# [Rd] akima 0.4-5, interpp() bug = COMMON block problem

Albrecht Gebhardt albrecht.gebhardt at uni-klu.ac.at
Wed Feb 1 11:31:17 CET 2006

```Hi,

I'm currently hunting a bug in the akima library, especially in the code
behind the interpp.old function (bi-variate linear interpolation).
It is based on a triangulation algorithm, interpolation at a given point
needs to know the triangle which contains this point, then the
interpolation is a straightforward calculation based on the three
vertexes.

The problem is: Sometimes the triangle indices are not given back
correctly, they just default to 1 leading to wrong results.

The following lines can be used to visualize it using the rgl library.
If the error occurs (may be architecture depending) the interpolation
"steepest" part of the interp() surface will not be hit by the interpp()
interpolation points.

library(akima)
library(rgl)
data(akima)
# data
rgl.spheres(akima\$x,akima\$z , akima\$y,0.5,color="red")
rgl.bbox()
# interp:
akima.li <- interp(akima\$x, akima\$y, akima\$z,
xo=seq(min(akima\$x), max(akima\$x), length = 200),
yo=seq(min(akima\$y), max(akima\$y), length = 200))
# interp surface:
rgl.surface(akima.li\$x,akima.li\$y,akima.li\$z,color="green",alpha=c(0.5))
# interpp:
akima.p <- interpp(akima\$x, akima\$y, akima\$z,
runif(2000,0,25),
runif(2000,0,20))
# interpp (partially wrong) points:
rgl.points(akima.p\$x,akima.p\$z , akima.p\$y,size=4,color="yellow")

The errors occurs at least with R 2.1.1 on a i386 Ubuntu Breezy system.
If I compile without "-O2" in FFLAGS the error vanishes.

Meanwhile I could track down the bug to the use of COMMON blocks in the
underlying Fortran code:

During the interpp() call a Fortran routine (IDLCTN called from IDBVIP)
is called several times. Only during the first call a COMMON block is
initialized, subsequent calls rely on the values initialized by the
first call. But these values just vanish after the first call.

I already have workaround: I call the initialization on every entry,
this fixes the problem.

===================================================================
RCS file: /home/cvs/math/stat/R/akima/src/idlctn.f,v
retrieving revision 1.2
diff -u -r1.2 idlctn.f
--- idlctn.f    19 Aug 1998 21:54:17 -0000      1.2
+++ idlctn.f    1 Feb 2006 10:28:53 -0000
@@ -49,7 +49,7 @@
X0 = XII
Y0 = YII
C PROCESSING FOR A NEW SET OF DATA POINTS
-      IF (NIT.NE.0) GO TO 80
+C      IF (NIT.NE.0) GO TO 80
NIT = 1
C - DIVIDES THE X-Y PLANE INTO NINE RECTANGULAR SECTIONS.
XMN = XD(1)

I guess, using COMMON blocks is generally a bad idea when using Fortran
code within R? Shall I leave it with my workaround or should I search
for more details of the COMMON block misuse?

Best regards

Albrecht Gebhardt

--
// Albrecht Gebhardt          Tel.: (++43 463) 2700/3118
// Institut fuer Mathematik   Fax : (++43 463) 2700/3198
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