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MathGroup Archive 2006

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Re: building a list containing elements f(i,j)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg71063] Re: building a list containing elements f(i,j)
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Wed, 8 Nov 2006 06:11:32 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <eimr3e$dhv$1@smc.vnet.net>

xarnaudx at gmail.com wrote:
> hi,
> i want to construct a list of the form:
> { f[1,1], ... f[n,m] }
> So the list contains elements of the form f[i,j] with i ranging from 0
> to n, and j from 0 to somebound[i].
> 
> What would be the most elegant way to this?
> thanks
> 
You can use either *Table* or *Array*, then *Flatten* the resulting 
2D-list to make it one dimensional. Beware that Mathematica use the one 
-- and not zero -- to index lists or arrays. You will find hereafter 
several example of list construction, some of them with the second index 
depending on the value of the first one.

In[1]:=
l1 = Flatten[Table[f[i, j], {i, 0, 3}, {j, 0, 2}]]

Out[1]=
{f[0, 0], f[0, 1], f[0, 2], f[1, 0], f[1, 1], f[1, 2],
   f[2, 0], f[2, 1], f[2, 2], f[3, 0], f[3, 1], f[3, 2]}

In[2]:=
l2 = Flatten[Array[f, {4, 3}, {0, 0}]]

Out[2]=
{f[0, 0], f[0, 1], f[0, 2], f[1, 0], f[1, 1], f[1, 2],
   f[2, 0], f[2, 1], f[2, 2], f[3, 0], f[3, 1], f[3, 2]}

In[3]:=
l1 == l2

Out[3]=
True

In[4]:=
Table[f[i, j], {i, 0, 3}, {j, 0, i/2}]

Out[4]=
{{f[0, 0]}, {f[1, 0]}, {f[2, 0], f[2, 1]},
   {f[3, 0], f[3, 1]}}

In[5]:=
somebound[i_] := {4, 1, 3, 2}[[i + 1]];
Table[f[i, j], {i, 0, 3}, {j, 0, somebound[i]}]

Out[6]=
{{f[0, 0], f[0, 1], f[0, 2], f[0, 3], f[0, 4]},
   {f[1, 0], f[1, 1]}, {f[2, 0], f[2, 1], f[2, 2],
    f[2, 3]}, {f[3, 0], f[3, 1], f[3, 2]}}

HTH,
Jean-Marc


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