MATLAB: Using assignin command in guide

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 %The below line is in base workspace
  C = char('red','red','red');
 %The below code is in gui created by guide.
 a = get(handles.popupmenu1,'Value') % Based on value from popup menu1 i have  to decide where i should set option selected in popup menu2
 val = get(handles.popupmenu2,'Value');
 switch val
    case 1
        assignin('base','C(a,:)','red');
    case 2
        assignin('base','C(a,:)','blue');
    case 3
        assignin('base','C(a,:)','black');
 end
It shows an error Invalid variable name "C(a,:)" in ASSIGNIN.

Best Answer

switch val
    case 1
        evalin('base',['C(' num2str(a) ',:)=''abc''']);
    case 2
        evalin('base',['C(' num2str(a) ',:)=''lmn''']);
    case 3
        evalin('base',['C(' num2str(a) ',:)=''xyz''']);
end
As your matrix C is predefined as 3x3, you can not assign C(a,:)='blue' because 'blue' has 4 columns while all rows in your matrix contain 3 columns.
Use cell array for such operations.
Using cell array, in base workspace
C={'red','red','red'};
In GUI function
 switch val
      case 1
          evalin('base',['C{' num2str(a) '}=''red''']);
      case 2
          evalin('base',['C{' num2str(a) '}=''blue''']);
      case 3
          evalin('base',['C{' num2str(a) '}=''black''']);
  end
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