# How to quickly do the calculation?

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Benson Gou on 30 Jun 2021
Edited: Matt J on 1 Jul 2021
Dear All,
I have a code to calculate an array using several arrays. The code is as follows:
Ieq1 = Haltmr(mmm(jbb2),IndBus(solvedbuses))*v(IndBus(solvedbuses));
Where Haltmr is a complex 6000x3000 sparse matrix, v is a complex 3000 column vector, mmm (for example, 20 rows in Haltmr) is a selected rows from Haltmr and jbb2 ( 5 rows in mmm) is the part of mmm, and IndBus is the order of columns in Haltmr, and solvebuses ( 15 indecies) is a part of the columns.
I am wondering if there is a faster way to do the calculation.
Thanks.
Benson
dpb on 30 Jun 2021
sparse saves memory; the cost is execution speed to decode.
If you have enough memory, you could try moving to full storage; H is "only" 138 MB on machines these days with GB memory.
I didn't try to see if could do something with the matrix algebra...

Matt J on 1 Jul 2021
Edited: Matt J on 1 Jul 2021
This might be faster.
r=false(6000,1); r(mmm(jbb2))=1;
c=false(3000,1); c(IndBus(solvedbuses))=1;
Ieq1=Haltmr*sparse(v.*c);
Ieq1=Ieq1(r);