PHP Math equation problem

Posted on 2011-03-02
Last Modified: 2012-05-11
Hi - I'm programming in PHP.  Here is some YTD information:
2011 YTD 14,989.3 2,877,688.8 $73,101.97
2010 YTD 18,409.8 1,911,203.4 $52,875.36
YTD Change Down 1.228% Up 1.506% Up 1.383%

The YTD Change % looks incorrect to me.  This is the formula I'm using to calculate the percentage.  I know you mathematical people will be horrified but I never did very well in math. :(  Help!

Thanks - Zack
if ($LMTDYYYAVG100 > $CMTDYYYAVG100) { $chMTDAVG100="<font color=red>Down " . number_format(($LMTDYYYAVG100/$CMTDYYYAVG100)*1, 3) . "%</font>"; } else { $chMTDAVG100="<font color=green>Up " . number_format(($CMTDYYYAVG100/$LMTDYYYAVG100)*1, 3) . "%</font>"; }
if ($LMTDYYYJet > $CMTDYYYJet) { $chMTDJet="<font color=red>Down " . number_format(($LMTDYYYJet/$CMTDYYYJet)*1, 3) . "%</font>"; } else { $chMTDJet="<font color=green>Up " . number_format(($CMTDYYYJet/$LMTDYYYJet)*1, 3) . "%</font>"; }
if ($LMTDYYYSPI > $CMTDYYYSPI) { $chMTDSPI="<font color=red>Down " . number_format(($LMTDYYYSPI/$CMTDYYYSPI)*1, 3) . "%</font>"; } else { $chMTDSPI="<font color=green>Up " . number_format(($CMTDYYYSPI/$LMTDYYYSPI)*1, 3) . "%</font>"; }

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Question by:zack4x4
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LVL 37

Accepted Solution

TommySzalapski earned 500 total points
ID: 35023968
The numbers are actually right, just the interpretation is off.
1.228 means you have currently 1.228 times last year, so that's a 22.8% increase.
If you get .9 then that's a 10% decrease. Take your number, subtract 1 and multiply by 100.
So (ThisYear/LastYear - 1)*100 gives the percentage change.
LVL 37

Expert Comment

ID: 35023977
For down, the formula is a little reversed. Basically, you just subtract from 1 instead of subtracting 1.
(1 - ThisYear/LastYear)*100
LVL 37

Expert Comment

ID: 35023984
I'll attempt to fix the code to show what I mean (I'm not the best at PHP so here goes):
if ($LMTDYYYAVG100 > $CMTDYYYAVG100) { $chMTDAVG100="<font color=red>Down " . number_format((1 - $LMTDYYYAVG100/$CMTDYYYAVG100)*100, 3) . "%</font>"; } else { $chMTDAVG100="<font color=green>Up " . number_format(($CMTDYYYAVG100/$LMTDYYYAVG100 - 1)*100, 3) . "%</font>"; }
if ($LMTDYYYJet > $CMTDYYYJet) { $chMTDJet="<font color=red>Down " . number_format((1 - $LMTDYYYJet/$CMTDYYYJet)*100, 3) . "%</font>"; } else { $chMTDJet="<font color=green>Up " . number_format(($CMTDYYYJet/$LMTDYYYJet - 1)*100, 3) . "%</font>"; }
if ($LMTDYYYSPI > $CMTDYYYSPI) { $chMTDSPI="<font color=red>Down " . number_format((1 - $LMTDYYYSPI/$CMTDYYYSPI)*100, 3) . "%</font>"; } else { $chMTDSPI="<font color=green>Up " . number_format(($CMTDYYYSPI/$LMTDYYYSPI - 1)*100, 3) . "%</font>"; }

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Author Closing Comment

ID: 35024313
That worked perfectly!!  Thanks!!  Just one question why do you have to -1 (subtract 1) from the back or from the front?  What does it do to the answer?

Apprecaite it!
LVL 37

Expert Comment

ID: 35028455
The number you generate is the ratio of the new number to the old number. To see the change, you subtract the old number. It's the same as (new - old)/old. With a little algebra, you can see that they are identical.
If you lost 10%, this would actually give you a value of -10% gain. But you want to show the number as positive, so we flip it - (new - old)/old = (-new + old)/old = (old - new)/old = 1 - new/old.
I just short cut the algebra.

Author Comment

ID: 35032400
Thanks for the explanation! I'm horrific in algebra. :(


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