question on passing variable to function by reference
- To: mathgroup at smc.vnet.net
- Subject: [mg113532] question on passing variable to function by reference
- From: "Nasser M. Abbasi" <nma at 12000.org>
- Date: Mon, 1 Nov 2010 05:01:51 -0500 (EST)
- Reply-to: nma at 12000.org
Suppose I want to pass an array to function to be filled in by some value.
We know that one can't write:
---------------------
foo[list_]:=Module[{somevalue=-1},
list[[All]] = somevalue
]
------------------
Because the default is pass by value.
On way, is to first assign the input to a local symbol, then use the
local symbol
---------------------
foo[list_]:=Module[{b=list, somevalue=-1},
b[[All]]=somevalue;
b
]
--------------------
But in the above, Mathematica allocated a new storage for b, copied list
to it, and on return, returned 'b'.
I wanted to avoid this extra storage, and use the input, so to make it
pass by reference, so I wrote
---------------------------
foo2[list_]:=Module[{ somevalue=-1},
list[[All]]=somevalue;
list
]
Attributes[foo2]={HoldFirst};
-----------------------------
And now:
list=Table[0,{i,5}];
foo2[list];
list
Out[55]= {-1,-1,-1,-1,-1}
So, this way, it was a pass by reference. Saving storage.
In this above the 'correct' way to handle this, i.e pass by reference?
Do I need to do something more after I change the Attributes? Any
possible problems with using this method?
thanks
--Nasser