# How do I sort data by appending a value to an array that may or may not exist?

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Matt Brown on 26 Jan 2018
Answered: Walter Roberson on 26 Jan 2018
I have a large data set that I need to sort into several categories. I have attempted to use the exist command and some if/else statements to determine if it already exists. If it does not, then the variable is created and stored. If it does exist, the value is appended onto the end of the existing array... You will have to forgive the trivialness of the example, it would have been too complicated to share my actual code on this. However, this example does illustrate the issue.
input={'A','A','B','B';5,6,7,8;50,60,70,80;500,600,700,800};
[m,n]=size(input)
for j=1:n
if strcmp(input{1,j},'A')==1
if exist('data.A.row2')==0
data.A.row2=input{2,j};
else
data.A.row2(end+1)=input{2,j};
end
if exist('data.A.row3')==0
data.A.row3=input{3,j};
else
data.A.row3(end+1)=input{3,j};
end
if exist('data.A.row4')==0
data.A.row4=input{4,j};
else
data.A.row4(end+1)=input{4,j};
end
elseif strcmp(input{1,j},'B')==1
if exist('data.B.row2')==0
data.B.row2=input{2,j};
else
data.B.row2(end+1)=input{2,j};
end
if exist('data.B.row3')==0
data.B.row3=input{3,j};
else
data.B.row3(end+1)=input{3,j};
end
if exist('data.B.row4')==0
data.B.row4=input{4,j};
else
data.B.row4(end+1)=input{4,j};
end
end
end
**
This is confusing to me because if I simply enter the following commands into the command line it will append values as I expected. I know it is something stupid on my end, I am just not sure what it is.
data.A.row2=5;
data.A.row2(end+1)=6;
data.A.row3=50;
data.A.row3(end+1)=60;
data.A.row4=500;
data.A.row4(end+1)=600;
data.B.row2=7
data.B.row2(end+1)=8
data.B.row3=70
data.B.row3(end+1)=80
data.B.row4=700
data.B.row4(end+1)=800

Walter Roberson on 26 Jan 2018
exist() applied to a structure reference returns 0. If you want to know whether a field exists use isfield:
if isfield(data.B, 'row4')