Thread
- To: mathgroup@smc.vnet.net
- Subject: [mg11869] Thread
- From: Des Penny <penny@suu.edu>
- Date: Fri, 3 Apr 1998 03:45:28 -0500
- Organization: Southern Utah University
Hi Groupers:
I've written a program to calculate the forces on charged particles. The
program works well except that parts are less than elegant. The key
questions I have concern the Thread function:
==========================================
Question 1:
------------
In[1]:=
Thread[f[a,{b,c}]]
Out[1]=
{f[a,b],f[a,c]}
We see that if "a" does not have head "List" it is automatically
replicated so that Thread works.
Now if a, b and c are defined as follows:
In[2]:=
a={q1,{x1,y1,z1}};b={q2,{x2,y2,z2}};c={q3,{x3,y3,z3}};
I would LIKE to have the following output when I apply Thread:
{f[{q1,{x1,y1,z1}},{q2,{x2,y2,z2}}],f[{q1,{x1,y1,z1}},{q3,{x3,y3,z3}]}
However, since "a" now has head "List", Thread now gives me:
In[3]:=
Thread[f[a,{b,c}]]
Out[3]=
{f[q1,{q2,{x2,y2,z2}}],f[{x1,y1,z1},{q3,{x3,y3,z3}}]}
Is there a way to force Thread to give me what I want?
================================================================ I can
make a workaround but its not very elegant:
In[ ]:=
t=Table[a,{2}];
Thread[f[t,{b,c}]]
Out[]=
{f[{q1,{x1,y1,z1}},{q2,{x2,y2,z2}}],f[{q1,{x1,y1,z1}},{q3,{x3,y3,z3}}]}
================================================================
Question 2:
-------------
I would like to be able to Thread g across two lists as follows:
In[1]:=
ccs={a,b}; oc={x,y};
Thread[g[ccs,oc]]
Out[1]=
{g[a,x],g[b,y]}
But unfortunately, since ccs and oc are lists, and since g is designed
to operate on lists, g executes before the thread takes place.
If g is undefined everything is ok:
In[3]:=
a={a1,a2}; b={b1,b2}; x={x1,x2}; y={y1,y2}; Thread[g[ccs,oc]]
Out[3]=
{g[{a1,a2},{x1,x2}],g[{b1,b2},{y1,y2}]}
If I now define g:
In[4]:=
g[x1_List,x2_List]:=x1+x2^2
I would now LIKE the output of the Thread statement to be:
In[6]:=
{g[{a1,a2},{x1,x2}],g[{b1,b2},{y1,y2}]}
Out[6]=
{{a1 + x1^2, a2 + x2^2}, {b1 + y1^2, b2 + y2^2}}
Instead the Thread statement now gives me:
In[7]:=
Thread[g[ccs,oc]]
Out[7]=
{{a1 + x1^2, b1 + y1^2}, {a2 + x2^2, b2 + y2^2}}
The reason (I believe) is that the g function executes before the Thread
operates. The function "List" is threaded over the sublists as in the
following:
In[9]:=
Thread[{{a1 + x1^2, a2 + x2^2}, {b1 + y1^2, b2 + y2^2}}]
Out[9]=
{{a1 + x1^2, b1 + y1^2}, {a2 + x2^2, b2 + y2^2}}
Is there a way of forcing Thread to keep its argument unevaluated until
after it operates?
=============================================== I am currently using the
following workaround, but I don't like it:
In[10]:=
Clear[f]
Thread[f[ccs,oc]] /. f->g
Out[11]=
{{a1 + x1^2, a2 + x2^2}, {b1 + y1^2, b2 + y2^2}}
===============================================
Is there a better way?
Cheers,
Des Penny
Physical Science Dept.
Southern Utah University
Cedar City, UT 84720
Office: (435) 586-7708
FAX: (435) 865-8051
email: penny@suu.edu
Home: (435) 586-2286
email: dpenny@iname.com