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: Wed, 28 Apr 2004 06:56:18 -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

```

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