Call by reference in Mathematica

Parameters values passed into Mathematica functions are immutable. However, one can fake "call by reference" behavior in Mathematica by giving the function a Hold* (HoldAll/HoldFirst/HoldRest) attribute. That way the function can directly modify the variable passed to it.

Attributes[MyFunc] = HoldAll;

MyFunc[a_Symbol]:=(a[[2]] = 2.2);

b = {1., 2., 3., 4.}

{1.`, 2.`, 3.`, 4.`}

MyFunc[b];

b

{1.`, 2.2`, 3.`, 4.`}

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The drawback to doing this is that you have to pass a variable into the function. Passing a literal value will not work.