I'm trying to define a pattern that will match lists of lists, where the inner lists contains only numbers (I want the sum of each list). Like this

{{1, 2}, {3, 4}}

But not like this

{1, {1, 3},{{3}}}

Why this is not working?

Clear[foo];
foo[x : {v___List}] := Plus @@@ x /; VectorQ[v, NumberQ]

Thank you!

  • your condition VectorQ[v, NumberQ] for v=Sequence[{1, 2}, {3, 4}] becomes VectorQ[{1, 2}, {3, 4}, NumberQ] thus you get the error message VectorQ::argt: VectorQ called with **3** arguments; 1 or 2 arguments are expected. – kglr Aug 21 at 20:00
up vote 6 down vote accepted
ClearAll[foo];
foo[x : {___List?(VectorQ[#, NumberQ] &)}] := Plus @@@ x

foo[{{1, 2}, {2, 3, 4}}]

{3, 9}

foo[{{1, 2}, {{3}}}]

foo[{{1, 2}, {{3}}}]

Note: You need to use NumericQ in place of NumberQ if you want foo[{{1, 2}, {2, 3, π}}] to return {3, 2 + 3 π}.

Another approach is to use Total, with a condition to forbid it from totaling entries with a single term:

f[x_] := If[Length[x] > 1, Total[x], x]

For instance:

f /@ {{1, 2}, {2, 3, 4}}
{3, 9}
f /@ {{1, 2}, {2, 3, 4}, {{3}}}
{3, 9, {{3}}}

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