questions usually ask you to clearly state your pivot so I just make a column for them and write them down for each pass.
3. Can someone please help me on question 7, thanks, I worked out the intersection of 3x=2y and 5x + 4y = 80, I got what the mark scheme has but their x is my y and my x is their y so I don't know what I'm doing wrong?, I figured that if you workout the intersection of y > 6 and 5x + 4y = 80 then you get the mark schemes answer but,isn't the y> 6 a inequality and not a line? therefore it doesn't make sense to find an intersection. PLEASE HELP, I'M WORRIED ABOUT THE EXAM, thanks!! =D

http://www.edexcel.com/migrationdocu...e_20100527.pdf

http://www.edexcel.com/migrationdocu...c_20100618.pdf
4. (Original post by IWantSomeMushu)
The questions usually ask you to clearly state your pivot so I just make a column for them and write them down for each pass.
Once you did it, the single elements can you make them all into pivot as well at the end
5. Learn everything from scratch again time to do the exams, also does anyone have a complete list of all the definitions which may come up, and what is the hardest D1 exam till date?
6. (Original post by posthumus)
To find the number of iterations in binary search, it came up in January (had no bloody clue how to do it), so I really don't think it will come up again.

the most common way is actually to half it, and see how many times you half it...

Another way is to use this

2n = x

where n is the number of iterations and x is the number of variables In QwertyG's case if it were binary search:

2n = 6

therefore : n = log26
which equals = 2.6

therefore 3 iterations are possible... I'll use the common method to confirm this:

6/2 = 3
3/2 = [1.5] = 2
2/2= 1
Thanks for your help Is there anything else similar to this that we need to know for other algorithms e.g. quick sort?
7. (Original post by Flounder1)
Guys when your doing quicksort is it better to keep the pivots and write them out each time or just do the line thing till the bottom l??

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I personally do circles around the chosen pivot... & then after I'm done with it, I put a box around it... to show it's been used I find it the most convenient way.
8. (Original post by Nitrogen)
Thanks for your help Is there anything else similar to this that we need to know for other algorithms e.g. quick sort?
Nope not really... This is the only thing that gets overlooked by many students... it isn't so obvious in the book either I wish it would come up this Friday as it's so simple & usually carries 3 marks ! But I highly doubt it will...
9. (Original post by posthumus)
I personally do circles around the chosen pivot... & then after I'm done with it, I put a box around it... to show it's been used I find it the most convenient way.
for the quick sort, at the end when you have the single elements can you classify them as a pivot, to show the examiner that at the end all of them become pivots?
10. (Original post by Dilzo999)
Learn everything from scratch again time to do the exams, also does anyone have a complete list of all the definitions which may come up, and what is the hardest D1 exam till date?
There's a list in the specification, and I'd say the January 2013 one was pretty hard, but only because the mark scheme was so so picky.

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11. (Original post by otrivine)
for the quick sort, at the end when you have the single elements can you classify them as a pivot, to show the examiner that at the end all of them become pivots?
Hmmm I was actually going to ask that question too I think to play it safe, make sure each variable has a chance to be a pivot even if it's obvious that you have the variables in the desired order

So I would hope I end up with all of them having square boxes around them do you do the square boxes & circles too ?
12. (Original post by posthumus)
Nope not really... This is the only thing that gets overlooked by many students... it isn't so obvious in the book either I wish it would come up this Friday as it's so simple & usually carries 3 marks ! But I highly doubt it will...
I hope it appears too since I know it now Now its just to learn the definitions
13. (Original post by posthumus)
Hmmm I was actually going to ask that question too I think to play it safe, make sure each variable has a chance to be a pivot even if it's obvious that you have the variables in the desired order

So I would hope I end up with all of them having square boxes around them do you do the square boxes & circles too ?
Yes cause in Maths247 video he made each element a pivot,

the pivot I use the circle and then I use the rectangles for the next stage (same pivot)
14. (Original post by Nitrogen)
I hope it appears too since I know it now Now its just to learn the definitions
As a common law in exams, what you've studied extensively right down to the last word = Not in the exam at all

What you aren't too confident on = 6 mark question

OH GOD THE DEFINITIONS, WHY THE DEFINITIONS
15. (Original post by Nitrogen)
I hope it appears too since I know it now Now its just to learn the definitions
I need to do this too ! I made little cards today... I find this the most annoying in D1 , will definitely be the first thing I get out of the way when I start my paper on Friday morning

(Original post by otrivine)
Yes cause in Maths247 video he made each element a pivot,

the pivot I use the circle and then I use the rectangles for the next stage (same pivot)
Ah great ! I think its the best method since you don't really need list which one you've chosen to be the pivot and all that... saves time
16. I just put the pivots on the side for every line.

ect

1 2 3 4 5 - Pivot 3

1 2 3 4 5 - Pivot 2, 5

There is meant to be a circle around the pivot chosen I just cannot do it via keyboard

but is this method correct?
17. (Original post by Westeros)
I checked the mark scheme and my graph is correct, so if i use the ruler method, the first point I hit is (11.2,6) and this works out £10400, but the mark scheme says (11,7) with a cost of £11100
AH! I hate Linear Programming.
Good Luck with your revision If you can look at this question tomorrow it would be much appreciated
- I put on on PostHumous Wall too. Thank me after the Exam.
- Hopefully I don't have to come on this Forum again for another few days.
- Good luck everyone. Keep calm, if you get stuck on a question in the Exam remember Exams are supposed to be challenging. Keep calm. Stop and think. DRINK Water, lots of it, and have breakfast!

Basically. When it is an Integer Value you must select whole numbers.
And sorry again not coming back for a while. I've explained it as fully as you can, even more expansive than the book. Go back to the book if your stuck. Catch you after the exam ideally. but here:

I.e. if your solution is x = 7.5, y=8.3

You must test you Objective Function to be Minimized or Maximized with
x=7 y=8
x=7 y=9
x=8 y=9
x=8 y=8

Which ever gives your the Highest Value. Then Great. Thats what you want.

But then here's the catch, those values must be in the feasible region, so you have to test them with all your Inequalities.

You can first look at your graph to get a vague idea whether your point will fit in the Feasible Region, but to be 100% certain just put the values in your inequalities. If they don't satisfy it then Reject.
18. (Original post by posthumus)
I need to do this too ! I made little cards today... I find this the most annoying in D1 , will definitely be the first thing I get out of the way when I start my paper on Friday morning

Ah great ! I think its the best method since you don't really need list which one you've chosen to be the pivot and all that... saves time

so we do chose everything as pivot at the end right!
19. (Original post by QwertyG)
I just put the pivots on the side for every line.

ect

1 2 3 4 5 - Pivot 3

1 2 3 4 5 - Pivot 2, 5

There is meant to be a circle around the pivot chosen I just cannot do it via keyboard

but is this method correct?
Yup it's correct Although I don't think it's necessary to do both As just circling it would tell the examiner which one you've chosen as the pivot
20. (Original post by otrivine)
so we do chose everything as pivot at the end right!
Yes I would definitely do that

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