Cell conversion to double

5,102 次查看（过去 30 天）

显示更早的评论

Steven ，2011-10-17

6
链接

Direct link to this question

https://ww2.mathworks.cn/matlabcentral/answers/18509-cell-conversion-to-double

评论： Hiril Patel ，2022-6-19

采纳的回答： Jan

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 个评论
显示 2更早的评论隐藏 2更早的评论

Chrysi K. 2019-2-5

@ayesha abbassi Thank you so much!!! You helped me!!! I had a similar problem!

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Jan 2011-10-17

33
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/18509-cell-conversion-to-double#answer_24842

编辑：MathWorks Support Team 2018-11-8

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.