Re: List of position-specifying functions

• To: mathgroup at smc.vnet.net
• Subject: [mg102410] Re: [mg102351] List of position-specifying functions
• From: Ray Koopman <koopman at sfu.ca>
• Date: Sat, 8 Aug 2009 04:39:06 -0400 (EDT)

```Suppose I have a data matrix in which row are cases and columns are
variables, with the variables names in a list:

varnames = {"id","sex","age","eth","dx1","dx2",<<others>>};

If I want to group the cases by the values of some of the variables
-- say sex, ethnicity, and primary diagnosis -- I might do:

vlist = {"sex","eth","dx1"};
plist = Flatten[Position[varnames,#]&/@vlist];
GatherBy[data,Function[p,#[[p]]&]/@plist]

----- <DrMajorBob at bigfoot.com> wrote:
> Here's the same thing with Map:
>
> plist = {2, 5, 4, 6, 3};
> Function[x, #[[x]] &] /@ plist
>
> {#1[[2]] &, #1[[5]] &, #1[[4]] &, #1[[6]] &, #1[[3]] &}
>
> I'm mystified by how you'd use this with SplitBy or GatherBy, though.
>
> Send an example, if you can!
>
> Bobby
>
> On Thu, 06 Aug 2009 05:32:16 -0500, Ray Koopman <koopman at sfu.ca> wrote:
>
>> I have a list of distinct positive integers, such as
>>
>> In[1]:= plist = {2, 5, 4, 6, 3};
>>
>> I want to convert it to a list of position-specifying functions,
>> such as might be used as the second argument in SplitBy or GatherBy.
>> One way that works is:
>>
>> In[2]:= plist /. p_Integer->(#[[p]]&)
>>
>> Out[2]= { #1[[2]]&, #1[[5]]&, #1[[4]]&, #1[[6]]&, #1[[3]]& }
>>
>> I've been trying to get the same thing using Map,
>> but I haven't been able to do it. Any suggestions?
>
> --
> DrMajorBob at bigfoot.com

```

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