SWITCH expression must be a scalar or a character vector. issue

Hi im trying to calculate price of 1,2,3 or more glass of milk. 1 glass of milk must be 1.75$ 2 is 3.0$ 3 is 3.5$ and for more glass 3.5$ + 0.5+ for every glass, but i am getting this error "SWITCH expression must be a scalar or a character vector". Any idea ? i check forum before but answer i get dont solve my problem thx for any help. Btw my proffesor only accept the homework if i use "inputdlg" "msgbox(sprintf(", and switch case stracture.

box=inputdlg('how many glass of milk');
switch box
    case '1'
        price=1.75;
    case '2'
        price=3;
    case '3'
        price=3.5;
    otherwise
        price=3.5+(0.5)*x;
end
msgbox(sprintf('you must pay %f',price));

Answer 1

[Alex Mcaulley]

inputdls is returning a cell, and also the variable x is not defined. Then you need to do something like this:

box=inputdlg('how many glass of milk');
switch box{1}
    case '1'
        price=1.75;
    case '2'
        price=3;
    case '3'
        price=3.5;
    otherwise
        price=3.5+(0.5)*str2double(box{1});
end
msgbox(sprintf('you must pay %f',price));

[Guillaume]

If you want the code to be robust (remember that user input can be completely different from what you expected), I would do the conversion to double before the switch, then check it is indeed a number and scalar, so:

box = inputdlg('how many glass of milk');
box = str2double(box{1});  %will be nan if box{1} is not a number
assert(~isnan(box) && isscalar(box), 'Invalid input');
switch box
    case 1  %no longer a char array
        %...
end