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In javascript, I am used to using the map function at this point. The callback for the map function takes two (really, three) arguments: the value and the index, like so: 

someArray.map((value, index) => { // do stuff })

I'm new to R, but have noticed that the apply method is similar in that you provide some sort of data structure and then a function to run on each data element.

However, I haven't seen in the docs anywhere where I can let that function be aware of the index. Something like:

lapply(some_list, function(val, idx) print(idx))

which I would hope to output:

1
2
.
.
.
n - length of list

Am I missing something, or is this not possible in R?

MrFlick
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fraxture
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  • Please show a small reproducible example and expected output – akrun Aug 06 '16 at 19:48
  • You could use a for loop, which might not be optimal from a performance point of view, but would let you keep track of the loop index. – Tim Biegeleisen Aug 06 '16 at 19:54
  • Yes @TimBiegeleisen, I thought about that as well, but was curious to know whether this is actually not possible in R. Would seem odd given that this "mapping" pattern is mostly there. – fraxture Aug 06 '16 at 19:55

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