# Methods to exporting data to Octave/Matlab

**URL:** <https://community.freefem.org/t/methods-to-exporting-data-to-octave-matlab/1833>\
**Category:** General Discussion\
**Created:** [June 21, 2022, 5:53pm UTC](https://community.freefem.org/t/methods-to-exporting-data-to-octave-matlab/1833 "2022-06-21T17:53:24Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![JRocha](https://avatars.discourse-cdn.com/v4/letter/j/da6949/32.png) [@JRocha](https://community.freefem.org/u/JRocha)\
**Post date:** [June 21, 2022, 5:53pm UTC](https://community.freefem.org/t/methods-to-exporting-data-to-octave-matlab/1833/1 "2022-06-21T17:53:24Z")

</div>

Hello all,

I am new to FreeFem++ and I hope anyone here could give some help.

I am trying to export the data from FreeFem++ however I am not able to replicate the results in Octave. Probably due to the way I am trying to export the results.

Taking this example: [https://www.um.es/freefem/ff++/pmwiki.php?n=Main.POD](https://www.um.es/freefem/ff++/pmwiki.php?n=Main.POD&fbclid=IwAR19fERWGxPzbbv33iXDuypxloQZOOkRj1VmrXaXV7k2Lm9K2tngd6lKb0I)

Considering only the High Definition Model (HDM) of the Lid-Driven Cavity:

* * *

load “lapack”

int n=20;  
int nev=4;  
int nsnsh=10;

mesh Th=square(n,n);  
//Th=movemesh(Th,[(1-cos(pi_x))/2.,(1-cos(pi_y))/2.]); // refine near the boundaries - refinement removed  
fespace Vh(Th,[P2,P2,P1]);  
Vh [u,v,p],[uu,vv,pp],[up,vp,q];  
// snapshot space:  
Vh[int] usnsh,vsnsh,psnsh;  
// average values:  
Vh [uavg,vavg,pavg];  
uavg=0.; up=0.;

macro div(u,v)(dx(u)+dy(v))//  
macro grad(u)[dx(u),dy(u)]//

int i,j,k;  
real Re=1,nu=1/Re,dt=.1;  
real[int] viso=[0.33,0.21,0.17,0.12,0.07,0.03,-0.017,-0.0065,-0.112,-0.16,-0.21,-0.25,-0.3,-0.35,-0.4,-0.44,-0.49,-0.54,-0.585,-0.66];

problem NSunst([u,v,p],[uu,vv,pp],solver=sparsesolver)=  
int2d(Th)(nu\*(grad(u)‘\*grad(uu)+grad(v)’\*grad(vv)))  
+int2d(Th)([up,vp]'_grad(u)uu+[up,vp]'grad(v)vv)  
-int2d(Th)(pdiv(uu,vv)+ppdiv(u,v))  
+int2d(Th)(1.e-10p_pp)  
+on(1,2,4,u=0,v=0)  
+on(3,u=1,v=0)  
;

for (j=0;j\<nsnsh;j++){  
for (i=0;i\<10;i++){ // here starts the fixed-point iteration for the nonlinearity  
NSunst;  
up=u;  
}  
usnsh[j]=u;  
uavg+=usnsh[j];  
plot([usnsh[j],vsnsh[j]],cmm=“Reynolds=”+Re,ps=“Re”+j+Re+“.eps”,wait=1);  
Re += 50;nu=1/Re;  
}

uavg/=nsnsh;  
plot([uavg,vavg],value=1,cmm=“uavg”,ps=“avg.eps”,wait=1);

for (int i=0;i\<nsnsh;i++)  
usnsh[i]-=uavg;

real[int,int] C(nsnsh,nsnsh);  
for (i=0;i\<nsnsh;i++)  
for (j=i;j\<nsnsh;j++)  
C(j,i) = int2d(Th)(usnsh[i]\*usnsh[j]+vsnsh[i]\*vsnsh[j]);  
for (j=0;j\<nsnsh;j++)  
for (i=j;i\<nsnsh;i++)  
C(j,i)=C(i,j);

cout \<\< "correlation matrix: "\<\< endl;  
cout \<\< C \<\< endl;

// identity matrx  
real[int,int] II(nsnsh,nsnsh);  
II=0;  
for (i=0;i\<nsnsh;i++) II(i,i)=1.;

real[int] ev(nsnsh);  
real[int,int] eVec(nsnsh,nsnsh);  
int k1=dsygvd(C,II,ev,eVec);  
cout \<\< "eigenvalues of the correlation matrix: " \<\< ev \<\< endl;

real[int,int] matVec(nsnsh,Vh.ndof);  
for (i=0;i\<nsnsh;i++)  
matVec(i,:)=usnsh[i];

real normsn=1;  
// recover the modes and store them in the usnsh matrix  
for (i=0;i\<nev;i++){

usnsh[i]=0.;  
for (j=0;j\<nsnsh;j++){  
usnsh[i]+=eVec(j,nsnsh-1-i)\*matVec(j,:); // reordering: most energetic modes first  
}  
normsn = sqrt(int2d(Th)(square(usnsh[i])+square(vsnsh[i])));  
usnsh[i]/=normsn;  
}

for(i=0;i\<nev;i++) {  
plot([usnsh[i],vsnsh[i]],cmm=“mode”+i,ps=“129PODmode”+i+“.eps”,wait=1);  
}

NSunst;

* * *

I am trying to export by ofstream file:

* * *

ofstream file(“datafile.dat”);

for (int i = 0; i \< Th.nv; i++) {

real xCoord = Th(i).x;

real yCoord = Th(i).y;

real val01 = usnsh[0](xCoord, yCoord); //[#] to choose the # of snsh so it retrieves the wanted Re response

real val02 = vsnsh[0](xCoord, yCoord); //[#] to choose the # of snsh so it retrieves the wanted Re response

## file \<\< xCoord \<\< " " \<\< yCoord \<\< " " \<\< val01 \<\< " " \<\< val02 \<\< endl;

However, the exported file doesn’t match the result given by FreeFem++ and the papers. For instance, at the lid, it should be 1 as u=1 v=0 was imposed at that boundary.

Many thanks in advance for your help.