switch case & operators

if (pop==1)
    w=str2double(get(handles.edit_kg,'string'));
    h=str2double(get(handles.edit_cm,'string'));
    body=1e4*(w)/(h^2);
    set(handles.textbmi,'string',sprintf('%.2f',body));
    switch (body)
    case (body < 17)
        s = 'Underweight';
    case (body >=17) && (body < 23)
        s = 'Normal';
    case (body >= 23) && (body < 27)
        s = 'Overweight';
    otherwise
        s = 'Obese';
    end
     set(handles.textstatus,'string',s);
     handles.textstatus = s;
end

[Sela Uguy]

I'm writing a BMI App, but the Obese kept printing. Why?

[dpb]

One would then conclude that the value of body is not numeric and under 27

>> body=1;    switch (body)
    case (body < 17)
        s = 'Underweight';
    case (body >=17) && (body < 23)
        s = 'Normal';
    case (body >= 23) && (body < 27)
        s = 'Overweight';
    otherwise
        s = 'Obese';
    end
>> s
s =
    'Underweight'
>> body=nan;
>>     switch (body)
    case (body < 17)
        s = 'Underweight';
    case (body >=17) && (body < 23)
        s = 'Normal';
    case (body >= 23) && (body < 27)
        s = 'Overweight';
    otherwise
        s = 'Obese';
    end
>> s
s =
    'Obese'
>> 

shows the SWITCH block works; ergo the value going in must not be what you think.

Set a breakpoint at the line computing body to see what's going on and find logic errors...

Answer 1

[Jan]

This is not the purpose of SWITCH/CASE. Please read the documentationm again:

doc switch

SWITCH evaluates its argument, ibn your case the variable body. Then it compares it with the expressions after the case statements. If body is e.g. 18, you get:

switch 18
    case FALSE   % This is the value of (body < 17)
        ...
    case TRUE    % Value of (body >=17) && (body < 23)
        ...            

But 18 is neither TRUE nor FALSE.

You want an IF command instead of SWITCH.

    if (body < 17)
        s = 'Underweight';
    elseif (body >=17) && (body < 23)
        s = 'Normal';
    elseif (body >= 23) && (body < 27)
        s = 'Overweight';
    else
        s = 'Obese';
    end    

You can simplify this: After body<17 has been excluded already, there is no need to check for body >= 17 again. So htis is sufficient:

    if (body < 17)
        s = 'Underweight';
    elseif (body < 23)
        s = 'Normal';
    elseif (body < 27)
        s = 'Overweight';
    else
        s = 'Obese';
    end