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ClearAll[f]
For[i = 1, i <= 20, i++,  
  f[x_, y_, z_] := 2 x + y + z;  
  data = RandomReal[{-1, 1}, {i, 3}];  
  mapping = Map[f @@ # &, data]  
  mapping[[i]]]  

In the above code, I want to run the loop 20 times, each time with different sets of random numbers, and finally print out the 20 values in the form of mapping [[i]]. Clearly I am not achieving that. Can anyone help me please?

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  • $\begingroup$ missing ; before the last mapping, also, For does not return so you need Print. $\endgroup$ – Kuba Jun 29 '17 at 12:10
  • $\begingroup$ Now about logic, each time you generate i rows, map every one but use only i-th, was that the goal? $\endgroup$ – Kuba Jun 29 '17 at 12:11
  • 3
    $\begingroup$ Check Table[f @@ RandomReal[{-1, 1}, 3], 20] or f @@@ RandomReal[{-1, 1}, {20, 3}]. $\endgroup$ – Kuba Jun 29 '17 at 12:12
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f[x_, y_, z_] := 2 x + y + z;

You really don't want to use looping constructs to evaluate f over 20 sets of randoms arguments. Mathematica has better tools for doing that kind of thing.

The Table function is one first functions beginners should learn about, so I recommend Kuba'a suggestion:

SeedRandom[42]; m = Table[f @@ RandomReal[{-1, 1}, 3], 20];

A somewhat more advanced approach is (also mentioned by Kuba):

SeedRandom[42]; m = f @@@ RandomReal[{-1, 1}, {20, 3}];

In both cases

m // Column

gives

result

Note: You can omit the expression SeedRandom[42]; in your work. It purpose here is to ensue the same result in both cases.

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