# Defining a sequence slot programmatically

This question is related to:

Composition of functions using slots

Slot (#) interfering with evaluation

Consider a toy example:

 list1 = { {a1, b1, c1, d1}, {a2, b2, c2, d2}, {a3, b3, c3, d3} };


I’d like to take elements from the list as follows example:

{#1, #2}@@@ list1


But I’d like to able to define #1, #2 programmatically.

I tried:

n=1;
{Slot@Evaluate[n], Slot@Evaluate[n + 1]} & @@@ list1


But it gives me an error…

Answers appreciated.Even better if the answer uses the # notation rather than the Slot function.

-

You can use With to insert values into held expressions:

With[{n=1,m=2},
{Slot[n], Slot[m]} & @@@ list1
]


{{a1, b1}, {a2, b2}, {a3, b3}}

If you're so inclined, you can do a nested With:

With[{n = 1},
With[{m = n + 1},
{Slot[n], Slot[m]} & @@@ list1
]
]

LetL[{n = 1, m = n + 1},
{Slot[n], Slot[m]} & @@@ list1
]

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oh. I was too late! –  acl Jul 17 '14 at 11:34
@acl Sorry, I cheated a bit by first posting half of my answer. Too bad you removed yours, as I wasn't aware that With[{n = n},...] was also allowed. –  Teake Nutma Jul 17 '14 at 11:41
+1. You can also use Where in V10 see my answer. –  RunnyKine Jul 17 '14 at 11:43
That's a very useful trick. Why don't you add it to your answer? –  acl Jul 17 '14 at 11:57

There's a nice function in the GeneralUtilities package included in Version 10 called Where. You can use this as:

Needs["GeneralUtilities"]

Where[n = 1, m = n + 1, {Slot[n], Slot[m]} & @@@ list1]


Gives:

{{a1, b1}, {a2, b2}, {a3, b3}}

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+1 Nice to hear indeed. I wonder if this is in fact based upon @LeonidShifrin's LetL. –  Teake Nutma Jul 17 '14 at 11:44
@TeakeNutma I wonder too. I just hope it is added as an internal function in a future release as I always use such functionality. –  RunnyKine Jul 17 '14 at 11:46
Thanks… I haven’t upgraded to MMA 10… but this is great…. –  Pam Jul 17 '14 at 11:52
@TeakeNutma AFAIK its not. LetL also supports shared local variables in definitions, as well as macro-expands at definition time, which makes it zero overhead w.r.t. manually-written nested With in function definitions (these are related). Again AFAIK, Where does not have this functionality. –  Leonid Shifrin Jul 17 '14 at 13:59
Where seems to have a definition intended to catch things like Where[{x,y}={2, 3}, f[x,y]] that they added last and is never tried. To fix it, use DownValues[Where] = RotateRight@DownValues@Where` –  Rojo Jul 24 '14 at 14:21