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How can we embed a counter in the Map function?
Consider a data set such as data = {{a, b, c, d}, {e, f}, {g, h, i}}. The desired function would output
{{0, a}, {0, b}, {0, c}, {0, d}, {1, e}, {1, f}, {2, g}, {2, h}, {2, i}}
Responders are not constrained to use Map.
N.B. The answer does not appear to be in Indexed Map Thread.
I believe you are looking for MapIndexed, and possibly Join/Apply:
Join @@ MapIndexed[{#2[[1]] - 1, #} &, data, {2}]
{{0, a}, {0, b}, {0, c}, {0, d}, {1, e}, {1, f}, {2, g}, {2, h}, {2, i}}
If this is not what you want or you find it difficult to apply please update the question.
Flatten[foo, 1] is not mapping Flatten at levelspec {1}, i.e. Map[Flatten, foo, {1}], just in case that should become a point of confusion. I typically find that Join @@ is faster, but often more importantly it preserves packed arrays. Since MapIndexed does not return packed arrays the latter is not evident here. Take for example x = Range @ RandomInteger[{1, 7}, 5*^4]; then RepeatedTiming[Developer`PackedArrayQ @ Flatten[x, 1]] vs RepeatedTiming[Developer`PackedArrayQ[Join @@ x]]
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Aug 11, 2016 at 14:15
{0.0207, False} and {0.00256, True}. So not only is Join nearly an order of magnitude faster, but it returns a packed array with all the benefits that entails. Out of habit I therefore use Join @@ even when it may not be strictly superior, though again I think you will often find it faster even when working with unpacked lists.
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Aug 11, 2016 at 14:17