Re: i don't understand mapping function over a long list

*To*: mathgroup at smc.vnet.net*Subject*: [mg47826] Re: [mg47802] i don't understand mapping function over a long list*From*: Sseziwa Mukasa <mukasa at jeol.com>*Date*: Thu, 29 Apr 2004 00:33:42 -0400 (EDT)*References*: <200404270847.EAA18860@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

On Apr 27, 2004, at 4:47 AM, sean kim wrote: > hello group. > please consider the following list of lists. > > > > l = Partition[Flatten[{Table[Unique[x], {n, 1, 30}]}], 3 ] > > above generates a list of 10 lists with 3 elements in each as in... > > { > {x$299, x$300, x$301}, {x$302, x$303, x$304}, > {x$305, x$306, x$307}, {x$308, x$309, x$310}, > {x$311, x$312, x$313}, {x$314, x$315, x$316}, > {x$317, x$318, x$319}, {x$320, x$321, x$322}, > {x$323, x$324, x$325}, {x$326, x$327, x$328} > } > > Now suppose I want to use each of the list( all 10 of them) as a part > of a function. . I want to "Apply" the function to every list(so, 10 > times in total) > for a simple examplel let's add 2 to the lists > > In[21]:= Apply[Plus@@xlist, 2] > Out[21]= 2 > > that didn't work. i wanted to get was > > {{x$299, x$300, x$301} + 2, > {x$302, x$303, x$304} + 2... > {x$326, x$327, x$328} + 2}} I am not sure how to explain your error to you. Suffice it to say this expression #+2&/@xlist will return what you want. > then i want to give each of the results unique names and use the > renamed list of lists as an argument in another function. > > { > uniquexname1 = {x$299, x$300, x$301} + 2, > uniquexname2 = {x$302, x$303, x$304} + 2... > uniquexname10 = {x$326, x$327, x$328} + 2} > } > Set[Evaluate[Unique[uniquename]],#]&/@xlist You'll have to use Names["uniquename*"] to see the names of the variables in this case. > and > > Map[Plus, xlist, 2] > > just bring back the list itself. Again, I'm not sure what you mean. > This problem recurs for me. and I think i have problems with it > because I just don't understand how Mathematica language works. It's a little different from other programming systems. > Reading the book an dhelp manual doesn't help me much in understanding > what lies underneath. Can you guys shed soem light on this with some > simple examples that use numerical operations? I'm not sure a numerical example would be useful, your problem seems to be a misunderstanding of the nature of Mathematica expressions and the use of the operators Map and Apply. The basic principle with Apply and Map is that every expression in Mathematica has a head (see help on Head) and body. Map uses the elements of the body as arguments to a function which is its first argument. Apply changes the head of an expression to another value. You may also want to understand the idea of levels in Mathematica expressions, I suggest studying the operators TreeForm and Level as well as section A 3.6 and section 2 of the Mathematica Book. > Maybe I'm asking a lot, but any and all insights are thoroughly > appreciated. I hope this helps, Ssezi

**References**:**i don't understand mapping function over a long list***From:*sean_incali@yahoo.com (sean kim)