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### Re^3: accurately rounding numbers for percentages

by ww (Bishop)
 on Aug 02, 2009 at 23:45 UTC ( #785301=note: print w/ replies, xml ) Need Help??

That's not what Trimbach said, by a long shot.

If you add the UNrounded numbers percentages, they should total 100% (except for the fact that you'll sometimes run into value/count pairs that are rounded at the end of whatever length decimal value you use: 100/6, for example).

But, for cases such as I infer yours is, a quite standard and commonly accepted practice is to include the disclaimer "Totals may not equal 100% because of rounding."

Update: For clarity (in light of OP's next reply), s/numbers/percentages/ at strikeout above.

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Re^4: accurately rounding numbers for percentages
by derekn (Initiate) on Aug 03, 2009 at 01:57 UTC
Hmm, I don't know that I made myself clear, so I will go into more detail to make sure we are on the same page. The users vote for which search engine they used to find site. (Google, Yahoo, Bing) The votes are stored in database as total votes for EACH search engine (Google=30,Yahoo=23,Bing=13) Then, I am figuring out the percentage of each search engine used by dividing by total number of votes (\$searchEngine/\$totalVotes*100) which of course does not always yield nice results. This number (\$google,\$yahoo,\$bing) is then rounded. The problem of course is that sometimes the 3 different percentages don't add up to 100%. Derek
Your made yourself clear enough in the OP but you don't seem to like the answers (or won't understand them?).

The numbers in your latest post, 30, 23, and 13, happen to be what I gather your mean by a "nice" case; that is, when calculated as percentages of their total and rounded, the percentages add up (nicely) to 100.

Here's a case where the numbers are not "nice" and where a disclaimer is the only reasonable solution (fudging is NOT, IMO, reasonable):

```#!/usr/bin/perl
use strict;
use warnings;
#785290

my \$Google=30; my \$Yahoo=23; my \$Bing=12;
my \$sum=\$Google + \$Yahoo + \$Bing;

my \$google_percent = \$Google/\$sum;
print "G: \$google_percent";
my \$yahoo_percent = \$Yahoo/\$sum;
print "\tY: \$yahoo_percent";
my \$bing_percent = \$Bing/\$sum;
print "\tB: \$bing_percent \n";

my \$total = (\$google_percent + \$yahoo_percent + \$bing_percent);

my \$roundedG = sprintf("%.2f", \$google_percent);
my \$roundedY = sprintf("%.2f", \$yahoo_percent);
my \$roundedB = sprintf("%.2f", \$bing_percent);
my \$rounded = sprintf("%.2f", \$total);
print "Google: \$roundedG; Yahoo: \$roundedY; Bing: \$roundedB; Total: \$r
+ounded \n";

printf ('%.0f', ((\$Google/\$sum)*100));
print "%\n";
printf ('%.0f', ((\$Yahoo/\$sum)*100));
print "%\n";
printf ('%.0f', ((\$Bing/\$sum)*100));
print "%\n";
print \$rounded*100 . "% (Percentages may not add up to 100 because of
+rounding errors!)\n";
[download]```
So do what ww said and simply put a disclaimer about the numbers being rounded and not necessarily adding up to 100%.

You can't introduce error to your numbers via rounding and then magically expect that error to disappear and have everything add up to 100 all the time. People are smart enough to understand rounding errors if you add a comment to that effect.

Gary Blackburn
Trained Killer

Ok. This is my first time to ever post on this board, and I may not know as much as you all do, which is why I posted here in the first place, to seek others' greater knowledge so that I might learn from them. I'm sorry if I sounded like I didn't LIKE any answers, because I assure you that was not the case. I just wanted to make sure that I made myself clear, as I wasn't sure that I had. After the replies you have shared with me, I will go with disclaimer. Derek

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