RE: Newbie question: pairswise function application

• To: mathgroup at smc.vnet.net
• Subject: [mg23868] RE: [mg23857] Newbie question: pairswise function application
• From: "David Park" <djmp at earthlink.net>
• Date: Thu, 15 Jun 2000 00:51:07 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

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> -----Original Message-----
> From: Sidney Cadot [mailto:sidney at janis.pds.tudelft.nl]
To: mathgroup at smc.vnet.net

> Hi,
>
> Wandering through the 1500-odd pages of the Mathematica book, I can't
> find the solution to the following rather silly problem:
>
> I have a list L consisting of (a lot of) integers, and I want to apply a
> function f to all pairs of numbers; furthermore, I don't want to
> calculate both f[a,b] and f[b,a]:
>
>   L = {1,5,19,100};
>
> ... and I want:
>
>   { f[1,5], f[1,19], f[1,100], f[5,19], f[5,100], f[19,100] }
>
> Can anyone please tell me how to do this? I'd prefer a smart solution
> (i.e., not just generating all pairs, then throwing away a lot of them)
> since my lists tend to get rather large.
>
>
>   sidney at janis.pds.twi.tudelft.nl
>

Sidney,

This is one method:

li = {1, 5, 9, 100};

Flatten[Table[f[li[[i]], li[[j]]],
{i, 1, Length[li] - 1}, {j, i + 1, Length[li]}]]
{f[1, 5], f[1, 9], f[1, 100], f[5, 9], f[5, 100], f[9, 100]}

David Park