Using assignin command in guide

 %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.

Answer 1

[TAB]

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

Answer 2

[C.J. Harris]

1. You cannot index into variables while using assignin. Therefore trying to assign the value 'red' to C(a,:) will not work.

2. The size of the char matrix C is a 3x3. So even if your use of assignin was correct you would get a size mismatch error when trying to assign the values of 'black' or 'blue' which are of length 5 and 4 respectively, and would not fit into your matrix.

You probably need to rethink the structure of your code. Do you really need to use assignin in this way?