Re: matrix columns

*To*: mathgroup at smc.vnet.net*Subject*: [mg30252] Re: [mg30247] matrix columns*From*: BobHanlon at aol.com*Date*: Sat, 4 Aug 2001 01:14:21 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

In a message dated 2001/8/3 1:02:29 AM, aaurba2 at pop.uky.edu writes: >According to the mathematica book there are 2 ways to access columns of >a matrix (Mathmatica V. 3) Transpose[m][[i]] and Map[#[[i]]&,m]. Are >the efficiencies of these two operations the same. Also, I can't figure >out how you would assign a column in a matrix (i.e. set a column in the >matrix equal to a specific vector or something). The two previously >mentioned functions return copies of the columns in the m and you can't >assign a new value to it as far as I can see. Is this possible? > $Version "4.1 for Power Macintosh (November \ 2, 2000)" m = Table[Range[1000], {500}]; n = 6; Timing[Union[Transpose[m][[n]]] == {n}] {0.31666666666660603*Second, True} Timing[Union[Map[#[[n]]&, m]] == {n}] {0.033333333333416704*Second, True} Timing[Union[m[[All, n]]] == {n}] {0.*Second, True} m[[All, n]] == Map[#[[n]]&, m] == Transpose[m][[n]] True m = Table[Range[5], {3}]; m[[All, 2]] = 20; m {{1, 20, 3, 4, 5}, {1, 20, 3, 4, 5}, {1, 20, 3, 4, 5}} m[[All, 2]] = {a, b, c}; m {{1, a, 3, 4, 5}, {1, b, 3, 4, 5}, {1, c, 3, 4, 5}} Bob Hanlon Chantilly, VA USA