Have a function check whether a variable already exists in the base workspace

if ~exist('A','var')
  A=[];
else
  A=evalin('base','A');
end

This code works great from a script, but not from within the function since it has its own workspace. I do not see an option to check in the base workspace.

[Joel Ngouadjeu]

ismember('var',evalin('base','who'))

this also work very fine where var is your variable.

[Steven Lord]

While it is possible to do this, it's not a good idea to do this. You're giving any function that has access to the base workspace (i.e. any function that can call assignin) the ability to control what your function does.

Imagine if you gave anyone with a web browser the ability to control the temperature setting of the burners on your stove. You'd come home one day to find the fire department outside your home (or perhaps the pile of ash and soot that used to be your home.)

Answer 1

[Sean de Wolski]

W = evalin('caller','whos'); %or 'base'
doesAexist = ismember('A',[W(:).name])

This won't check if what it is, just that it exists, but you could figure that out by looking at the other fields of W.

[Jan Berling]

This answer is not fully correct, because it checks if there is any 'A' in all variable names of the caller workspace. The square bracket creates an array of chars and should be replaced by curly brackets which create a cell array of strings.

W = evalin('caller','whos'); %or 'base'
doesExist = ismember('A',{W(:).name})

Otherwise stay with the Comment from Joel Ngouadjeu on 28 Dec 2016.

Answer 2

[Paulo Silva]

In a script you don't need the evalin because the variables are in the same scope as the workspace ones, in a function do this:

try
A=evalin('base','A');
catch
A=[];
end

The code also works in the workspace or a script but it's best just to

if ~exist('A','var'),A=[];end

Answer 3

[Tony]

function InBase=fnBaseExist(var1)
W = evalin('base','whos');
InBase=0;
for ii= 1:length(W)
  nm1=W(ii).name;
  InBase=strcmp(nm1,var1)+InBase;
  %InBase = ismember(var1,W(ii).name)+InBase
end
InBase(InBase>0)=1;

This stops an issue where the variable name was part of an existing variable.

[Jan Berling]

 function InBase=fnBaseExist(var1)
      W = evalin('base','who');
      InBase=0;
      for ii= 1:length(W)
        nm1=W{ii};
        InBase=strcmp(nm1,var1)+InBase;
      end
  InBase(InBase>0)=1;

This modification of Tonys' function reduces the calculation time, especially if there is a lot of data in the workspace. The "who" command only lists the names of the variables, whereas the "whos" command also lists the size of the variables, etc.. (With Matlab under memory stress, whos took about 1.000s and who nearly 0s.)

[Gaganjyoti Baishya]

try

x=evalin('base', 'yourVar');

catch

x=0

if ~x x is not present in base

else x is present