This is *just* what happens when you convert between binary and decimal - see the links I provided in the post below.

update:
Think of it like this: decimal floats are inaccurate too, since they can't represent a rational number like 1/3 in finite notation: 

1/3 = 0.333333333333333333....

[download]

If all arithmatic was done using decimal notation, assuming a maximum of 10 places you get: 

$x = 1/3; #  0.3333333333
$y = $x * 3;
$y == 1 -> false  # $y == 0.9999999999

[download]

The same thing happens here, only since humans have a hard time thinking in floating binary, it's much harder to predict where.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re^3: [s]printf %d oddity by Joost
in thread [s]printf %d oddity by sscecina

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