Error when using logical indexing to get a value from a table

I have the following snippet of code, which I created based on online tutorial:

theta = pi/4

%Create the table
Angles = {pi/2;pi/3;pi/4;pi/6;pi/12};
wValues = {0;0.8846;1.318;1.973;3.782};

%Create a table of w values at specific angles
T = table(Angles,wValues);

%Read the value of w from the table
logical_index = T.Angles == theta;
w = T.wValues(logical_index);

I keep getting the following error on the line:

logical_index = T.Angles == theta;

Operator '==' is not supported for operands of type 'cell'.

I don't understand why this is happening since I am following a code example.

Answer 1

[Stephen23]

"Operator '==' is not supported for operands of type 'cell'."

Why are you using a cell array to store scalar numeric data?

"I don't understand why this is happening since I am following a code example."

Cell arrays are not defined for logical operations like EQ. Use numeric vectors instead:

theta = pi/3;
Angles = [pi/2;pi/3;pi/4;pi/6;pi/12]; % numeric vector, NOT a cell array!
wValues = [0;0.8846;1.318;1.973;3.782]; % numeric vector, NOT a cell array!
T = table(Angles,wValues);
logical_index = T.Angles == theta;
w = T.wValues(logical_index)

w = 0.8846

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

https://www.mathworks.com/help/matlab/cell-arrays.html

[Robert Demyanovich]

Thank you. This works. I am new to MatLab and didn't realize that using curly brackets instead of regular brackets would result in a "cell array" (I don't even know what that is). As I said, I followed a code example which used curly brackets and didn't get the error.

Answer 2

[Walter Roberson]

Examine the table that gets created:

%Create the table
Angles = {pi/2;pi/3;pi/4;pi/6;pi/12};
wValues = {0;0.8846;1.318;1.973;3.782};

%Create a table of w values at specific angles
T = table(Angles,wValues)

T = 5×2 table
      Angles       wValues  
    __________    __________

    {[1.5708]}    {[     0]}
    {[1.0472]}    {[0.8846]}
    {[0.7854]}    {[1.3180]}
    {[0.5236]}    {[1.9730]}
    {[0.2618]}    {[3.7820]}

Notice that it is a 5 x 2 table, with the entries being cell arrays. Those cell array entries cannot be compared to numeric values.

There is a connection between cell arrays and tables, but it has to do with appending data to tables.

T1 = table((1:5).', (101:105).')

T1 = 5×2 table
    Var1    Var2
    ____    ____

     1      101 
     2      102 
     3      103 
     4      104 
     5      105 

data_to_add = [Angles,wValues]

data_to_add = 5×2 cell array
    {[1.5708]}    {[     0]}
    {[1.0472]}    {[0.8846]}
    {[0.7854]}    {[1.3180]}
    {[0.5236]}    {[1.9730]}
    {[0.2618]}    {[3.7820]}

T1(end+1:end+5,:) = data_to_add

T1 = 10×2 table
     Var1      Var2 
    ______    ______

         1       101
         2       102
         3       103
         4       104
         5       105
    1.5708         0
    1.0472    0.8846
    0.7854     1.318
    0.5236     1.973
    0.2618     3.782

So, an existing table can be extended by assigning a cell array.

Or you can

T2 = cell2table(data_to_add)

T2 = 5×2 table
    data_to_add1    data_to_add2
    ____________    ____________

       1.5708               0   
       1.0472          0.8846   
       0.7854           1.318   
       0.5236           1.973   
       0.2618           3.782   

[Robert Demyanovich]

When I look at Table "T" in the workspace I do not see any brackets whatsoever around the data. To me it looked like a typical table with data values.