Re: Problems ....
- To: mathgroup@smc.vnet.net
- Subject: [mg11559] Re: [mg11532] Problems ....
- From: Sean Ross <seanross@worldnet.att.net>
- Date: Sat, 14 Mar 1998 13:56:12 -0500
- References: <199803131722.MAA03809@smc.vnet.net.>
Tie wrote:
>
> I am a new user. need help to know more about mathematica. Anyone can
> help me do the following:
>
> 1. Let Y={a,b} and let X be an arbitrary numeric vector. Replace all
> occurances of a in X with b.
>
> 2. Sort the elements of X in ascending order of absolute value.
>
> 3. Square each elements of a vector X but keep its sign.
>
> 4. Remove second element of the vector X without using the Table
> function
>
> thanks.
1. I am confused by X and Y. What does Y have to do with this question?
If you want to replace a symbol or number with another in an
expression, use a transformation rule.
X/.a->b
2. Sort[X]
you can change the criteria by which the sort occurs by using the
option p. Using this option usually requires the use of "pure
functions" as in Sort[x,If[#1>#2,True,False]&], where the second
argument gives a true if the elements as sorted in descending order and
false otherwise. The net effect of this sorting function is to sort in
reverse order.
Sort[{1,3,4,5,8,2},If[#1>#2,True,False]&]
returns
{8,5,4,3,2,1}
while
Sort[{1,3,4,5,8,2},If[#1<#2,True,False]&] or
Sort[{1,3,4,5,8,2}]
returns
{1,2,3,4,5,8}
3. If x is a 1-D list,
y = x^2 * Sign[x]
4. Drop[x,{2}]
Finally, a comment. From your questions, it is clear you are just
starting to become familiar with the language and your programming will
be seriously hampered without a basic knowledge of the features of
mathematica and you will have to wait 2 days for someone to answer your
question via mathgroup. If you have not done so, I recommend you
purchase a hard copy of the mathematica book and fall asleep a few
nights in a row looking through it just to see what is out there. At
least go through all the examples in the "tour of mathematica" section.
You have some potentially dangerous concepts implicit in your
questions, such as calling a 1-D list a vector. You may think of them
as vectors or may use them to represent vectors, but without special
handling, 1-D lists don't multiply, transform or behave like vectors.
Another concept is "arbitrary numerical vector". There is no such thing
in mathematica. You can have a symbol with or without assigned values.
You can make a list composed of symbols which have not been assigned
values yet, but you cannot tell mathematica that something is a list
without putting something in the elements of said list.(If I am wrong
on this point, somebody please tell me how to do it) If a list is
arbitrary, it is not yet numeric. If it has been assigned numbers to
its elements it is no longer arbitrary.
Good luck.
--
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- References:
- Problems ....
- From: tie@cscn.com (Tie)
- Problems ....