Re: A problem with Thread[]

*To*: mathgroup at smc.vnet.net*Subject*: [mg111414] Re: A problem with Thread[]*From*: Themis Matsoukas <tmatsoukas at me.com>*Date*: Sat, 31 Jul 2010 02:42:03 -0400 (EDT)

Here is a workaround: Thread[f[{-1, 0, 1}, 0]] /. f -> MyArcTan {\[Pi], 0, 0} The problem is that Thread first evaluates and then threads (see ref/Thread under possible issues). To see what is going on, redefine your function in the simpler form: MyArcTan2 := Function[{x, y}, If[x == 0 && y == 0, 0, ArcTan[x, y]]] and execute Thread[MyArcTan2[{-1, 0, 1}, 0]] If[{-1, 0, 1} == 0, 0, ArcTan[{-1, 0, 1}, 0]] This shows that instead of threading, Mathematica tries to evaluate the function with first argument {-1,0,1} and second argument 0, causing the If statement to choke. If at least one of these argument is not zero, then the If[] executes ArcTan[x, y], which is why the following still works: Thread[MyArcTan[{-1, 0, 1}, 1]] {(3 \[Pi])/4, \[Pi]/2, \[Pi]/4} Themis