Re: how to transform a list in a sequence of arguments?

• To: mathgroup at smc.vnet.net
• Subject: [mg77330] Re: how to transform a list in a sequence of arguments?
• From: dh <dh at metrohm.ch>
• Date: Thu, 7 Jun 2007 03:37:24 -0400 (EDT)
• References: <f43i7q\$3lt\$1@smc.vnet.net>

```
Hi Alessandro,

if I understand correctly, you have a matrix m and indices given as list

{i,j,k} and you want to get m[[i,j,k]]. This can be done e.g. by:

m[[Sequence@@#]]&

hope this helps, Daniel

alexxx.magni at gmail.com wrote:

> I believed it was fairly trivial, yet I was unable to find a

> solution ...

>

> say I have a 3d matrix,

>

> In[64]:= s

> Dimensions[s]

>

> Out[64]= {{{-1, 1, 1}, {-1, 1, 1}, {-1, 1, 1}}, {{1, 1, 1}, {1, 1, 1},

> {1, -1, -1}}, {{1, -1, 1}, {-1, 1, -1}, {-1, 1, -1}}}

> Out[65]= {3, 3, 3}

>

> and I want to access&modify it in a procedure, in which the indices

> i,j,k are found sequentially: {1,1,1}, {1,1,2}, ....

>

> How can I do it, since s[[{i,j,k}]] is not the same - of course - of

> s[[i,j,k]] ???

>

> I finally wrote the construct:

>

> Apply[s[[##]] &, {i,j,k}]

>

> ant this works OK in reading, but not in writing since it reports:

>

> s[[##]] & @@ {1,2,1} = "0"

> Set::write: Tag Apply in (s[[##1]]&)@@{1,2,1} is Protected. >>

>

> any idea on how to make it work read/write???

>

> P.S.

> it <does> work, for now, because I'm using this (where i0 is the

> indices list):

> s = ReplacePart[s, i0 -> - Extract[s, i0]];

>

> but I fear that it must be much slower than a Part[] access

>

>

> Thank you!

>

> Alessandro Magni

>

>

```

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