PROGRAM MYPROG

INTEGER NX, NY ! Variables to hold number of intervals in each direction
REAL XSTART, XEND ! Variables defining the interval in x-direction
REAL YSTART, YEND ! Variables defining the inverval in y-direction
REAL DX, DY
REAL PHI(21,21)

CALL DEFINEMATRIX(XSTART, XEND, YSTART, YEND, NX, NY) ! Lets the user specify the interval for which the problem will be solved, and the number of intervals for discretisation

DX=(XEND-XSTART)/NX
DY=(YEND-YSTART)/NY

CALL ENTERBOUNDARYVALUES(PHI,XSTART,DX,NX,YSTART,DY,NY)

WRITE(*,*) ' xs ys ', PHI(1,1)
WRITE(*,*) ' xs+d ys ', PHI(1,1+NX)
WRITE(*,*) ' xs ys+d ', PHI(1+NY,1)
WRITE(*,*) ' xs+d ys+d ', PHI(1+NY,1+NX)


END PROGRAM


!---------------------------------!
SUBROUTINE DEFINEMATRIX(XS,XE,YS,YE,NX,NY)
REAL XS, XE, YS, YE
INTEGER NX, NY

write(*,*) 'Choose the interval in x-direction for which you wish to solve the problem. Input format <x_min> <x_max>'
CALL READINTERVAL(XS, XE) ! Prompt user to input x-interval
write(*,*) 'Choose the interval in y-direction for which you wish to solve the problem. Input format <y_min> <y_max>'
CALL READINTERVAL(YS, YE) ! Prompt user to input y-interval
write(*,*) 'Specify number of intervals you want in the x-direction'
read(*,*) NX
write(*,*) 'Specify number of intervals you want in the y-direction'
read(*,*) NY

END SUBROUTINE
!----------------------------------!
SUBROUTINE READINTERVAL(START, END)
REAL START, END
read(*,*) START, END ! Assignes the values to the corresponding variables
END SUBROUTINE
!----------------------------------!
FUNCTION G(X,Y)
REAL X,Y
REAL G
G=1
RETURN
END FUNCTION
!----------------------------------!
REAL FUNCTION PHIBOUNDARY(X,Y)
REAL X, Y
PHIBOUNDARY=(X**2+Y**2)/4.0
RETURN
END FUNCTION
!----------------------------------!
SUBROUTINE ENTERBOUNDARYVALUES(PHI, XSTART, DX, NX, YSTART, DY, NY)
REAL PHI(21,21)
REAL XSTART, DX, YSTART, DY
INTEGER NX, NY

INTEGER COL
INTEGER ROW
DO WHILE(COL<=NX+1)
PHI(1,COL)=PHIBOUNDARY(XSTART+(COL-1)*DX,YSTART)
PHI(NY+1,COL)=PHIBOUNDARY(XSTART+(COL-1)*DX,YSTART+NY*DY)
COL=COL+1
END DO
ROW=2
DO WHILE(ROW<=NY)!DENNE GÅR FRA 2 TIL OG MED NY, DA 1 OG NY+1 ER FYLT UT AV LØKKEN OVER
PHI(ROW,1)=PHIBOUNDARY(XSTART,YSTART+(ROW-1)*DY)
PHI(ROW,NX+1)=PHIBOUNDARY(XSTART+NX*DX,YSTART+(ROW-1)*DY)
ROW=ROW+1
END DO
END SUBROUTINE