# Access matrix element
**URL:** https://community.freefem.org/t/access-matrix-element/885
**Category:** General Discussion
**Created:** [March 31, 2021, 6:55pm UTC](https://community.freefem.org/t/access-matrix-element/885 "2021-03-31T18:55:50Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@yongxing](https://community.freefem.org/u/yongxing)
#### Post date: [March 31, 2021, 6:55pm UTC](https://community.freefem.org/t/access-matrix-element/885/1 "2021-03-31T18:55:50Z")
Dear all,
I can understand we can create the system matrix in FreeFEM++, my question is:
Can we modify the element of the matrix, such as putting a penalty (a large number) on the diagonal for a specific degree of freedom?
Best,
Yongxing.
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### Author:  [@Daoudi](https://community.freefem.org/u/Daoudi)
#### Post date: [March 31, 2021, 11:39pm UTC](https://community.freefem.org/t/access-matrix-element/885/2 "2021-03-31T23:39:38Z")
[https://doc.freefem.org/tutorials/heatEquationOptimization.html](https://doc.freefem.org/tutorials/heatEquationOptimization.html)
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### Author:  [@yongxing](https://community.freefem.org/u/yongxing)
#### Post date: [April 1, 2021, 6:17am UTC](https://community.freefem.org/t/access-matrix-element/885/3 "2021-04-01T06:17:33Z")
Thank you, but this is still operating the whole matrix rather than a particular element of the matrix. I am thinking a penalty method to enforce Drichlet BC at a point?
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### Author:  [@Daoudi](https://community.freefem.org/u/Daoudi)
#### Post date: [April 1, 2021, 7:11am UTC](https://community.freefem.org/t/access-matrix-element/885/4 "2021-04-01T07:11:11Z")
Can you send me the copy of your freefem code
and the specific point?
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### Author:  [@yongxing](https://community.freefem.org/u/yongxing)
#### Post date: [April 1, 2021, 10:43am UTC](https://community.freefem.org/t/access-matrix-element/885/5 "2021-04-01T10:43:03Z")
[LidDrivenCavityDisc.edp](https://community.freefem.org/uploads/short-url/mVxEAKRyW70t2q7NvV2zpqWdm3i.edp) (4.4 KB)
Thanks very much for your help Daoudi.
Can you please fix the velocity (u,v)=(0,0) at point (0.6, 0.5) on mesh Th – expecting that the solid disc would keep rotating stably without shifting away?
Best,
Yongxing.
---
### Author:  [@LucasPa](https://community.freefem.org/u/LucasPa)
#### Post date: [April 2, 2021, 4:53pm UTC](https://community.freefem.org/t/access-matrix-element/885/6 "2021-04-02T16:53:38Z")
I am not 100% sure but here is what I would do (supposing that (0.6, 0.5) is a node of your mesh):
I did not look in details your script, but let’s assume we want to modify the matrix Aadj to impose a condition on the first component u (it would then be easy to do the same for other components) where Aadj is defined by:
`fespace RhAdj(Th,[P2,P2,P2,P2,P1,P1]);`
`varf NSAdj([u,v,uhat,vhat,p,phat],[uh,vh,uhath,vhath,ph,phath]) = ...;`
`matrix Aadj = NSAdj(RhAdj,RhAdj,tgv=tgv);`
The question is which entry (I,I) of the matrix should we modify?
`int I; // index in the matrix corresponding to (0.6, 0.5) --> to find`
Let’s do the following:
fespace Vh(Th,P2);
Vh xx = x, yy=y;
RhAdj [w1x, w2x, s1x, s2x, wpx, spx] = [xx,0,0,0,0,0];
RhAdj [w1y, w2y, s1y, s2y, wpy, spy] = [yy,0,0,0,0,0];
```
for (int k = 0; k < w1x[].n; ++k)
{
if (w1x[][k] == 0.6 && w1y[][k]=0.5)
{
I = k;
}
}
```
And that should do it!
---
### Author:  [@yongxing](https://community.freefem.org/u/yongxing)
#### Post date: [April 4, 2021, 8:36pm UTC](https://community.freefem.org/t/access-matrix-element/885/7 "2021-04-04T20:36:53Z")
Thank you very much! I will try this out.
Best,
Yongxing.