how to get element from matrix

function btn_forward_Callback(hObject, eventdata, handles)
% hObject    handle to btn_forward (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
Th_1 = str2double(get(handles.Theta_1,'string'))*pi/180;
Th_2 = str2double(get(handles.Theta_2,'string'))*pi/180;
Th_3 = str2double(get(handles.Theta_3,'string'))*pi/180;
Th_4 = str2double(get(handles.Theta_4,'string'))*pi/180;
L_1 = 10;
L_2 = 20;
L_3 = 30;
L_4 = 10;
L(1) = Link([0 L_1 0 pi/2]);
L(2) = Link([0 0 L_2 0]);
L(3) = Link([0 0 L_3 0]);
L(4) = Link([0 0 L_4 0]);
Robot = SerialLink(L);
Robot.name = 'anosh';
Robot.plot([Th_1 Th_2 Th_3 Th_4]);
T = Robot.fkine([Th_1 Th_2 Th_3 Th_4]);
handles.Pos_X.string = num2str(floor(T(1,4)));

function Pos_X_Callback(hObject, eventdata, handles)

there is error

Index exceeds matrix dimensions.

Error in ggg>btn_forward_Callback (line 188)
handles.Pos_X.string = num2str(floor(T(1,4)));

Error in gui_mainfcn (line 95)
        feval(varargin{:});

Error in ggg (line 42)
    gui_mainfcn(gui_State, varargin{:});

Error in
matlab.graphics.internal.figfile.FigFile/read>@(hObject,eventdata)ggg('btn_forward_Callback',hObject,eventdata,guidata(hObject)) 
Error while evaluating UIControl Callback.

[Geoff Hayes]

ans - what can you tell us about T? From the code

T = Robot.fkine([Th_1 Th_2 Th_3 Th_4]);
handles.Pos_X.string = num2str(floor(T(1,4)));

you are assuming that T has (at least) four columns but the error message is telling you that the index exceeds the matrix dimensions...so T may not have four columns.

[ans sobhi]

T is th transformation matrix of robot arm and it has fixed columns that equal four

[Adam Danz]

Please copy the value of T from the command window and paste it here. I'm guessing T does not have 4 columns.

[ans sobhi]

T =

1 0 0 60

0 0 -1 0

0 1 0 10

0 0 0 1

[ans sobhi]

this is value of T at theta equal zero

[Geoff Hayes]

Is this always true? Is there perhaps some cases where T is not a 4x4 matrix? Perhaps try adding the following checks before updating the position

T = Robot.fkine([Th_1 Th_2 Th_3 Th_4]);
if size(T,2) >= 4 % ensure that there at least four columns
    handles.Pos_X.string = num2str(floor(T(1,4)));
else
    fprintf('T does not have four columns\n ');
end

[ans sobhi]

but in all cases and all values the matrix has four columns

[Adam Danz]

According to the error message, at least sometimes T does not have 4 columns. I would add an sanity check similar to what Geoff suggested. If you put this conditional statement in your code just prior to the line causing error, you'll know if T is not the expected size.

T = Robot.fkine([Th_1 Th_2 Th_3 Th_4]);

if size(T,2) < 4
    error('T does not have at least 4 columns.')
end

handles.Pos_X.string = num2str(floor(T(1,4)));

After doing so, run your code and I bet you'll see the error message indicated above.

If you do not see the error message indicated above, there's something fishy going on.

• Perhaps 'floor' is a global variable and you're trying to index it. (unlikely but something to check)
• The same could be true for 'num2str' (again, unlikely).

[ans sobhi]

i solved it by this code

T = T.T;

set(handles.Pos_X,'string',T(1,4));

set(handles.Pos_Y,'string',T(2,4));

set(handles.Pos_Z,'string',T(3,4));

[Geoff Hayes]

you many want to rename the T in

T = Robot.fkine([Th_1 Th_2 Th_3 Th_4]);

to avoid confusion with the field that you are accessing of the same name.

[Adam Danz]

This suggests that T was not originally a matrix with 4 columns but instead, a structure. When you tried to get data from the 4th "column" of a structure, you got the error.

[Arianna Arianna Clemente]

Hi, have you managed to run also the inverse kinematics?

Answer 1

[Adam Danz]

Here's a summary of the comments under the question so that this question is marked as answered.

The variable "T" is a structure but was being accessed as if it were a matrix.

Here's a replication of the error

T.T1 = magic(4);          % a 4x4 matrix stored in field 'T1' of structure "T"
T(1,4)

Index exceeds matrix dimensions.   %matlab 2017b

Index in position 2 exceeds array bounds (must not exceed 1).   %Matlab 2018a

Here's the solution

T.T1(1,4)
ans =
    13