Thread Subject: check if variable exists

Hi,

I want to check if a variable exists before i make a decision to add
data to that variable, or create the variable if it does nopt already
exist.

in very simple terms:

if A exists
    A=A+data
else
    A=data
end

I expect my data will be in structure format, but haven't decided yet,
as cell format might be better.

my question is how do you check if the variable exists or not? all the
data will be stored in a .mat file, so i will probably have to load it
at the start of the script. If there is a way to check if variables
exist within a mat file without having to load it i would be
interested too.

Thanks
Adam

"Adam Chapman" <adam.chapman@student.manchester.ac.uk> wrote in message
news:d0fada10-feff-42f8-
>
> my question is how do you check if the variable exists or not?

help exist ?

Adam Chapman <adam.chapman@student.manchester.ac.uk> wrote in message <d0fada10-feff-42f8-a7dc-64e891aab7c4@i18g2000prf.googlegroups.com>...
> Hi,
>
> I want to check if a variable exists before i make a decision to add
> data to that variable, or create the variable if it does nopt already
> exist.
>
> in very simple terms:
>
> if A exists
> A=A+data
> else
> A=data
> end
>
> I expect my data will be in structure format, but haven't decided yet,
> as cell format might be better.
>
> my question is how do you check if the variable exists or not? all the
> data will be stored in a .mat file, so i will probably have to load it
> at the start of the script. If there is a way to check if variables
> exist within a mat file without having to load it i would be
> interested too.
>
> Thanks
> Adam

doc exist
doc isfield

the load function can be told to only load specific variables from a mat file, you could then check to see if they exist to see if they were in the file... i don't know of any way to query a mat file otherwise.
i

In article <d0fada10-feff-42f8-a7dc-64e891aab7c4
@i18g2000prf.googlegroups.com>, adam.chapman@student.manchester.ac.uk
says...
> Hi,
>
> I want to check if a variable exists before i make a decision to add
> data to that variable, or create the variable if it does nopt already
> exist.
>
> in very simple terms:
>
> if A exists
> A=A+data
> else
> A=data
> end
>
> I expect my data will be in structure format, but haven't decided yet,
> as cell format might be better.
>
> my question is how do you check if the variable exists or not? all the
> data will be stored in a .mat file, so i will probably have to load it
> at the start of the script. If there is a way to check if variables
> exist within a mat file without having to load it i would be
> interested too.
>
> Thanks
> Adam
>

help exist


--
Loren
http://blogs.mathworks.com/loren

On Dec 29, 6:13=A0pm, Loren Shure <lo...@mathworks.com> wrote:
> In article <d0fada10-feff-42f8-a7dc-64e891aab7c4
> @i18g2000prf.googlegroups.com>, adam.chap...@student.manchester.ac.uk
> says...
>
>
>
>
>
> > Hi,
>
> > I want to check if a variable exists before i make a decision to add
> > data to that variable, or create the variable if it does nopt already
> > exist.
>
> > in very simple terms:
>
> > if A exists
> > =A0 =A0 A=3DA+data
> > else
> > =A0 =A0 A=3Ddata
> > end
>
> > I expect my data will be in structure format, but haven't decided yet,
> > as cell format might be better.
>
> > my question is how do you check if the variable exists or not? all the
> > data will be stored in a .mat file, so i will probably have to load it
> > at the start of the script. If there is a way to check if variables
> > exist within a mat file without having to load it i would be
> > interested too.
>
> > Thanks
> > Adam
>
> help exist
>
> --
> Lorenhttp://blogs.mathworks.com/loren- Hide quoted text -
>
> - Show quoted text -


got it:
exist('varname','var')

didn't notice the 'var' bit in the help in my stressed out state.
Thanks.

Adam Chapman <adam.chapman@student.manchester.ac.uk> wrote in message <d0fada10-feff-42f8-a7dc-64e891aab7c4@i18g2000prf.googlegroups.com>...
> Hi,
>
> I want to check if a variable exists before i make a decision to add
> data to that variable, or create the variable if it does nopt already
> exist.
>
> in very simple terms:
>
> if A exists
> A=A+data
> else
> A=data
> end
>
> I expect my data will be in structure format, but haven't decided yet,
> as cell format might be better.
>
> my question is how do you check if the variable exists or not? all the
> data will be stored in a .mat file, so i will probably have to load it
> at the start of the script. If there is a way to check if variables
> exist within a mat file without having to load it i would be
> interested too.
>
> Thanks
> Adam

Another option to consider,

 try
     A=A+data
 catch
    A=data
end

In article
<d0fada10-feff-42f8-a7dc-64e891aab7c4@i18g2000prf.googlegroups.com>,
 Adam Chapman <adam.chapman@student.manchester.ac.uk> wrote:

[snip]

> If there is a way to check if variables
> exist within a mat file without having to load it i would be
> interested too.


  filename = 'myfile.mat';
  w = whos('-file',filename);

--
Doug Schwarz
dmschwarz&ieee,org
Make obvious changes to get real email address.

"Adam Chapman" <adam.chapman@student.manchester.ac.uk> wrote in message
news:d0fada10-feff-42f8-a7dc-64e891aab7c4@i18g2000prf.googlegroups.com...
> Hi,
>
> I want to check if a variable exists before i make a decision to add
> data to that variable, or create the variable if it does nopt already
> exist.
>
> in very simple terms:
>
> if A exists
> A=A+data
> else
> A=data
> end
>
> I expect my data will be in structure format, but haven't decided yet,
> as cell format might be better.

Others have mentioned EXIST, but if your data is in structure format you may
also need ISSTRUCT and ISFIELD.

*snip*

--
Steve Lord
slord@mathworks.com

On Dec 31 2008, 6:01=A0am, "Steven Lord" <sl...@mathworks.com> wrote:
> "Adam Chapman" <adam.chap...@student.manchester.ac.uk> wrote in message
>
> news:d0fada10-feff-42f8-a7dc-64e891aab7c4@i18g2000prf.googlegroups.com...
>
>
>
>
>
> > Hi,
>
> > I want to check if a variable exists before i make a decision to add
> > data to that variable, or create the variable if it does nopt already
> > exist.
>
> > in very simple terms:
>
> > if A exists
> > =A0 =A0A=3DA+data
> > else
> > =A0 =A0A=3Ddata
> > end
>
> > I expect my data will be in structure format, but haven't decided yet,
> > as cell format might be better.
>
> Others have mentioned EXIST, but if your data is in structure format you =
may
> also need ISSTRUCT and ISFIELD.
>
> *snip*
>
> --
> Steve Lord
> sl...@mathworks.com- Hide quoted text -
>
> - Show quoted text -

awesome! thanks fro the extra help there
Adam