Using the exist function, on a user defined variable in a function.

So I am creating a function with a user input. In order to get full credit we have to check first to see if the input exists. The issue is that to use the exist function the variable name has to be made into a string exist('variable','var'), but the variable is an input. So my question is, how I do use the exist function in this scenario.

function M=get_mass_matrix(vibration_model, FSAE_Race_Car)
if 1 == exist('FSAE_Race_Car', 'var')
  M=FSAE_Race_Car.mass
else
  M='error, input does not exist'
end
end

Example script:

X=get_mass_matrix('1_DOF', car_2010)

where car_2010 is a predefined structure and exists and should run and give a value but

X=get_mass_matrix('1_DOF', car_2009)

where car_2009 is not a predefined structure and does not exist and should give an error message.

Answer 1

[Walter Roberson]

Your existing code does as much as can be done in MATLAB. If car_2009 does not exist, then it is not legal to make the call

   X=get_mass_matrix('1_DOF', car_2009)

so your get_mass_matrix routine would not be run.

The assignment is talking about the case where the user might call

X = get_mass_matrix

or

X = get_mass_matrix('1DOF')

with no second argument.

Note: it would also be a good idea to check isstruct() before using a field in the structure, and it would also be a good idea to check to see that the field exists in the structure before using it.

[Joshua Barton]

I used isstruct later in the code, I just didn't want to clutter up my question! Thanks for the help!

Answer 2

[dpb]

Use inputname to find the name of the variable passed to the function and check its existence. But, of course, it has to have existed or the call will have failed when it attempts to run so the test inside the function is useless.

Example from the workspace here--

>> whos
  Name                Size             Bytes  Class      Attributes

    STRS                1x2                154  cell                 
    ans                 2x2                294  cell                 
    data                2x1                 16  double               
    ds                  2x2               1290  dataset              
    filecontent         1x264              528  char                 
    fmt                 1x13                26  char                 
    fmts                1x2                128  cell                 
    i                   1x1                  8  double               
    keys                9x1                694  cell                 
    keyvaluepairs      13x2               1848  cell                 
    rows               13x1                104  double               
    s                  17x1               1480  cell                 
    txt                 2x1                138  cell                 
    values              9x1               2388  cell                 
    x                   2x2                 32  double               
    y                   2x2                 32  double               

>> mean(z)
Undefined function or variable 'z'. 
>>

There is no z so if try to call a function with it; no can do. So, mean in this case checking on the existence of z is futile; it'll never get there to do so.

[Joshua Barton]

So I use exist(inputname(2),'var')?

I have to have a user output error message, not a matlab designated one.

[dpb]

The point is that the function can't ever get called if the argument(s) don't exist. That particular error can never happen.

[Joshua Barton]

Thanks for the help!