Re: How to manipulate globals in a function?

*To*: mathgroup at smc.vnet.net*Subject*: [mg73710] Re: How to manipulate globals in a function?*From*: "oshaughn" <oshaughn at northwestern.edu>*Date*: Sun, 25 Feb 2007 04:36:55 -0500 (EST)*References*: <erop34$94v$1@smc.vnet.net>

This is an order-of-evaluation issue : when mathematica evaluates a function, the arguments are evaluated *first*. So for example if I have f[a_,b_] := a=b then a=3 Trace[ f[a,2]] returns an error, because the first step in evaluation is to replace 'a' by 3, before the function is called. {{a, 3}, f[3, 2], 3 = 2, {Message[Set:: setraw, 3], {Set::setraw, Cannot assign to raw object `1`.}, However, the mathematica attribute HoldFirst fixes this problem, preventing the first argument from being evaluated "too soon" SetAttributes[f, HoldFirst] Trace[f[a,2]] --> {f[a, 2], a = 2, 2} On Feb 24, 1:22 am, davi... at gmail.com wrote: > I am trying to write code to manipulate a global array in a function. > How do something like this: > > Moo [A_List, i_Integer] := > > A[[i]] = 7; > > Moo[A, 1] > > This fails. But, if I do this without a funciton it works. > > i=1 > A[[i]] = 7 > > JD