Why does this print output -3? xpos can be any value.
void loop() {
xPos = analogRead(xPin);
yPos = analogRead(yPin);
pwm = xPos*(255/1023);
Serial.println(pwm);
}
-1
1
The problem is that (255/1023) is being treated as an integer calculation, meaning the result gets truncated to 0. You can make the compiler promote it to a floating-point calculation by explicitly making at least one of the values floating-point. For example:
pwm = xPos*(255.0f/1023.0f);
answered Jul 1 '15 at 10:46
-
Is the
fdesignator necessary? Without it does the compiler treat the calculation asdoubles and then re-cast it tofloator something? Also, is it not sufficient to add the decimal point to just the numerator or denominator? I know it works in 'regular C++ on a computer', but in an embedded environment would it bloat the program memory? – CharlieHanson Jul 1 '15 at 12:40 -
2@chaaarlie2 Without the
fdesignator, a C++ compiler would treat it asdoubleduring the calculation, which normally requires slightly more resources. Most Arduinos treatfloatanddoublethe same though so it wouldn't necessarily make a practical difference. It's not usually a good idea to rely on platform quirks like that though. – Peter Bloomfield Jul 1 '15 at 13:45 -
@chaaarlie2 I would always put the
fdesignator on both values, simply for consistency. The chances are that the compiler would be smart enough to convert the integer to floating-point at compile-time (rather than run-time), so it probably doesn't matter from a technical point-of-view. – Peter Bloomfield Jul 1 '15 at 13:46 -
I prefer the format pwm = xPos * val1 / (float) val2; which works equally for hardcoded values, constants and variables. – Igor Stoppa Jul 1 '15 at 14:25
1
The previous answer correctly suggested writing
pwm = xPos*(255.0f/1023.0f);
in place of the original form pwm = xPos*(255/1023);. But because 255/1023 - 1/4 = -1/1364 which is fairly small (ie it differs by 1 part in 1364), just saying
pwm = xPos/4;
will usually give the correct result or nearly so.
Note, the following python program shows that this result is exact 514 times as xPos ranges from 0 to 1023, and is one too large 510 times, which for many processes being controlled by PWM signals is close enough.
#!/usr/bin/env python
c = [0]*8
for i in range(1024):
vf = int(i*255.0/1023.0)
vi = i/4
d = vi-vf
c[d+3] += 1
#print '{:5} {:5} {:5} {:5}'.format(i, d, vi, vf)
print c
answered Jul 1 '15 at 21:42
int,float, etc) – Peter Bloomfield Jul 1 '15 at 10:23setup()loop and the declarations ofxPos,yPos, andpwm. There could be something there that you're overlooking. – CharlieHanson Jul 1 '15 at 12:35pwm = xPos*255/1023;instead. – Gerben Jul 1 '15 at 12:41