Simple FOR loop does not reach the end?!

Some kind of rounding error?!

[penders]

An example...

(1) This works OK and outputs the nos 0, 0.1, 0.2, ...., 0.9, 1

for($myvar=0; $myvar<=1; $myvar+=0.1) { 
 echo $myvar.', '; 
}

(2) This does not reach the end (IMO) - outputs the nos 0, 0.01, 0.02, ...., 0.98, 0.99

for($myvar=0; $myvar<=1; $myvar+=0.01) { 
 echo $myvar.', '; 
}

Why does example (2) not reach 1 ?

[Little_G]

Hi,

Seems to be a problem with 'hidden precision', see PHP: Floating point numbers - User Contributed Notes [php.net]

This works:

for($myvar=0; round($myvar,2)<=1; $myvar+=0.01) {  
  echo $myvar.', ';  
}

Andrew

[edited by: Little_G at 11:49 am (utc) on Oct. 14, 2008]

[penders]

Ah OK, thanks for that! The penny has finally dropped! Mmmm... appears to be an issue checking floats for equality period. Dang, I've encountered this many times before but I hadn't appreciated what was really going on! (Incidentally, it's the same with JavaScript ...and other languages I guess.)

This particular example could be changed to check for integer equality, which obviously works:

for($myvar=0; $myvar<=100; $myvar++) {  
 echo ($myvar/100).', ';  
}

Regarding this floating point (im)precision, you can 'see' it in action if you do something like:

for($myvar=0; $myvar<=1; $myvar+=0.01) {  
 echo number_format($myvar,20).'<br />';  
}

Which outputs...

0.00000000000000000000
0.01000000000000000021
0.02000000000000000042
0.02999999999999999889
0.04000000000000000083
0.05000000000000000278
0.05999999999999999778
0.07000000000000000666
0.08000000000000000167
0.08999999999999999667
0.10000000000000000555
:
etc.

$myvar never really equals anything you might expect, unless you intervene and round() the precision.