Preallocation Problem in for loop

I have a function which takes a cell array "c" and checks for the length of x and a part of the code is:

str = [];
for i=1:size(c,1)
    str = [str;extractBetween( func2str(c{i,1}),'x(',')')];
end

I get the warning that str is not preallocated.

%Examples, how str looks like
c = cell(1,1);
c{1} = @(x) 10*x(1)-x(2)^2
%here str would be a 5 x 1 cell array :
    {'1'}
    {'2'}
    
%or:
c = cell(2,1);
c{1} = @(x) 10*x(1)-x(1)^2
c{2} = @(x) x(1)^2+4*x(2);
%here str would be a 4 x 1 cell array :
    {'1'}
    {'2'}
    {'1'}
    {'2'}

Would it be possible to preallocate str?

Answer 1

[Voss]

If you know that every element of c has the same number of instances of 'x(...)', i.e., you know extractBetween returns the same size output for each element of c, then it's easy to pre-allocate str:

c = cell(2,1);
c{1} = @(x) 10*x(1)-x(1)^2;
c{2} = @(x) x(1)^2+4*x(2);

num_x = 2; % two x(...) matches in each element of c
str = cell(num_x*numel(c),1); % pre-allocate
for i=1:size(c,1)
    str(num_x*(i-1)+(1:num_x)) = extractBetween( func2str(c{i,1}),'x(',')');
end
disp(str);

    {'1'}
    {'1'}
    {'1'}
    {'2'}

However, it may be better to do it in a way that handles differing numbers of matches in each element of c and doesn't require pre-allocation because there's no loop:

% make a new element of c with 3 x's:
c{end+1} = @(x) x(1)^2+4*x(2)+10*x(3);
% use cellfun instead of a loop:
str = cellfun(@(x)extractBetween(func2str(x),'x(',')'),c,'UniformOutput',false);
str = vertcat(str{:})

str = 7×1 cell array
    {'1'}
    {'1'}
    {'1'}
    {'2'}
    {'1'}
    {'2'}
    {'3'}