How can I trust Visual FoxPro arithmetics after this?


This is a design VFP behaviour.

Run this:

CLEAR
SET FIXED OFF
SET DECIMALS TO RAND()*18  && THIS IS insignificant
y = 100.00000000
for n = 1 to 1000
 y = y - .10000000
NEXT
? y=0,y*3=0,y*4=0,Y+Y+Y+Y=0

y is zero, y*3 is zero, but y*4 is not zero ( and this is a bug )


VFP store any variable decimal number with a IEEE float format.
0.1 have not a exact IEEE value, and therefore every calculation introduces round errors.

You can use ROUND(), but it is not perfect.