Re: extracting column data from 2D Matrix

*To*: mathgroup at smc.vnet.net*Subject*: [mg23428] Re: extracting column data from 2D Matrix*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Wed, 10 May 2000 02:32:14 -0400 (EDT)*References*: <8f54qe$487@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Chris, Please enter the following into Mathematica and evaluate. First question: xyData = Table[Random[Integer, 9], {3}, {5}] xyData[[All, {1, 3, 5}]] Second question: first a step by step solution. a0 = Rest[Transpose[xyData]] a1 = Partition[a0, 2] a2 = Map[Sequence @@ Append[#, (Plus @@ #)/Length[#]] &, a1, {1}] a3 = Prepend[a2, xyData[[All, 1]]] Transpose[a3] Now put this together in a function with some input tests. columnMeans[data_, n_Integer] /; IntegerQ[(Dimensions[data][[2]] - 1)/n ] := Transpose[ Prepend[Map[Sequence @@ Append[#, (Plus @@ #)/n] &, Partition[Rest[Transpose[data]], n], {1}], data[[All, 1]]]] Check the function for correctness columnMeans[xyData, 2] Time on a largish example. tst = Table[Random[], {1000}, {101}]; (res = columnMeans[tst, 10]); // Timing {1.26 Second, Null} Dimensions[res] {1000, 111} 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 "Krautschik, Chris G" <krautschikc at intel.co.jp> wrote in message news:8f54qe$487 at smc.vnet.net... > I have a 2D Matrix of say 101 columns and 1000 rows. Let's call the data > matrix xyDATA. The first column represents the X coordinate and all other > columns represent Y coordindates for different input conditions. > > Let's say I need to extract every 10th y column as well as the X column (1st > column) so I can plot the data in another program. I have been using the > following expression for the extraction process: > > colmnIndex={#1, #11,#21,#31#41,#51,#61,#71,#81,#91,#101}; > xy10 = columIndex & @@@ xyDATA; > > How do I automate the process for the column Index without have to type it > each time? That is perhaps something like the following I had in mind: > > yindex= #*Table[i,{1,101,10}]; > columnIndex2=Append[{#1},yindex]; > > This though doesn't seem to work even when I used: > > ToExpression[columnIndex2] > > or tried various other string to expression schemes. > > Also I like to average each 10 consequitive rows. That is I have written a > RowMean function that generate the column vector of the 1000 elements. How > do I then insert after each 10 y column the columns that holds the average? > So the matrix now grows from 101 colums to 111 colums (still 1000 rows) but > the averaged columns need to be inserted at (12,22,33,...). > > thanks, > Chris > > >