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printf (awk,perl,shell) float rounding issue

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Top Forums Shell Programming and Scripting printf (awk,perl,shell) float rounding issue
# 1  
Old 10-14-2012
printf (awk,perl,shell) float rounding issue
Hi guys,

could someone throw some light on the following behaviour of printf (I'll start with info about the system and the tool/shell/interpreter versions)?:
Code:
$ uname -a
Linux linux-86if.site 3.1.0-1.2-desktop #1 SMP PREEMPT Thu Nov 3 14:45:45 UTC 2011 (187dde0) x86_64 x86_64 x86_64 GNU/Linux

$ bash --version
GNU bash, version 4.2.10(1)-release (x86_64-suse-linux-gnu) 

$ awk --version
GNU Awk 4.0.0 

$ perl --version
This is perl 5, version 14, subversion 2 (v5.14.2) built for x86_64-linux-thread-multi 

$ ksh ### using ksh now
$ echo ${.sh.version}
Version JM 93t+ 2010-06-21 

$ exit ### back to bash

$ type printf
printf is a shell builtin

$ printf "%.0f\n" 4.5
4

$ printf "%.0f\n" 5.5
6

$ awk 'BEGIN{printf "%.0f\n",4.5}'
4

$ awk 'BEGIN{printf "%.0f\n",5.5}'
6

$ perl -e 'printf "%.0f\n",4.5'
4

$ perl -e 'printf "%.0f\n",5.5'
6

$ ksh ### now to ksh
$ whence -v printf
printf is a shell builtin

$ printf "%.0f\n" 4.5
5

$ printf "%.0f\n" 5.5
6

Only the printf builtin in ksh93t+ seems to give me what I expect (4.5 being rounded to 5 and 5.5 to 6).

Why so? And if this has anything to do with the internal representation of numbers, why does ksh93t+ make an exception?
Last edited by elixir_sinari; 10-14-2012 at 12:48 AM..
# 2  
Old 10-14-2012
With perl try these:
Code:
$ perl -e 'printf "%.4f\n",4.5'
4.5000

$ perl -e 'printf "%1.1f\n",4.5'
4.5

$ perl -e 'printf "%1.0f\n",4.5'
5

$ perl -e 'printf "%f\n",4.5'
4.500000

# 3  
Old 10-14-2012
Quote:
Originally Posted by elixir_sinari
Only the printf builtin in ksh93t+ seems to give me what I expect (4.5 being rounded to 5 and 5.5 to 6).

Why so?
Because floating point doesn't work that way. It doesn't count in nice 1.0 jumps, and forcing it to do so removes a lot of the point of having a floating point -- both figuratively and literally.

It's not really a missing feature, anyway. Add 0.5 before you print the decimal and that gets you rounding.
# 4  
Old 10-14-2012
Quote:
Originally Posted by Corona688
Because floating point doesn't work that way.
And why does it seem to work in ksh93?

---------- Post updated at 12:54 AM ---------- Previous update was at 12:48 AM ----------

Quote:
Originally Posted by Corona688
It doesn't count in nice 1.0 jumps, and forcing it to do so removes a lot of the point of having a floating point -- both figuratively and literally.
Could you elucidate?
Last edited by elixir_sinari; 10-14-2012 at 03:08 AM..
# 5  
Old 10-14-2012
Quote:
Originally Posted by spacebar
[..]
Code:
$ perl -e 'printf "%1.0f\n",4.5'
5
[..]

[/FONT]
My perl 5.12.4 gives:

Code:
$ perl -e 'printf "%1.0f\n",4.5'
4
$ perl -e 'printf "%.0f\n",4.5'
4

So does bash 3.2.48 and dash 0.5.7:
Code:
$ printf "%1.0f\n" 4.5
4

but ksh93u:
Code:
$ printf "%1.0f\n" 4.5
5

They employ different tie-braking rules for rounding of half-integers: "round half up" vs. "round half to even". It is tie-breaking since for example 1.5 is as far removed from 1 as it is from 2.
# 6  
Old 10-14-2012
There are some statistics books that recommend rounding even values down (4.5 -> 4) and odd values up (5.5 -> 6) assuming that doing so will produce sums of rounded numbers closer to the value of the sums of the unrounded values. But I doubt that ksh is trying to provide that capability. I would guess that this could easily be caused by some programs using float, others using double, and still others using long double and having the process of converting 4.5 to the closest value in those formats sitting on opposite sides of the not quite exact value 4.5 representable in the various formats.
# 7  
Old 10-14-2012
Quote:
Originally Posted by elixir_sinari
And why does it seem to work in ksh93?
They must have taken effort to make it do so. I don't know why they bothered, since it's not a feature really needed, undesirable in most cases, and easy to do yourself if you really want it.

Quote:
Originally Posted by elixir_sinari
Could you elucidate?
Floating point numbers are represented internally as an exponent, like 1.5*10^3 or so forth.
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