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Simple PHP math problem trying to get a percentage and failing at it.

Posted on 2013-06-29
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Last Modified: 2013-06-30
I'm sorry to have to ask. I should've listened up in math class:

I'm comparing open rates for 2 different email campaigns.

For example:
$current_campaign = 3
$previous_campaign = 2

So I'm trying to do the math to show what percentage of increase or decrease the current campaign has compared to the previous. Seemed simple enough when I started....

My math is:
$sent_percent = number_format(($previous_campaign / $current_campaign) * 100 - 100,0);
echo $sent_percent."%";

This echos out: -33% where based on the above it should actually be a 33% increase (no negative sign)

If I take out the '-100' in the formula, I get 67% but that isn't how much of an improvement the current campaign is compared to the previous. See what I mean?

This can't be that hard and I'm chalking it up to my inexperience and failure to take math seriously in high school.

Help...
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Question by:tjyoung
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9 Comments
 
LVL 14

Expert Comment

by:Giovanni Heward
ID: 39287188
Try...

$current_campaign = 3;
$previous_campaign = 2;
$sent_percent = round(( $current_campaign - $previous_campaign ) / abs( $current_campaign ) * 100, 2);
if (preg_match('/\A(\d+.\d{2})\Z/', $sent_percent)) {
	# Successful match
        echo "$sent_percent%";
} else {
	# Match attempt failed
        echo "Error...";
}

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Author Comment

by:tjyoung
ID: 39287203
trying yours right now
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LVL 14

Expert Comment

by:Giovanni Heward
ID: 39287205
Or...

<?php
$current_campaign = 3;
$previous_campaign = 2;
$sent_percent = round( ( $current_campaign - $previous_campaign ) / 
		abs( $current_campaign ) * 100, 0 );
if (preg_match('/\A(\d+)\Z/', $sent_percent)) {
	# Successful match
        echo "$sent_percent%";
} else {
	# Match attempt failed
        echo "Error...";
	unset($sent_percent);
}
?>

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0
 
LVL 1

Author Comment

by:tjyoung
ID: 39287210
Hi,
The last version works fine when the current_campaign is a larger number.
I get the error when its less though. Can't figure out what the preg_match is doing?
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LVL 58

Accepted Solution

by:
Gary earned 500 total points
ID: 39287211
x66's calculations are wrong.

Initial figure / 100 - gives you what 1% represents (of the old campaign)
New figure divide by the (1%) - gives you the percentage of the new value (campaign)
Take away 100 gives you the actual increase/decrease.

So in math
$diff = (newcampaign / (oldcampaign/100)) - 100
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LVL 58

Expert Comment

by:Gary
ID: 39287213
An increase from 2 to 3 is a 50% increase, not 33.3%.
0
 
LVL 1

Author Closing Comment

by:tjyoung
ID: 39287217
doh... it is 50% increase.
Thanks this works.
0
 
LVL 58

Expert Comment

by:Gary
ID: 39287222
:o)
You were working out the difference between the two numbers.
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LVL 14

Expert Comment

by:Giovanni Heward
ID: 39289024
Good catch.

If you switch my variables around you get the gain; just another way to do it.

<?php
$current_campaign = 3;
$previous_campaign = 2;
$diff = round( ( $current_campaign - $previous_campaign ) /
            abs( $current_campaign ) * 100, 0 );
$gain = round( abs( ( $previous_campaign - $current_campaign ) /
            $previous_campaign ) * 100, 0 );
echo "diff: $diff% | ";
echo "gain: $gain%";
?>
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