Question

When I run:

for($o=1;$o<=655;$o++){
$r = $r+0.01;
echo $r." ";}

at some point I get:

...4.29 4.3 4.31 4.32 4.33 4.34 4.35 4.36 4.37 4.38 4.39 4.4
4.41 4.42 4.4299999999999 4.4399999999999 4.4499999999999
4.4599999999999 4.4699999999999 ...

But when I run:

for($o=1;$o<=5;$o+=0.01){
echo $o." ";
}

anomaly starts at:

4.34 4.35 4.36 4.37 4.38 4.3899999999999 4.3999999999999

Why is there a difference between the addition being part of the for loop, or within the for loop?

Answer 1

Welcome to the wonders of rounding errors.

0.01 is not exactly expressible as a finite binary fraction, so you’ll pick up rounding errors eventually if you add it repeatedly to another floating point number.

The specific point at which you see the rounding error depends on how the binary expansion of the numbers involved play out to produce the rounding error.