Text directly to Matlab code

Hi, I was wondering if I have some text, they can be read from file or created within Matlab, can I possibly turn them directly to Matlab code, for example, I have a text

'a+b'
%And I want to assign the expression in this text to a variable, is it possible, 

c=a+b
%Or if there is some alternative ways, thank you very much

A more complex version is as follows, where I want to generate a bunch of expression using loop

fprintf(file,'[');
flag=0;%This flag determines whether this is the first time th condition is satisfied.

for i=1:size_con
    %Traverse through every Qc and lc
    for row=1:size_con
        for col=1:size_con
            if(Qc{i}(row,col)~=0)

                  if(flag>0)
                      fprintf(file,'+');
                  end
                  flag=1;

                  if(Qc{i}(row,col)==1)
                      fprintf(file,'x%d^2',row);
                  else
                      fprintf(file,'x%d*x%d',row,col);
                  end

              end
          end
      end

      if(i~=size_con)
          fprintf(file,';');
      end
  end
  fprintf(file,']');

fclose(file);

Then I will obtain a array like

[x1^2+x1*x4+x2^2+x2*x5+x3^2+x3*x6+x4*x1+x4^2+x5*x2+x5^2+x6*x3+x6^2,...
Then I will assign this to 

nlcon=x1^2+x1*x4+x2^2+x2*x5+x3^2+x3*x6+x4*x1+x4^2+x5*x2+x5^2+x6*x3+x6^2;

[Stephen23]

Related:

http://www.mathworks.com/matlabcentral/answers/263009-generate-a-vector-of-equations-using-loop