PROGRAM random_walk2
! random walk in two dimensions
DIM x(500),y(500)
LIBRARY "csgraphics"
CALL initial(x(),y(),N,nwalkers,walker$)
FOR step = 1 to N
    SET CURSOR 1,1
    PRINT step
    CALL move(x(),y(),N,nwalkers,walker$)
NEXT step
END

SUB initial(x(),y(),N,nwalkers,walker$)
    RANDOMIZE
    INPUT prompt "number of walkers = ": nwalkers
    INPUT prompt "total number of steps = ": N
    CLEAR
    CALL compute_aspect_ratio(N,xwin,ywin)
    SET WINDOW -xwin,xwin,-ywin,ywin
    ! place N particles at origin
    SET COLOR "red"
    LET r = 0.5
    BOX AREA -r,r,-r,r
    BOX KEEP -r,r,-r,r in walker$
    SET COLOR "black"
    FOR i = 1 to nwalkers
        LET x(i) = 0
        LET y(i) = 0
    NEXT i
END SUB

SUB move(x(),y(),N,nwalkers,walker$)
    FOR i = 1 to nwalkers
        CALL choice(x(),y(),i)
        BOX SHOW walker$ at x(i)-0.5,y(i)-0.5
    NEXT i
END SUB

SUB choice(x(),y(),i)
    LET p = rnd
    IF p <= 0.25 then
       LET x(i) = x(i) + 1
    ELSEIF p <= 0.5 then
       LET x(i) = x(i) - 1
    ELSEIF p <= 0.75 then
       LET y(i) = y(i) - 1
    ELSE
       LET y(i) = y(i) + 1
    END IF
END SUB