# Reversing parts of (even) items in a listbox (cellarray)

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Jason on 18 Jan 2021
Edited: Cris LaPierre on 19 Jan 2021
Hello, I have items in a listbox (so cell array) - the integers before the "r" can be any number from 1-12.
My aim is to take any line containing an even number after r (r2, r4 etc) and reverse the other numbers and then put back into the list box
So line 2 & 4 would change to
1 2 3 r2 -> 3 2 1 r2
1 2 3 r4 -> 3 2 1 r4
This was my attempt.
items=app.ListBoxPos.Items
nl=numel(items);
rstr=["r2","r4","r6","r8"];
s1=[];
out={};
for i=1:nl
line=char(items(i));
if contains(line, rstr)
tokens = strsplit(line, 'r' )
t=fliplr(tokens{1,1})
new=[t,' r',tokens{1,2}]
app.ListBoxPos.Items{i}=new
%Not sure what to do now!
end
end
Not sure if this is the most elegant way, and it falls over trying to overwrite the line in the listbox.
Thanks for any help
Jason

Cris LaPierre on 18 Jan 2021
You can use mod to determine if the last number is even or not. I would then use indexing to reverse the order of the rows that end in an even number.
items=app.ListBoxPos.Items
nl=numel(items);
for i=1:nl
if mod(items{i}(end),2)==0
out{i}=items{i}([end-3:-1:1 end-2:end]);
else
out(i)=items(i);
end
end
app.ListBoxPos.Items = out;
Cris LaPierre on 19 Jan 2021
Edited: Cris LaPierre on 19 Jan 2021
What version of MATLAB are you using?
It's good to see you coming up with your own approach. Nice job. Just be aware that this solution suffers from the original problem - it doesn't work when the final number has 2 digits (10 or 12).
Jason on 19 Jan 2021
Im using 2020b

Jason on 19 Jan 2021
Edited: Jason on 19 Jan 2021
Here is my solution based on Cris' answer:
val = app.SNAKESCANCheckBox.Value;
switch val
case 1
items=app.ListBoxPos.Items;
nl=numel(items);
out={};
for i=1:nl
L=str2num(items{i}(end));
if mod(L,2)==0
out{i}=items{i}([end-3:-1:1 end-2:end]);
else
out{i}=items{i};
end
end
app.ListBoxPos.Items=out;
case 0
end
I thought this worked, but when I have a number 10,11 or 12 (my numbers can be 1-12), then it doesn't quite work - Its reversing the order of the double digits
Cris LaPierre on 19 Jan 2021
When the number at the end is 2 characters, it would require a different approach. Perhaps finding the 'r' and using that to set the index rather the 'end'.

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