how to display a error message then proced further when the string doesnot matches by using for and if loop

this is my program

list={'AOA','CAS','IAS','ALT'};
prompt='Enter variable name';
a=input(prompt,'s');
b=strcmp(a,list);

if input does not matches with the list of variables then it must show an error 'Variable not found, please try again' (for three times) & if the user enters invalid variable for three times the program should be terminated.

Answer 1

[Chandrasekhar]

try
....your code
catch exception
disp(exception.message)
end

Answer 2

[Ravikant Singh]

Its not working

Answer 3

[Walter Roberson]

trycount = 0;
while trycount < 3
  ....
  if no match
     trycount = trycount + 1
  end
end
if trycount >= 3
  user blew it
end

[Ravikant Singh]

just have a look on my program once again the last statement will store 1 or 0 in variable now i want to display an error when m=0 so how to do ??