Programming - The Boundary Element Method with Programming - page 18

C= SUM (A, MASK= A>0.0)

would sum only coefficients of A which are greater than “zero”.

Gathering and scattering: A feature in F90 makes the 'gathering' and 'scattering' of
values, which we will need later, very simple. To explain these operations consider a
two-dimensional mesh of boundary elements in Figure 2.1.
The nodes of the mesh where elements are connected with each other can be
numbered in two different ways: locally and globally. When referring, for example, to
the unknown u (e.g. temperature in the case of heat conduction problems) one has two
vectors, a global one

u
u
u
uu
u
u
u

-½
° °°
° °°
®¾
° °°
° °°
¯¿

1

2

3

{}

(2.7)

4

5

6

7

and a local one defined at element level, for example, for the two nodes of element 3:

3

-

u

½

}

1

{ u (2.8)

u

2

For element 3 we may define a 'connectivity vector' of dimension 2, which contains
the global node numbers of the two nodes of the element

CONNECTIVITY=(/3,4/)

The 'scatter' operation is where the locally defined unknowns are put into the global
vector

U_GLOBAL(CONNECTIVITY)= U_LOCAL

This statement would put u 1 and u 2 of element 3 into locations 3 and 4 of the global
vector. The 'gather' operation would do the opposite, i.e.

U_LOCAL= U_GLOBAL(CONNECTIVITY)

would put the global values of u 1 and u 2 into the local positions 1 and 2.