Why printf (Unix) uses the round half?

Question

Why does printf behave in such an unusual way?

> printf %.0f 2.5
> 2
> printf %.0f 2.51
> 3

Is there an advantage to behavior that compensates for possible misunderstandings (like this one )?

Answer 1

This is not strictly round:

> printf '%.0f\n' 2.5
2
> printf '%.0f\n' 3.5
4

This is a rounding form used to combat bias if you round a large number of values; approximately half of them will be rounded, the other half rounded. The rule is rounded if the integer part is even, up if the integer part is odd.

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This, however, is only an explanation of the specific rounding scheme, which is not guaranteed for all implementations printf.

Answer 2

POSIX printf:

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: %f ; , , .

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, , POSIX printf():

f, F

"[-] ddd.ddd", radix . , 6; , '#' , . , . .

, , "[-] inf" "[-] "; . , NaN, "[-] nan (n- char -sequence)" "[-] nan"; n- char -, . F "INF", "INFINITY" "NAN" "inf", "" "nan" .