"Function definitions are not supported in this context. Functions can only be created as local or nested functions in code files."
Like the error message says, function definitions are not supported inside control blocks (if, else, while, for, etc.). You'd have to define the function processKey outside any control block in your script.
Since your intent is for processKey to set variables that need to be accessed within the while loop of the script (snakex, snakey, LastInput), it is convenient to make processKey a nested function (which requires making your script into a function) since nested functions share variables with their parent function.
Some construction as follows might be close to what you had in mind:
function snake_game()
fig = figure();
% Only need to set the figure's KeyPressFcn once, before the while loop.
% (processKey() will be called whenever the user presses a key while the
% figure has focus.)
set(fig, 'KeyPressFcn', @processKey)
axis([0, 20, 0, 20]);
snakex = randi([1,19],1);
snakey = randi([1,19],1);
foodx = randi([1,19],1);
foody = randi([1,19],1);
alive = true;
% initialize LastInput here in the parent function so that
% processKey can modify it
LastInput = '';
while alive
figure(fig);
Food = plot(foodx,foody,'db'); hold("on");
Snake = plot(snakex,snakey,'og'); pause(0.1); hold("on");
grid("on"); axis("tight"); axis([0, 20, 0, 20])
if snakex == 0
alive = false;
end
if snakex == 20
alive = false;
end
if snakey == 0
alive = false;
end
if snakey == 20
alive = false;
end
end
function processKey(~,evnt)
if length(evnt.Key) >= 1
switch evnt.Key
case 'leftarrow'
snakex = snakex - 1;
LastInput = 'left';
case 'rightarrow'
snakex = snakex + 1;
LastInput = 'right';
case 'downarrow'
snakey = snakey - 1;
LastInput = 'down';
case 'uparrow'
snakey = snakey + 1;
LastInput = 'up';
otherwise
switch LastInput
case 'left'
snakex = snakex - 1;
case 'right'
snakex = snakex + 1;
case 'down'
snakey = snakey - 1;
case 'up'
snakey = snakey + 1;
end
end
end
end
end
