Using SelectMany

Hi, I have a list xxx which likes

Code:

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List<string> xxx = new List<string>();
// add elements to xxx.
xxx ={a,b,c,t} //xxx is very long thousands of elements
// read a matrix from a text file.
yyy[0] ={a,1,2,3,5};
yyy[1] ={b,2,4,6,7};
yyy[2] ={c,2,7,9,7};
yyy[3] ={d,21,4,16,7};
yyy[4] ={e,12,34,65,77};
yyy[5] ={t,2,4,6,7};
yyy[6] ={z,52,4,6,7};
// etc... just an example

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I want to retrieve a matrix from yyy in which the first column is contained in the list xxx. How to do it? I splitted each line of yyy.

Code:

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string[] fields =yyy[i].split('\t');
if(xxx.contains(fields[0])
//...

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But it is very slow, is there a better way such as SelectMany in linq?
Thanks!