Re: Problem with using Symbol[] in a loop

*To*: mathgroup at smc.vnet.net*Subject*: [mg111842] Re: Problem with using Symbol[] in a loop*From*: Murray Eisenberg <murray at math.umass.edu>*Date*: Sun, 15 Aug 2010 07:36:36 -0400 (EDT)

Easy answer: because in the first version you included Print but in the second you did not! If you change to ... For[i = 0, i < Length[dirnames], i++; Print[Symbol[dirnames[[i]]]]] ... then you'll get the expected result. The underlying reason -- from the ref page on For: Unless an explicit Return is used, the value returned by For is Null. Without the Print wrapper, all you get is a Null value returned, which means nothing is printed as the output. BTW, this, but usually for Do, is one of the more common questions we see in this news group. On 8/13/2010 6:57 AM, Bob F wrote: > Can anyone tell me why this works as expected: > > dirnames = Names["$*Directory*"]; > For[i = 0, i< Length[dirnames], i++; Print[dirnames[[i]]]] > > and gives the following results: > > $AddOnsDirectory > > $BaseDirectory : : > > But that the following doesn't give any results: > > dirnames = Names["$*Directory*"]; > For[i = 0, i< Length[dirnames], i++; Symbol[dirnames[[i]]]] > > but if I go thru the dirnames array elements manually it works as I > would expect, e.g. > > In[161]:= Symbol[dirnames[[1]]] > > Out[161]= "/Library/Mathematica" > > Why does the Print[dirnames[[i]]] work OK but the > Symbol[dirnames[[i]]] fail to give anything? > > Thanks... > > -Bob > -- Murray Eisenberg murray at math.umass.edu Mathematics & Statistics Dept. Lederle Graduate Research Tower phone 413 549-1020 (H) University of Massachusetts 413 545-2859 (W) 710 North Pleasant Street fax 413 545-1801 Amherst, MA 01003-9305