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Tuesday, March 17, 2009

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

Download Mathematica notebook

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


Anonymous said...

Great tip, thanks. swgreen at mit

Anonymous said...

Thank! Awesome post!

Anonymous said...

many thanks!!!!!!!!!

nice post : )