# Bug in printf ?

Peter J. Acklam pjacklam@online.no
Thu Jun 30 12:14:00 GMT 2005

```haro@kgt.co.jp wrote:

> ::How come "0.125" gets printed as "0.12", and not "1.3"?
>                                                     ^^^ "0.13", off cource ;-)

Dealing with integers illustrates the matter more clearly. When
the decimal value is exactly 0.5, then printf should round to the
nearest *even* integer, as far as I know, so you should get

0   -> 0
0.5 -> 0
1   -> 1
1.5 -> 2
2   -> 2
2.5 -> 2
3   -> 3
3.5 -> 4
4   -> 4
4.5 -> 4
5   -> 5

Now I realize that Cygwin's printf doesn't get it right, because

seq 0 .5 5 | while read x; do printf '%-3s -> %.0f\n' \$x \$x; done

gives

0   -> 0
0.5 -> 1
1   -> 1
1.5 -> 1
2   -> 2
2.5 -> 2
3   -> 3
3.5 -> 3
4   -> 4
4.5 -> 4
5   -> 5

Both Solaris' /bin/printf and Perl's printf() give the right
output.

In your examples there is also an additional issue, which is how
numerical values are represented.  Here is Perl:

seq .105 .01 .155 | while read x; do printf '%-5s -> %.2f\n' \$x \$x; done

0.105 -> 0.10
0.115 -> 0.12
0.125 -> 0.12
0.135 -> 0.14
0.145 -> 0.14
0.155 -> 0.15

They all seem correct expect the last one.  The reason why 0.155
becomes 0.15 and not 0.16 is that 0.115 can not be represented
exactly.  It is represented as a number which is slightly
*smaller* than 0.155, so it becomes 0.15 after rounding.

Peter

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