Re: Indexed element treated as number?

*To*: mathgroup at smc.vnet.net*Subject*: [mg80079] Re: Indexed element treated as number?*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Sun, 12 Aug 2007 07:12:52 -0400 (EDT)*Organization*: The Open University, Milton Keynes, UK*References*: <f9jl5e$3c6$1@smc.vnet.net>

Jung-Tsung Shen wrote: > To get an array of length n, where n is large and post-given, one > issues the command, for example, > > n=100; > kList1=Array[a, n]; > > but it seems that there are some limitations to this approach. For > example, in cases that I know all of the elements are real, and would > like to issue, say, > > a[37]/:Im[a[37]]=0; > > I get an error message saying > "TagSet::sym: Argument a[37] at position 1 is expected to be a > symbol." To declare the element to be explicitly zero sometimes speed > up the computation. In this case, the correct syntax for a tag set is the name of the array without any index followed by the tag set to the desired element as in In[1]:= a /: Im[a[2]] = 0; Alternatively, we could use *UpSet* as in In[2]:= Im[a[3]] ^= 0; Now we can compare the imaginary part of the elements. In[3]:= Im[a[2]] == Im[a[3]] Out[3]= True In[4]:= Im[a[2]] == Im[a[4]] Out[4]= 0 == Im[a[4]] > For a list such as > > kList2={k1, k2, k3, ..., k100}; > > there's no such limitations, but since the number of elements is > large, and is post-given, I couldn't have this before hand. > > Is there any solution(s) to this "problem"? Well, you could use a function like the one below to create your array of symbol when you know what the length of the list is. In[1]:= createArray[n_Integer /; n > 0, sym_Symbol: k] := Table[ToExpression[StringJoin[ToString /@ {sym, i}]], {i, n}] In[2]:= createArray[5, a] Out[2]= {a1, a2, a3, a4, a5} In[3]:= kList2 = createArray[10] Out[3]= {k1, k2, k3, k4, k5, k6, k7, k8, k9, k10} HTH, -- Jean-Marc