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Horus

Sat Dec 20, 2008 8:55 pm


2008 junior CCC question #4

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so i was just looking at last year's question here:
http://cemc.math.uwaterloo.ca/contests/computing/2008/stage1/juniorEn.pdf

and i couldn't figure out what question #4 means.

i pretty much get polish notation but i don't understand how postfix notation works...

and btw does  --3+219 means (-3)-(2+1+9) ?
just making sure...


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jbking

Sat Dec 20, 2008 10:06 pm


RE:2008 junior CCC question #4

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Not quite....

Remember that - a b means a - b and - + a b c means a+b - c as the + a b is evaluated first  and then that answer is used an input for the next operator.

- - 3 + 2 1 9

which equals :-  - 3 (2+1) 9

which equals: - (3 - (2+1)) 9

which equals: 3 - (2+1) - 9 in infix notation which is different than you wrote.


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Horus

Sat Dec 20, 2008 10:16 pm


RE:2008 junior CCC question #4

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oh ok, kinda get it now

but what about postfix notation?


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OneOffDriveByPoster

Sat Dec 20, 2008 10:49 pm


Re: RE:2008 junior CCC question #4

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oh ok, kinda get it now

but what about postfix notation?Postfix is like how old calculators work:
3 2 -
means you are calculating (3 - 2); the calculation happens as soon as the operator (-) is used.

3 2 - 1 + would be (3 - 2) + 1.

3 2 - 1 9 * + would be (3 - 2) + (1 * 9).


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Horus

Sat Dec 20, 2008 10:54 pm


RE:2008 junior CCC question #4

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ok thx i finally understand it now.  now i can finally code haha.



lol if this type of question appears on CCC in 2009 i'm probably screwed...


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Euphoracle

Sun Dec 21, 2008 12:31 am


RE:2008 junior CCC question #4

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When I did this, I was silly, and I converted it to our "regular" notation (in a sense), and then to the target format.  I found it easier to work with a format I was comfortable with, and, although it wasn't the most efficient way of doing it, it got the job done correctly.  I probably should have kept those solutions.  Oh well.