ATAN

The QDOS ATAN function only works in 90 degree sectors (PI/2 radians).

Minerva's ATAN function can take 2 parameters to provide a 4 quadrant result. If x is greater than 0, ATAN(x,y) gives the same result as ATAN(y/x). Otherwise it returns values in the other quadrants (>PI/2 and < -PI/2)

Here is a function from Steve Poole to improve and enhance this:

100 REMark ATAN__bas by Steve Poole v6mar17
110 CLS : FOR x = -1 to 1 : print : for y = -1 to 1 : print DEG(ATAN_(y,x))
120 :
130 DEFine FuNction ATAN_(y,x)
140 IF x>0 : RETurn ATAN(y/x)
150 IF x=0 : RETurn PI * ((y>0) - (y<0)) /2
160 IF y=0 : RETurn PI
170 RETurn ATAN(y/x) + PI * ((y>0) - (y<0))
180 END DEFine

Or a slightly faster version:

100 DEFine FuNction ATAN_(y,x)
110 IF x>0 : RETurn ATAN(y/x)
120 if x<0 : IF y<0 : RETurn ATAN(y/x) - PI : ELSE RETurn ATAN(y/x) + PI
130 IF y>0 : RETurn PI / 2
140 IF y<0 : RETurn PI / -2 : ELSE RETurn 0
150 END DEFine