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Let's say I have some data:

data={{a1,b1},{a2,b2},{a3,b3},{a4,b4},...}

and I would like to apply the function:

f1[x_]=x*5 to every a_i element.

How can I do this?

The best solution I've found so far is to do:

MapAt[f1, data, {{1, 1}, {2, 1}, {3, 1}, {4, 1}...}]

Up to the length of my data, but I'm sure there's a more efficient solution for this.

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    $\begingroup$ MapAt[f1, data, {All, 1}]? $\endgroup$ – J. M. will be back soon♦ May 24 '16 at 16:42
  • $\begingroup$ Map[Apply[{f[#1], ##2} &], data], if you insist... but J.M.'s approach is best. $\endgroup$ – Emilio Pisanty May 24 '16 at 16:44
  • $\begingroup$ Transpose@{f1[#1], #2} & @@ Transpose@data If your f1 is Listable (and yours is ) that can give you significant speed increase. $\endgroup$ – BlacKow May 24 '16 at 16:50
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    $\begingroup$ also: {f1@#, #2} & @@@ data $\endgroup$ – kglr May 24 '16 at 23:44
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    $\begingroup$ @BlacKow #1 and # are equivalent. $\endgroup$ – LLlAMnYP May 25 '16 at 7:16
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There are a number of ways (if I understand aim),e.g.:

lst = data = {{a1, b1}, {a2, b2}, {a3, b3}, {a4, b4}};
lst /. {x_, y_} :> {f[x], y}
MapAt[f, lst, {All, 1}]
{f@#1, #2} & @@@ lst
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