Calculating the percentage difference between two values

Question

Before you start laughing at such a simple question let me explain:

I am trying to determine how much change (percentage %) there is in an account over various indicators. This is not particularly hard. But how do you generally handle the cases where the current value is zero or the previous value is zero?

i.e.

This week: Earnings = $25.6

Last week: Earnings = $0.0

I currently calculate the % difference by the following formula:

If (CurrentValue > 0.0 && PreviousValue > 0.0) {
   return (CurrentValue - PreviousValue) / PreviousValue;
}  return 0.0;

if the earnings were zero in the previous week - what should the % difference be? +Infinity?
And inversely if the current week is zero? -Infinity?

Then to complicate things how would you handle this in a Linq-To-SQL query

Upside_Earnings = (statistics.Where(d => d.DateTime > first_startdate && d.DateTime <= first_enddate).Average(e => (double)e.Earnings) > zero &&
                                     statistics.Where(d => d.DateTime > second_startdate && d.DateTime <= second_enddate).Average(e => (double)e.Earnings) > zero) ?
                                   ((statistics.Where(d => d.DateTime > first_startdate && d.DateTime <= first_enddate).Average(e => (double)e.Earnings) -
                                     statistics.Where(d => d.DateTime > second_startdate && d.DateTime <= second_enddate).Average(e => (double)e.Earnings)) /
                                     statistics.Where(d => d.DateTime > second_startdate && d.DateTime <= second_enddate).Average(e => (double)e.Earnings)) : zero,

Answer 1

The change can be positive or negative. So formula is:

var change = ((V2 - V1) / Math.Abs(V1)) * 100;

Example:

1. Calculate percentage change from V1 = 110 to V2 = 220 (positive change)

[ ((V2 - V1) / |V1|) * 100 ]

= ((220 - 110) / |110|) * 100 = (110 / 110) * 100 = 1 * 100 = 100% change

= 100% increase

2. Calculate percentage change from V1 = 150 to V2 = 75 (negative change)

[ ((V2 - V1) / |V1|) * 100 ]

= ((75 - 150) / |150|) * 100 = (-75 / 150) * 100 = -0.5 * 100 = -50% change

= 50% decrease

NOTE: this doesn't handle zeros. You could do that yourself