Trying to access element of a matrix

Question

Possible Duplicate:
Question about MapThread and Dynamic

I'm trying to extract element of a matrix. Let's suppose that the matrix is

ACC = {{1, 0, 0, 1}, {0, 0, 0, 2}, {0, 0, 0, 3}, {0, 0, 0, 1}}

then

ACC[[1,4]]

will return 3. I don't understand why Mathematica is giving me the error Part::partw when trying to get the value [[1,x]] with x>1 of the following matrix Rbar: (sorry for the code but it wont work otherwise)

Avz[\[Theta]z_, 
   dz_] := {{Cos[\[Theta]z], -Sin[\[Theta]z], 0, 0}, {Sin[\[Theta]z], 
    Cos[\[Theta]z], 0, 0}, {0, 0, 1, dz}, {0, 0, 0, 1}};

Avx[\[Theta]x_, 
   dx_] := {{1, 0, 0, dx}, {0, Cos[\[Theta]x], -Sin[\[Theta]x], 
    0}, {0, Sin[\[Theta]x], Cos[\[Theta]x], 0}, {0, 0, 0, 1}};

Linea[tz_, dz_, tx_, dx_] := Avz[tz, dz].Avx[tx, dx];

Grid[{{"", "Theta", "d", "Alpha", "a"}, {"Link1", 
   InputField[Dynamic[t1], FieldSize -> 3], 
   InputField[Dynamic[d1], FieldSize -> 2], 
   InputField[Dynamic[alpha1], FieldSize -> 4], 
   InputField[Dynamic[x1], FieldSize -> 1]}}, 
 Frame -> Darker[Gray, .6], Alignment -> {{Left, {Center}}}]


Rt = {{1, 0, 0, -L1/2}, {0, 1, 0, 0}, {0, 0, 1, 0}, {0, 0, 0, 1}};

Rbar = Dynamic[Linea[t1, d1, alpha1, x1].Rt]

here Rbar[[1,4]] for example returns error, but why?

hi @narutov6, don't forget that MMA is case sensitive. You use ACC and Acc. It's a good practice to use all user function as lower case to avoid conflict. — Murta, Jan 23 '13 at 01:23
The reason why Rbar[[1,4]] does work the way you expect is because of the incorrect use of Dynamic in Rbar. — Szabolcs, Jan 23 '13 at 01:25
Duplicate: How do I troubleshoot when I get a Part::partd or a Part::partw error? — rm -rf, Jan 23 '13 at 01:25
@rm-rf I'd say it's a duplicate of a question where someone tried using Dynamic in a similar incorrect way. I was just trying to find it, but no luck so far. — Szabolcs, Jan 23 '13 at 01:27
@Szabolcs Here: http://mathematica.stackexchange.com/q/15090/5 — rm -rf, Jan 23 '13 at 01:28
Jens's answer on your earlier question should solve this one too: "The Dynamic should wrap the expresion you want to display" — Szabolcs, Jan 23 '13 at 01:42
In addition to the above if you are using Dynamic and not extracting what you expect then try wrapping your expression in FullForm to see what you are actually operating on. — Mike Honeychurch, Jan 23 '13 at 02:43
Possible duplicate: http://mathematica.stackexchange.com/q/2972/121 — Mr.Wizard, Jan 23 '13 at 05:14
Thank you solved using: `Rbar = Linea[t1, d1, alpha1, x1].Rt;
d=Dynamic[Rbar[[1,4]]];` Thanks — UserK, Jan 23 '13 at 14:26

Trying to access element of a matrix

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