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.

[Steven Lord]

A cell array is a data type that can store different types and sizes together in one array. This flexibility (you can store anything in it) comes with some cost (you can store anything in it, so certain operations don't make sense to perform on it and aren't defined on it, and some operations have extra overhead as they check if the data in the array has certain characteristics the operation requires the data to have.)

For example, want to check the length of pieces of text in an array? If you use the strlength function on a string array, MATLAB knows (from its class) that it does contain text data rather than (say) numeric data.

S = "abracadabra";
strlength(S)

ans = 11

But if you pass in a cell array, it might work or it might not, depending on whether the cell array contains text data. MATLAB has to check and throw an error if the cell doesn't have text data. Imagine you had a large cell array where only the last element was not text data; MATLAB would have to look through the whole thing before deciding "The question doesn't make sense, I'll throw an error."

C = {'abracadabra'};
strlength(C) % works

ans = 11

D = {42};
strlength(D) % errors

Error using strlength
First argument must be text.

Trying to figure out what == equality means for a cell (which again could contain just about anything) and another piece of data would have so many edge/corner cases that I believe trying to explain it would just confuse people even more. So == isn't defined for cell arrays.

[Robert Demyanovich]

Thank you for that very informative yet succinct explanation of cell arrays.

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.