# Logical indexing: Find row in table by text in column

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Moshe Flam on 6 Feb 2018
a = {'hi', 'bye'; 'don', 'tcry' };
t = array2table(a);
u = t(t.a1 == 'don', :); % error: Undefined operator '==' for cell
How do I do a logical search for the row where a1 is 'bye'?
If it was numbers, it would be easy:
b = [1,2 ; 3,4];
q = array2table(b);
r = q(b1 == 3, :); # works perfectly
OK I found one way: Use string().
g = t(string(t.a1)=="don", :); % works! ;-)
Are there other ways, better ways, nicer ways?
Stephen23 on 6 Feb 2018
Edited: Stephen23 on 6 Feb 2018
"Are there other ways, better ways, nicer ways?"
Yes: use strcmp, strcmpi, etc. Do not use == for testing for string/character array equivalence.

Star Strider on 6 Feb 2018
To compare strings use the strcmp (link) or strcmpi function:
u = t(strcmp(t.a1, 'don'), :);
u =
1×2 table
a1 a2
_____ ______
'don' 'tcry'
##### 2 CommentsShowHide 1 older comment
Guillaume on 21 Jun 2019
Note that beginners tend to use find more than they should, you typically see:
indices = find(somearray == somevalue);
result = somerarray(indices);
where find wasn't needed at all and was just a waste of time:
isfound = somearray == somevalue;
result = somearray(isfound);
It's actually rare that you do need the indices.

Guillaume on 6 Feb 2018
Edited: Guillaume on 6 Feb 2018
Comparison of char arrays is always done with strcmp, never with ==. Thus:
u = t(strcmp(t.a1, 'don'), :)
The string class introduced in r2016b overloads == so that it can be used for comparison , so converting the char arrays to strings is indeed another solution.

Peter Perkins on 7 Feb 2018
Two other things worth considering:
1) if {'hi', 'bye'; 'don', 'tcry'} are always unique, consider making them row names. Then t('don', :) is what you would use.
2) If {'hi', 'bye'; 'don', 'tcry'} could be a large list of repeated values, consider making them a categorical vector. Then u = t(t.a1 == 'don', :) is what you would use.
But even if those are not an option, if you are using R2016b or later, consider using a string array instead of a cellstr, as Guillaume says.
##### 2 CommentsShowHide 1 older comment
Peter Perkins on 12 Feb 2018
To make them row names, it's just
r.Properties.RowNames = t.a1;
t.a1 = []; % delete the original variable
but if the data are coming from a file you may well be able to read them in as row names. See the doc for readtable.
To make a categorical, it's just
t.a1 = categorical(t.a1)
but you will probably want to look over the documentation for categorical arrays. Once t.a1 is categorical, you can use ==. It's not the main reason for using categoricals, but it's a convenience (one that string also provides).

Gabor on 3 Mar 2021
b = [1,2 ; 3,4];
q = array2table(b);
r = q(b1 == 3, :);
Unrecognized function or variable 'b1'.
Gabor on 3 Mar 2021
b = [1,2 ; 3,4];
q = array2table(b);
r = q(q.b1 == 3, :);
This is how it works

Kristupas Karcemarskas on 17 Apr 2022
I found that it is really easy to use categorise() function
for example:
u=categorise(u);