fortran95 - what does this piece of fortran mean? -

im trying go through little algorithm in fortran (im not fortran programmer unfortunately) need understand doing: here is,

  omega = 0.d0   s = 1.d0   = 1   j = 2   k = 3 101 iperm = 1, 3      omega = omega + s * a1 (i) * a2 (j) * a3 (k)      l =      = j      j = k      k = l   enddo   = 2   j = 1   k = 3   s = - s    if (s.lt.0.d0) goto 101    omega = abs (omega) * alat**3 

a1,a2,a3 vectors (three elements each, real values, representing vectors in 3d space) s unit integer (can 1 or -1 alternately) , i,j,k integers while omega (which need understand how arrived at) floating point value, alat. going on there? iperm =1,3 part, vector being created? @ first thought iperm might fancy function/routine or iterator, after search think thats not case, whats purpose of iperm? there looping on iperm between "do" , "enddo" ?

all you've got sequence of assignments loop thrown in fun. guess understand statement such as

lhs = rhs 

evaluates rhs , assigns result variable lhs.

the line

101 iperm = 1, 3 

starts do loop. 101 statement label, it's used later. loop comprises statements line line enddo. loop executed 3 times (once each of integers in sequence starting @ 1 , ending @ 3). loop control variable iperm assigned these values in turn. loop little unusual in loop variable not used inside loop. statement

omega = omega + s * a1 (i) * a2 (j) * a3 (k) 

updates value of omega. term a1(i) (the space in original immaterial) means the i-th element of array a1. et cetera

when line

if (s.lt.0.d0) goto 101 

is executed if s less 0 control goes bak line labelled 101.

finally, term alat**3 calculates cube of alat.

so piece of paper , figure out value omega gets.