Find elements in table without looping (Matlab)

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NC
NC 2015-6-5
评论： Francesco Giuseppe Fornari 2019-11-11
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.

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采纳的回答

Azzi Abdelmalek
Azzi Abdelmalek 2015-6-5
编辑：Azzi Abdelmalek 2015-6-5
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'),:)
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显示 1更早的评论
Edward Moran
Edward Moran 2019-7-14
% 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
Francesco Giuseppe Fornari 2019-11-11
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

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更多回答（2 个）

Gabor
Gabor 2019-3-9
rows = T.id=='DHK';
T(rows, :)
Peter Perkins
Peter Perkins 2015-6-5
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!

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