MATLAB Answers

Cell conversion to double

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Steven
Steven on 17 Oct 2011
Commented: Nishaban P.K. on 21 Apr 2020
Greetings,
Let's say a is a 11x1 cell like this
a =
'0.000000'
'1.000000'
'2.000000'
'3.000000'
'4.000000'
'5.000000'
'6.000000'
'7.000000'
'8.000000'
'9.000000'
'10.000000'
and I want to convert it in double. If I try b=cell2mat(a), I got the following error :
??? Error using ==> cat
CAT arguments dimensions are not consistent.
Error in ==> cell2mat at 85
m{n} = cat(1,c{:,n});
However, I know I can bypass it with a loop with 2 conversion as:
for i = 1:length(a)
b(i) = str2num(cell2mat(a(i)));
end
Thus, I wonder if there is a simpler way to do this with an easy one-step function.
Regards,
Steven

  3 Comments

Barry Swindler
Barry Swindler on 30 May 2016
not sure if you found an answer yet, but a simple thing like this works.
m=zeros(size(a,1),size(a,2));
m=str2double(a);
still not just one step, but there's no need for a loop.
hope this helps!
ayesha abbassi
ayesha abbassi on 24 Feb 2018
B = cell2mat(A). now check its type by writing whos B in command window
Chrysi K.
Chrysi K. on 5 Feb 2019
@ayesha abbassi Thank you so much!!! You helped me!!! I had a similar problem!

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Accepted Answer

Jan
Jan on 17 Oct 2011
Edited: MathWorks Support Team on 8 Nov 2018
To convert a cell array of character vectors to numbers, you can use the str2double function. This function is the simplest method.
C = {'0.000000'; '10.000000'; '100000.000000'};
M = str2double(C)
The cell2mat function converts a cell array of character vectors to a character array, and only if all the character vectors have the same length. cell2mat also preserves the data type of the contents of the cells, so it does not convert characters to numbers.
If you need your code to be fast, then use the following code instead. This code is faster than str2double:
C = {'0.000000'; '1.000000'; '2.000000'; ...
'3.000000'; '4.000000'; '5.000000'; '6.000000'
'7.000000'; '8.000000'; '9.000000'; '10.000000'};
S = sprintf('%s*', C{:});
N = sscanf(S, '%f*');
Unfortunately sprintf seems to forget a proper pre-allocation. This C-Mex is 4 times faster: FEX: CStr2String:
S = CStr2String(C, '*');
N = sscanf(S, '%f*');
Timings in 2011b, Core2Duo:
n = 100;
C = cell(1, n);
for iC = 1:n; C{i} = sprintf('%f', i); end
tic; for i=1:1000; N = cellfun(@(x)str2double(x), C); end; toc
>> 3.61 sec
tic; for i=1:1000; N = cellfun(@(x) sscanf(x, '%f'), C); end; toc
>> 3.01 sec
tic; for i=1:1000; N = str2double(C); end; toc
>> 2.79 sec
tic; for i=1:1000; N = cellfun(@str2double, C); end; toc
>> 2.49 sec
tic; for i=1:1000;
N = zeros(1,100); for j=1:100; N(j) = sscanf(C{j}, '%f'); end;
end; toc
>> 1.40 sec
tic; for i=1:1000; N = sscanf(sprintf('%s*', C{:}), '%f*'); end; toc
>> 0.14 sec
tic; for i=1:1000; N = sscanf(CStr2String(C, '*'), '%f*'); end; toc
>> 0.071 sec
To my surprise a full implementation in C is slower than sscanf(sprintf()), see FEX: String to double. Matlab's sscanf seems to be much better than the MSVC implementation.

  7 Comments

Show 4 older comments
Jan
Jan on 18 Oct 2011
Inserted: N(j)=sscanf(C{j}, '%f').
hello_world
hello_world on 4 Jul 2018
@Jan great example, but it does not help if the cell contains strings. I have raised a question with that problem at: https://www.mathworks.com/matlabcentral/answers/408675-how-to-convert-a-csv-file-of-cell-array-type-to-double
Can you please help with that?
Nishaban P.K.
Nishaban P.K. on 21 Apr 2020
sir, I use str2double function ,it return some binary values like '00000111' into 111. why?

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More Answers (1)

Daniel Shub
Daniel Shub on 17 Oct 2011
It appears your cell array contains strings and not numbers (doubles).
b = cellfun(@(x)str2double(x), a);

  6 Comments

Show 3 older comments
Jan
Jan on 17 Oct 2011
"cellfun(@str2double, C)" is faster than "str2double(C)". Surprising! But the indirection "cellfun(@(x)str2double(x), C)" wastes time.
Fangjun Jiang
Fangjun Jiang on 17 Oct 2011
+1, For none-time-critical task, I still vote for using str2double().
hello_world
hello_world on 4 Jul 2018
@Daniel Shub: It converts all cell entries (which are string) into NaN values. I have raised a question at: https://www.mathworks.com/matlabcentral/answers/408675-how-to-convert-a-csv-file-of-cell-array-type-to-double
Can you please look into that?

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