Find elements in table without looping (Matlab)

As this seems to be a quite common problem, I have spent a while trying to find a suitable answer for my case in other posts. However, most of them are full of maths and I cannot see the solution very clear.

This is my case:

I generated a function that imports data into Matlab in form of a table. The table looks like:

This table has around one million of rows. Basically, I need to extract from this table just the rows where the table1.ID matches 'DHK' and leave the result in another table.

This is the code I have just now:

 cont=0;

 for p=1:length(table1.ID)

            ID=table1.ID(p,1);
            IDstr=ID{1,1};

            if strcmp(IDstr,'DHK')==1
                cont=cont+1;
                var1(cont,1)=table1.var1(p,1);
                var2(cont,1)=table1.var2(p,1);
                var3(cont,1)=table1.var3(p,1);
                var4(cont,1)=table1.var4(p,1);
            end
  end

 table2=table(var1,var2,var3,var4);

This code works but it takes extremely long time as I am dealing with a million of rows and many import files. There migth be a better way or command that automatically finds the information that I need instead of looking for it line by line?. I found this MATLAB link on the documentation but I am not sure how to apply it for what I need: http://uk.mathworks.com/help/matlab/matlab_prog/find-array-elements-that-meet-a-condition.html

I would be very grateful if somebody could help me.

Answer 1

[Azzi Abdelmalek]

v={'ID'   'var1' 'var2' ;'DHK' 12 14; 'ABC',15 20;'DHK',15 13}
w=cell2table(v(2:end,:),'variablenames',v(1,:))   % Example
out=w(ismember(w.ID,'DHK'),:)

[Edward Moran]

I want to do the same thing as asked in this question except that I want to look for more than just 'DHK'. I want to search for 'DHK' and, say, 'ABC' at the same time without writing a loop.

I want to avoid having to write the following:

out=w(ismember(w.ID,'DHK'),:)

out2=w(ismember(w.ID,'ABC'),:)

I have several thousand of these variable like 'DHK' and 'ABC' in a table. I want to search without writing a loop to create one table with all the occurrences of these variables. I need it in just a few lines if possible. I need the results of 'DHK' and 'ABC' in the same table.

Thanks!

[Edward Moran]

% this is the solution I've come up with and it works! Thank you, Edward!

myKeys = string({'DHK';'ABC'});
idx = ismember(T.Var1, myKeys);
% Access the table for these specific rows
out=T(idx,:)

[Francesco Giuseppe Fornari]

hi,

what if I wanted to find 'DHK' without knowing in which columns it can be?

I've got multiple csv files, where I need to find the same string.

thanks

Answer 2

[Gabor]

rows = T.id=='DHK';
T(rows, :)

Answer 3

[Peter Perkins]

First convert those highly repetitive strings to a categorical variable:

table1.Var1 = categorical(table1.Var1);

Type whos before and after doing that and watch your memory use decrease. Then do more or less what Azzi suggests, leveraging the categorical equality operator:

   table2 = table1(table1.ID == 'DHK',:);

That's it!