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Is there Mathematica analogue of Python _ (underscore) or Matlab ~, a special variable, that can be used when a function returns a list and only some of the list elements are needed while others just ignored. E.g.:

f[x_] = {x^ 2, x^ 3};
{a1, _} = f[2]
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    $\begingroup$ In this particular case, you could use a1 = f[2][[1]]. $\endgroup$
    – bbgodfrey
    Commented Nov 18, 2015 at 22:20
  • $\begingroup$ Have a look at Part or Extract. Part would be shorter code. Both can select multiple items from the returned list. $\endgroup$
    – Edmund
    Commented Nov 18, 2015 at 23:01
  • $\begingroup$ Is this ok: a /: Set[a, _] := Null? No value can be set to a, but can be SetDelayed etc. $\endgroup$
    – Kuba
    Commented Nov 19, 2015 at 7:28

2 Answers 2

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Instead of an underscore, a "throwaway" symbol can be used. For instance,

f[x_] = {x^2, x^3, x^4, x^5};
{a1, z, z, a2} = f[2]

a1 and a2 have their desired values, and the others are overwritten into z at the cost of only a single symbol.

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  • $\begingroup$ That seems to be the easiest way. I figured out the dollar sign is a legal variable name and probably might as well be used for trash-variable. $\endgroup$
    – Student4K
    Commented Nov 18, 2015 at 22:46
  • $\begingroup$ Yes, $z would work just fine. Welcome to Mathematica.StackExchange. $\endgroup$
    – bbgodfrey
    Commented Nov 18, 2015 at 22:48
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    $\begingroup$ @Kuba, Using a Formal Symbol is a clever idea (+1), although Off[Set::wrsym] would be necessary to avoid what could be many error messages. $\endgroup$
    – bbgodfrey
    Commented Nov 18, 2015 at 23:34
  • $\begingroup$ or f[x_] = {x^2, x^3, x^4, x^5}; Module[{z}, {a1, z, z, a2} = f[2]] $\endgroup$
    – Dr. belisarius
    Commented Nov 19, 2015 at 0:18
  • $\begingroup$ @bbgodfrey I had messages window hiden so I haven't noticed it :P but anyway, what was I expecting? :P $\endgroup$
    – Kuba
    Commented Nov 19, 2015 at 7:25
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I would recommend to write

a1 = f[2][[1]]

4

Clear[a1]

{a1, _} = f[2];

also assigns 4 to a1 but produces an error message which you can turn off temporarily with Quiet:

Quiet[{a1, _} = f[2]];

a1

4

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