Possible Duplicate:
Math - mapping numbers
I have value "x" that can be from 0 to 127 and a value "y" that can be from -1000 to 0. I need to make that if x = 0 than y = -1000 and if x = 127 than y = 0... How can i make it?
+3
A:
It sounds like you just want a linear equation (y = mx + b). In your case, this would be
y = x*(1000/127) - 1000
Niki Yoshiuchi
2010-06-09 17:08:32
Depending on the language this might yield inaccurate results (if it takes the 1000 and 127 and does an integer divide). You might want to mention that.
Andrew Rollings
2010-06-09 17:14:00
Works fine! Many thanks to you :D
FBSC
2010-06-09 17:14:05
And this is why modern programming courses don't have anywhere near enough math components :) (grumble grumble). Or enough onions on your belt.
Andrew Rollings
2010-06-09 17:16:17
@FBSC - you may want to check that... if you're using that directly, then the 1000/127 will evaluate to (int)7, whereas you want it to evaluate to about 7.87... See my answer for more information.
Andrew Rollings
2010-06-09 17:20:22
@Andrew i don't actually need that accuracy, since y is an integer, but i changed it :) thanks
FBSC
2010-06-09 17:22:44
A:
linear interpolation...
slope = (0 - -1000) / (127 - 0) = (1000.0/127.0) y-intercept = 127
y = (1000.0/127.0) * x - 1000
of course this assumes x and y can take on "real" values and not just integers.
vicatcu
2010-06-09 17:09:39
+1
A:
y = x * (1000.0/127.0) - 1000.0
Make sure you use float values in your calculation otherwise you will get inaccurate answers.
EDIT: And if you're really picky about accuracy, then this is better still:
y = (int) (0.5 + (x * (1000.0/127.0) - 1000.0))
(which will do correct rounding).
Andrew Rollings
2010-06-09 17:10:44