Mathematica Stack Exchange is a question and answer site for users of Wolfram Mathematica. It only takes a minute to sign up.
Sign up to join this community
how to give some elements of a list a value that be same always, and cannot change anyway.
a = {1, 0, 1, 1};
SetAttributes[a[[2]], Constant]
SetAttributes::sym: Argument a[[2]] at position 1 is expected to be a symbol. >>
I want to a[[2]] be always 0 , I used SetAttributes but it didn't work.
a, the list variable, is a variable, so by definition it can vary. But you could always write your own function as wrapper to do the setting and inside the function, you can check for this.
setMyList[m_List, idx_, value_] := Module[{m0 = m},
If[idx == 2, m, m0[[idx]] = value; m0]
]
and now call it
a = {1, 0, 1, 1};
setMyList[a, 1, 99]
(* {99, 0, 1, 1} *)
But when the index is 2, then it will not change
setMyList[a, 2, 99]
(* {1, 0, 1, 1} *)
The index that is supposed not to be change is hard coded inside the function. But you can change that if you want and pass that as well as an extra parameter.
but there should be simple way, the simple way is to use an if statement. This is what one does in any programming language really. I do not see why it would be different in Mathematica. Trying to play tricks like what you wanted to do is the wrong approach to solve the problem. To get better help, please post a minimal example of the actual problem you are working on.
$\endgroup$
– Nasser
Jan 2 '16 at 9:58
