Conversion problem of data types of an array

Hi all, I read with textscan from a text file to get an array. I received this error report at first

"All contents of the input cell array must be of the same data type."

for my array "data_A".

data_A:
522x1 double       522x1 double       522x1 cell       522x1 double

I tried to do this:

data_A(:,3) = str2double(data_A(:,3));

and I received this report:

"Conversion to cell from double is not possible."

Hence, I tried to use cell2mat:

data_A(:,3) = cell2mat(data_A(:,3));

for which I got the same error again:

"All contents of the input cell array must be of the same data type."

It would be so kind, if anyone could look into the matter. Any help and tips are very much appreciated. =)

Best regards,

Boon

[KL]

and what is inside data_A(:,3)?

[cbh0307]

It's the wavelength Index and it is written as like ',0' as normal %f cannot read it. I read it using %s instead compared to other column.

[dpb]

So, what is in the data record that is screwing up the reading of what looks as should be number? The ',' is a normal delimiter so that shouldn't be the problem.

Fix the problem at its root rather than try to post-process a mess created where needn't be (at least until can prove it must be a mess).

Show an offending record and how you tried to read it.

[cbh0307]

Thank you dpb!

Answer 1

[Guillaume]

I'm with dpb, fix the code so that it does not make a mess in the first place rather than trying to fix the mess.

It sounds like you're importing some data with something like textscan which needs you to explicitly specify the format of the text file. Since R2013b, matlab has included readtable which is usually capable of figuring out the format (and headers) on its own. Perhaps, moving to more modern constructs would solve the problem.

Most time all that is needed is:

t = readtable(yourfile);

Nothing more, and if readtable gets it wrong, you can guide it with detectImportOptions and specify the actual format for these columns where it gets it wrong.

[cbh0307]

Hello Guillaume, thank you for the tips on 'readtable'. It is indeed helpful.

I have solved the problem after I noticed that what I have read from data_A is all the datas along the row (in the same column) being saved as 522x1 double in the first row only (one cell) instead of displaying all datas along the row. I have tried this

for i = 1:4
     data_A_new(:,i) = cell2mat(data_A(1,i));
end

I have made this the length to be 4 here instead of 499867 x 23 for a better view and discussion here. Although it somehow shows this warning:

The size of the indicated variable or array appears to be 
changing with each loop iteration. Consider preallocating for speed.

The codes do run smoothly and the plot is nice in the end.

I honored the answers and tips from you guys. Thanks again for your attention and time.

Best,

Boon

[Guillaume]

Consider preallocating for speed. Most likely, you haven't preallocated data_A_new, so at each step of your loop, matlab resizes it make room for the row you add.

In addition, you need to learn how to access the content of a cell array. data_A{1, i} (note the curly braces) is a lot more efficient than cell2mat(data_A(1, i)).

So, with that said:

data_A_new = zeros(numel(data_A{1, 1}, size(data_A, 2));  %preallocation
for i = 1 : size(data_A, 2)   %don't hardcode sizes
   data_A_new(:, i) = data_A{1, i};
end

But, as usual with matlab, the above can be achieved more simply without a loop:

%if the content of data_A{1, i} is all column vectors:
data_A_new = cell2mat(data(1, :));

%if the content of data_A{1, i} is all row vectors:
data_A_new = cell2mat(data(1, :)).';

%otherwise:
data_A_new = cellfun(@(x) x(:), data_A(1, :), 'UniformOutput', false);  %to reshape every matrix into column vectors
data_A_new = cell2mat(data(1, :));

[cbh0307]

Thanks again Guillaume!! I did learned a lot from your comment. The warning is no more as well. My gratitude.