CODEWITHSUNDEEP
listfunctiondictionarywolfram-mathematica
Ivan
Ivan

Reputation: 172

Mathematica: Applying a function to a list of list

I have a function f[d_]. My intention is to apply this function to a list as a whole. Say, d={1,2,3,...}, then f[d] gives me a result (a number or whatever). Until here, everything is clear.

Now lets say I have the following list of lists: p={l1,l2,l3,...} Is there a more efficient way than Map to compute f[p], where the expected result is {f[l1],f[l2],f[l3],...}?

For example, with the Sin[x] function, Mapping it over a list is way slower than just putting the list inside its argument. This doesn't seem to work with my function f[d] and the list of lists p. What should I do to make that work? Would it be faster than Map?

To make myself clearer, say

f[d_]:=Total[d]
Then, f[{a1,a2,a3}] gives me a1+a2+a3, as expected.
But, f[{{a1, a2, a3}, {b1, b2, b3}, {c1, c2, c3, c4}}] kills the machine.

Thank you!

Upvotes: 2

Views: 325

Answers (2)

agentp
agentp

Reputation: 6989

Sorry Listable doesnt do the job here.. You can put map inside the function if it helps..

ClearAll[f]
f[d_List] := Map[ Total, d, {-2}]
f[{1, 2, 3}]
f[{{1, 2, 3}, {4, 5}}]
f[{{{1, 2, 3}, {4, 5}}, {{6}, {7, 8}}}]

(* 6 *)
(* {6, 9} *)
(* {{6, 9}, {6, 15}} *)

Upvotes: 0

tintin
tintin

Reputation: 1519

When I change your code, It gives:

     Total[{{a1, b1, c1}, {b1, b2, b3}, {c1, c2, c3}}]
     {a1 + b1 + c1, b1 + b2 + c2, b3 + c1 + c3}

When Total take a list argument, It just add the members, in your case, It adds 

    {a1, b1, c1},{b1, b2, b3}, {c1, c2, c3}

But they are not the same dimension, so you cant get the right answer.

In this case,Map should be used.

    Map[Total,{{a1, b1, c1}, {b1, b2, b3}, {c1, c2, c3, c4}}]

or

    Plus @@@ {{a1, b1, c1}, {b1, b2, b3}, {c1, c2, c3, c4}}

Upvotes: 1

Related Questions