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Re: Mapping to Create Nested Loops

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  Re: Mapping to Create Nested Loops Bob Hanlon <hanlonr@cox.net> - 2011-06-01 11:27 +0000
    Re: Mapping to Create Nested Loops Szabolcs Horvát <szhorvat@gmail.com> - 2011-06-02 11:16 +0000

#2881 — Re: Mapping to Create Nested Loops

GatherBy[myArray, #[[3]] &]

GatherBy[#, #[[3]] &] & /@ myListOfArrays


Bob Hanlon


---- Gregory Lypny <gregory.lypny@videotron.ca> wrote: 

=============
Hello everyone,

I'm trying to learn how to use pure functions and mapping to create what amounts to nested loops. 

Suppose I have a 50 x 6 array called myArray.  The third element of each row is the word red, blue, or green.  If I wanted to break down the array into three according to the value of the third element, I could do this:

Cases[myArray, {__, __, #, __, __, __}] & /@ {red, blue, green}

But what if I needed to do this for more than one array?  Suppose I had three 50 x 6 arrays as

myListOfArrays = {myArray1, myArray2, myArray3} or perhaps the arrays might be imported from files. 


Regards,

Gregory

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#2889

On 2011.06.01. 13:27, Bob Hanlon wrote:
> GatherBy[myArray, #[[3]]&]
>
> GatherBy[#, #[[3]]&]&  /@ myListOfArrays
>
>

Is this kind of use of nested functions with # guaranteed to be safe 
(let's disregard readability concerns for now)?

It appears that # indeed is always the argument of the innermost 
function, but I couldn't find this stated in the docs.  Instead, it's 
suggested in the docs (page for Slot) to use named arguments with nested 
functions (but those aren't without name-conflict / localization issues 
either)

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