printf (awk,perl,shell) float rounding issue


 
Thread Tools Search this Thread
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 01:48 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 04:08 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.
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

[awk] rounding a float number?

Heyas Trying to calculate the total size of a file by reading its bitrate. Code snippet: fs_expected() { # # Returns the expected filesize in bytes # pr_str() { ff=$(cat $TMP.info) d="${ff#*bitrate: }" echo "${d%%,*}" | $AWK '{print $1}' | head -n 1 } t_BYTERATE=$((... (9 Replies)
Discussion started by: sea
9 Replies

2. UNIX for Dummies Questions & Answers

How to restrict Rounding using Printf?

Hello, I am using bash shell on Linux OS, May i please know why is it rounding for big numbers but not for others, is there a workaround to print it as it is with out round off? printf '%'\''.2f\n' 9999999999999999999.99 10,000,000,000,000,000,000.00 printf '%'\''.2f\n' 99999999999999.99... (1 Reply)
Discussion started by: Ariean
1 Replies

3. Shell Programming and Scripting

PERL - rounding fractional number

It seems that perl sprintf uses the round-to-even method: foreach my $i ( 0.5, 1.5, 2.5, 3.5 ) { printf "$i -> %.0f\n", $i; } __END__ 0.5 -> 0 1.5 -> 2 2.5 -> 2 3.5 -> 4 4.5 -> 4 Where we probably wants to use round-half-up, i.e. output should be as below: 0.5 -> 1 1.5 -> 2... (8 Replies)
Discussion started by: ganapati
8 Replies

4. Shell Programming and Scripting

AWK rounding up numbers

Hi, I have managed to round up numbers by using the following command: echo "5.54" | awk '{printf "%.0f\n", $1}' result 6 How can I round up all the numbers in a column in a file and print the lines with the new calculated totals? Thanks, (3 Replies)
Discussion started by: keenboy100
3 Replies

5. Shell Programming and Scripting

rounding time in perl

Is there a way I can round time in perl to the nearest five minutes? For example if I have log giving the following time stamps 23,52,30 it would rounded up to 23,55,00 and 23,50,01 would be rounded to 23,50,00 (3 Replies)
Discussion started by: borderblaster
3 Replies

6. Shell Programming and Scripting

Rounding issue with awk

Hi Friends, I am trying to round following number. 0.07435000 echo "0.07435000"|awk '{printf "%s\n",$1*100}'|awk '{printf "%.2f\n",$1}' It returns: 7.435 It should return: 7.44 Any suggestion please? Thanks, Prashant (2 Replies)
Discussion started by: ppat7046
2 Replies

7. Shell Programming and Scripting

How to format the output using float in awk{printf}

Hi I'm using awk to manipulate the data in the 6th field of the file xxx_yyy.hrv. The sample data that is available in this field is given below 220731.7100000000000000 When i tried using this command cat xxx_yyy.hrv | awk '{printf("%23.16f\n",$6*-1)}' I get the output as... (4 Replies)
Discussion started by: angelarosh
4 Replies

8. Shell Programming and Scripting

Rounding off decimals to the nearest number in PERL

Hi Guys, I am generating a statistical report , below is the snippet of the code : Now, $nSlices stands for the time duration,meaning,the statistics will be displayed for that particular time duration. Trouble is, for certain values of $totalTime (which is the end time - start time ), i... (9 Replies)
Discussion started by: rdlover
9 Replies

9. UNIX for Dummies Questions & Answers

Annoying rounding issue in awk

Hello I am getting this very annoying issue in awk: awk '{a=12825;b=a*1.25; print b}' test 16031.2 Thing is the multiplication result is wrong... Result should be 16031.25. I think the issue only happens on bigger numbers. What can I do to get passed this? Thanks by advance (3 Replies)
Discussion started by: Indalecio
3 Replies

10. Shell Programming and Scripting

Float and Double in Perl

HI! What is the notation which correspond to C's doubleandfloatin Perl?? Please tell me. Thanks. Well what I want to do is, for example, calculatingsqrt(2)in two way: float and double. (5 Replies)
Discussion started by: Euler04
5 Replies
Login or Register to Ask a Question

Featured Tech Videos