Editing the function argument

Asked Aug 12 '18 at 21:41

Active Aug 12 '18 at 21:41

Viewed 46 times

I'd like to construct a function, that changes its own input. The code

SetAttributes[f, HoldFirst];   l={1,2,3};
f[l_List] := Module[{i}, For[i=1, i<=Length@l, i++, Print[i]; l[[i]]=1;]];
f[l];   l

returns {1,2,3} instead of {1,1,1}. What am I doing wrong?

This and this post did not help.

asked Aug 12 '18 at 21:41

Leo

Hmm, strange, if instead I write f[l_]:=..., then it works. :/ – Leo Aug 12 '18 at 21:44
Yes, because the HoldFirst attribute means that the Symbol is passed, not an expression with a head List. – Michael E2 Aug 12 '18 at 21:45
Aha, so the solution is to always omit _List? – Leo Aug 12 '18 at 21:47
2

Do[l[[i]] = 1, {i, Length@l}] seems like better code for the body of f. (I realize it's just a test example, but For is rarely used by Mathematica programmers.) – Michael E2 Aug 12 '18 at 21:47
3

Yes, as you discovered (by your first comment). – Michael E2 Aug 12 '18 at 21:47
3

You can also use l_?ListQ which will test without the Hold – b3m2a1 Aug 12 '18 at 22:08

0 Answers0