Logical indexing: Find row in table by text in column

    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]

"Are there other ways, better ways, nicer ways?"

Yes: use strcmp, strcmpi, etc. Do not use == for testing for string/character array equivalence.

Answer 1

[Star Strider]

To compare strings use the strcmp (link) or strcmpi function:

u = t(strcmp(t.a1, 'don'), :);

u =

    1×2 table

       a1        a2  
      _____    ______

      'don'    'tcry'

[KAE]

Just another way to use Star Strider's solution. If you want the index to the row containing your desired value,

iRow = find(strcmp(t.a1, 'don')==1);

[Guillaume]

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.

Answer 2

[Guillaume]

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.

Answer 3

[Kristupas Karcemarskas]

I found that it is really easy to use categorise() function

for example:

u=categorise(u);

Answer 4

[Peter Perkins]

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.

[Moshe Flam]

1) Could you edit your answer and show how to make them row names?

2) Could you edit your answer and show how to make 'hi' and 'don' a categorical vector?

And in case 2 - two questions:

2a. Will I not get the same as I had with my original question? Reminder

u = t(t.a1 == 'don', :);  % error: Undefined operator '==' for cell

2b. Will it be correct to use t.a1 == 'don' or should I use strcmp here too?

3. Just a last remark: Guillaume confirmed what I said and explained when and why it was made to work, but did not suggest it.

[Peter Perkins]

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).

Answer 5

[Gabor]

b = [1,2 ; 3,4];

q = array2table(b);

r = q(b1 == 3, :);

Unrecognized function or variable 'b1'.

[Gabor]

b = [1,2 ; 3,4];

q = array2table(b);

r = q(q.b1 == 3, :);

This is how it works