This is something I struggled with when I first learned how to build GUIs. The way you have your code set up, a call to Question() runs through and does not call helpme() because the key press is never registered. In other words, Question() executes to the end and realizes that "answer" isn't assigned, so you see the error. What you actually need to do is move helpme() outside of Question() and use helpme() to return "answer".
function Question
hfig = figure('KeyPressFcn',@helpme);
end
function answer = helpme(src,event)
if strcmp(event.Key,'a')
disp('a key pressed')
answer = 'answer';
end
end
In this code, Question() is creating some GUI with the a key press callback. The callback function is the one generating the output.
