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functionwolfram-mathematicavariable-assignmentsymbolic-mathsparse-array

Assignments in Wolfram Mathematica

Question about assignments and variables

(* For example *) SP = SparseArray[{},5] or SP = Range[5]

now we want to work with this array in some another function : 

(* example *) Fun[array_]:= array[[3]] = 100 ; (* set cell №3 equal to 100*)

then we eval

Fun[SP]

ERROR! output will be an Error like: Set::write Tag SparseArray in ... is Protected.

So what is the right way to change the arguments of function in function (non-pure-functions)? How to creare analog-like of Part[]?

maybe smth like:

Clear[f]; f[a_]:=Set[Symbol[a][[3]],100]; A =SparseArray[{},5]; f["A"];

But it's error again

Solution

  • I believe that Chris Degnen's method should generally be avoided.
    Mathematica provides a better way: the Hold attributes.

    a = Range[5];

SetAttributes[fun, HoldFirst]

fun[array_] := array[[3]] = 100

fun[a];

a

    {1, 2, 100, 4, 5}

    As a "pure function":

    b = Range[5];

fun2 = Function[array, array[[3]] = 100, HoldFirst];

fun2[b];

b

    {1, 2, 100, 4, 5}