Problem over switch function

So this is what I have so far.

and here is the problem:

I've made the switches, and when I go to HELP SWITCH, it shows me how to do the switch, but I don't really understand how to integrate it within the problem. I was trying to figure out if there was a way to input the element needed, and then it would add the values of a and b, and then I could execute the equation, but I can't really figure out how to go about doing that (if I'm right).

[Joel]

So this is what I have so far now. I'm trying to use an input function so you can choose what element you want, and apparently I'm doing something wrong... or maybe I'm using an input function in the wrong way, but every time I type one of my elements it says it doesn't have a value.

 T = 300 ; V = 20; R = .08206;
myelement = input('My element: ');
switch (myelement)
    case 'he'
        a= .0341 ; b = .0237;
    case 'hydrogheen'
        a = .244 ; b = .0266;
    case 'oxygen' 
        a = 1.36 ; b = .0318;
    case 'chlorine' 
        a = 6.49 ; b = .0562;
    case 'carbondioxide'
        a = 3.59 ; b = .0427;
end 
P = (R*T)/ (V - b) - a / V.^2;

Answer 1

[Adam]

Your code works, but you have to input the element as:

'he'

(with the quotation marks), not just

he

which Matlab tries to evaluate as a function or variable rather than a string.

If you want to get fancy you can also use

validatestring

to ensure a sensible input string (this also allows you to input, for example just 'o' if one of your defined valid strings is 'oxygen' and it will convert 'o' to 'oxygen' on output, though, of course, just 'h' would be ambiguous.

By the way, your spelling of 'Hydrogen' is a little off too which may cause a failure if the user types it in correctly at the command line.

Answer 2

[Guillaume]

Please don't post images of code when you could just copy and paste the code.

Your switch syntax is correct. However, for it to be useful, you need to put it before the equation not after.

If you'd copy-pasted the code, I would have copied it below and rearranged it for you. Can't do that with a picture.

[Joel]

So this is what I have so far now. I'm trying to use an input function so you can choose what element you want, and apparently I'm doing something wrong... or maybe I'm using an input function in the wrong way, but every time I type one of my elements it says it doesn't have a value.

 T = 300 ; V = 20; R = .08206;
myelement = input('My element: ');
switch (myelement)
    case 'he'
        a= .0341 ; b = .0237;
    case 'hydrogheen'
        a = .244 ; b = .0266;
    case 'oxygen' 
        a = 1.36 ; b = .0318;
    case 'chlorine' 
        a = 6.49 ; b = .0562;
    case 'carbondioxide'
        a = 3.59 ; b = .0427;
end 
P = (R*T)/ (V - b) - a / V.^2;

Answer 3

[Julia]

Hi,

you have to write a function:

function P = VanDerWaals(T,V,e)
% switch-case
% computation of P

Since R is the same for every gas you can enter the value directly in the equation.

[Shammi Doly]

hi,

I am trying to read data from the RADAR sensor through the USB port. I used switch function to read the real time data. But I am getting some error like this.'Not enough input arguments.Error in f_serial (line 2).

function p = f_serial(p,str)
    switch(str)
        case 'clear'
            a = instrfindall();
            if(isempty(a)==0)
                fclose(a);
                delete(a);
            end
        case 'openBGT'
            try
                p.sr = serial(p.serialPortBGT,'BAUD',9600);
                set(p.sr,'Timeout',0.5);
                set(p.sr,'InputBufferSize',8192);
                fopen(p.sr);
            catch
                disp('Error while opening USB Connection. Check the connection or the portname');
                return
            end
        case 'closeBGT'
            fclose(p.sr);
            delete(p.sr);
    end

[Steven Lord]

That suggests you called f_serial with only one input. If you did, when MATLAB tries to evaluate the switch expression str is undefined. MATLAB can see that str was defined to be an input argument to your function and so gives you a (hopefully) more informative error, that you called your function with fewer inputs than your function requires.

Call your function with the two inputs that it requires.