Re: Re: making a column into a list

*To*: mathgroup at smc.vnet.net*Subject*: [mg23050] Re: [mg22990] Re: [mg22964] making a column into a list*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Thu, 13 Apr 2000 02:43:33 -0400 (EDT)*References*: <LP9J4.62$UI4.3344@ralph.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Mark, Thanks for bringing this to my attention. We can get a flat list directly: mat = {{1, 2, 3}, {4, 5, 6}}; mat[[All, 2]] {2, 5} The brackets also allow quit a bit of construction: mat[[{2, 1, 1}, {2, 1, 2}]] {{5, 4, 5}, {2, 1, 2}, {2, 1, 2}} -- Allan --------------------- Allan Hayes Mathematica Training and Consulting Leicester UK www.haystack.demon.co.uk hay at haystack.demon.co.uk Voice: +44 (0)116 271 4198 Fax: +44 (0)870 164 0565 "Mark Harder" <harderm at ucs.orst.edu> wrote in message news:LP9J4.62$UI4.3344 at ralph.vnet.net... > Allan; > Thanks for the suggestion to use Part to extract a column from a matrix; > it is indeed simpler and faster when you want to keep the extracted column > in column-vector form. To get a flat list from your result, requires that > TakeColumn be followed by Flatten[]. > I must confess that this aspect of Mathematica -- the correspondences > between "vector" and "list" and "matrix" and "2-dimensional list"-- is > still a bit confusing to me, so let me run through my reasoning. The result > that you obtained with TakeColumns[], remains in the form of a column matrix > (list of single-element lists), and I was attempting to produce a "list", as > per the poster's request, which I interpreted to be a singly dimensioned > list, which I thought corresponded to a row-vector from some matrix. To see > how this latter is incorrect, I now note the difference between using > Flatten and Transpose: > > In[6]:=Flatten[{{1}, {1}, {1}} ] > > Out[6]={1, 1, 1} > > , a flat list. > > > In[7]:=Transpose[{{1}, {1}, {1}} ] > > Out[7]={{1, 1, 1}} > > , a list-of-lists, or a one-by-three matrix. > > In working out my posting, I used Column[], from the > Statistics`DataManipulation` package, which I had forgotten was loaded at > the time. Sorry I did not mention that; details, details.... > > -mark harder > harderm at ucs.orst.edu > > > -----Original Message----- > From: Allan Hayes <hay at haystack.demon.co.uk> To: mathgroup at smc.vnet.net > Subject: [mg23050] [mg22990] Re: [mg22964] making a column into a list > > > >Mark: > >You report > >> To get a column from a matrix in list form: > >> IN> > >> matrix = {{1, 2}, {1, 2}, {1, 2}} > >> column = Column[matrix, 1] > >> OUT> > >> {{1, 2}, {1, 2}, {1, 2}} > >> {1, 1, 1} > > > >This doesn't work for me with Mathematica 4.0. > > > >matrix = {{1, 2}, {1, 2}, {1, 2}} > >column = Column[matrix, 1] > > > >{{1, 2}, {1, 2}, {1, 2}} > > > >Column[{{1, 2}, {1, 2}, {1, 2}}, 1] > > > >Did you mean TakeColumns from the package? > > > ><< LinearAlgebra`MatrixManipulation` > > > >TakeColumns[matrix, {1}] > > > > {{1}, {1}, {1}} > > > >------------------------------------- > > > >But in Mathematica 4.0 we can use > > > >matrix[[All, {1}]] > > > >{{1}, {1}, {1}} > > > >And this is much faster: > > > >matrix = Table[Random[], {50000}, {10}]; > > > >TakeColumns[matrix, {1}]; // Timing > > > >{1.37 Second, Null} > > > >matrix[[All, {1}]]; // Timing > > > >{0.11 Second, Null} > > > > > >-- > >Allan > >--------------------- > >Allan Hayes > >Mathematica Training and Consulting > >Leicester UK > >www.haystack.demon.co.uk > >hay at haystack.demon.co.uk > >Voice: +44 (0)116 271 4198 > >Fax: +44 (0)870 164 0565 > > > > > > > > > > > > > > >