Calculate Percent Increase and Decrease

Question

I'm likely over-thinking this, but I'm looking for something a bit more elegant than what I currently have.

I have this method called CalculatePercentageTrend, which takes in a long "previous" value and "current" value. As it currently stands :

public static string CalculatePercentageTrend(long previousValue, long currentValue)
{
    var trend = "0.00%";

    // calculate percentage increase
    if (previousValue < currentValue)
    {
        if (previousValue != 0)
        {
            var difference = previousValue - currentValue;
            var pctDecrease = (double)(difference / previousValue) * 100;
            trend = pctDecrease.ToString("P");
        }
        else
        {
            trend = currentValue.ToString("P");
        }
     }
    // calculate percentage decrease
    else if (currentValue < previousValue)
    {
        if (previousValue != 0)
        {
            var difference = currentValue - previousValue;
            var pctIncrease = (double)(difference / previousValue) * 100;
            trend = pctIncrease.ToString("P");
        }
        else
        {
            trend = currentValue.ToString("P");
        }
    }
    return trend;
}

This feels very repetitive, and has a few short comings. Negative trends aren't calculated properly, as they always result in a 0.00% change - what would be great is if I could get a negative percentage IF in fact the previous value is greater than the current value.

Also, I'm handling any possible 0's before dividing, but I'm wondering if there's a better approach to that as well.

My Question :

How can I get negative percentages to calculate correctly, and how can I improve this code overall?

Duomo

You need to get rid of your first if else if. Simply check if either variable is 0. Then do (previous - current) / previous * 100. This will return positive if it is an increase and negative if it is a decrease. – Grillroom

At least, change (double)(difference / previousValue) * 100 to ((double) difference / previousValue) * 100; – Maxwell

Important to understand that you should NOT multiply this result by 100 if you intend to use it in any fashion other than displaying a more 'friendly' value to the user. 31% == .31, 31% != 31. 31 == 3100%. – Folacin

Answer 1

You're way over thinking it.

double CalculateChange(long previous, long current)
{
    if (previous == 0)
        throw new InvalidOperationException();

    var change = current - previous;
    return (double)change / previous;
}

To display this data to a user in a friendly 'percentage' format, you can use:

string DoubleToPercentageString(double d)
{
    return "%" + (Math.Round(d, 2) * 100).ToString();
}

Folacin

you need to multiply the result by 100 to get the number as a percentage – Sledge

That is strictly a display concern. If you multiply this result by 100 and then try to use it to operate on anything, you will not get the desired result. 50% == .5 – Folacin

Yes, but his method indicates the answer will be returned as a percentage not a decimal. – Sledge

Again, 50% equals .5, not 50. – Folacin

1 meter == 100 cm, yet if a function was named GetMeters I would expect it to return 1 not 100 – Sledge

What??? Look the original method was named 'CalculatePercentageTrend' and its return type is a string, so I'm not putting a lot of stock into the original design. That doesn't even make any sense. If I called this function to perform some sort of mathematical operation and it returned the value multiplied by 100, I would trash the entire library immediately. – Folacin

Also, this is pretty basic mathematics. Open Excel, put a value of '.5' into a cell, then change the formatting to Percentage. Let me know what you get. – Folacin

you are not the designer, excel also stores dates as numbers. That is an implementation detail, and shouldn't factor into this discussion. 0.5 == 50% sure, the method name includes Percent, which is the decimal representation multiplied by 100. – Sledge

Let us continue this discussion in chat. – Sledge

I deleted my last comment, as it was inappropriate and not constructive. Just consider this: p = 1; c = 1; x = GetPercChange(p, c); y = p * x; y != x, if you multiply by 100 with the function. – Folacin

those are equal... p = 1; c = 1; x = GetPercChange(p,c) = 0, y = p * x = 0... 100 * 0 = 0 – Sledge

Hah. Of course I choose the corner case in my haste. Try p = 2. – Folacin

this should be in chat, want to clear the air there? – Sledge

(13,11) => -15.38 %, your result => %-15 – Selfpreservation

If you want to display the fractional result of the %, change the 2 to a 4 when you call Math.Round in the ToString method. – Folacin

@JamesRalston I know this is ancient history, but I stumbled across this question and immediately recollected this conversation. Thought you might be interested in how MS views the issue: #1791475 – Folacin

I have never seen a program return an integer for a percent. It's always a decimal with 1 representing 100. I know these comments are dated, but I'm a bit blown away that anyone would expect a percent to be returned as 50 rather than .50. – Incommodious

Answer 2

I used following function with 100% accuracy.

    decimal CalculateChange(decimal previous, decimal current)
    {
        if (previous == 0)
            return 0;

        if (current == 0)
            return -100;

        var change = ((current - previous) / previous) * 100;

        return change;
    }

Einhorn

This is exactly what I need! Works like a charm! – Puttier

if the previous is 0 then the changes is 100% not 0 if 0 becomes any number then there is a change by 100% – Stateroom

Answer 3

I think this will resolve your issue, all you need is calculating the difference and it's ratio with respect to previous value.

public static string CalculatePercentageTrend(long previous, long current)
{
    return ((current - previous) / (double)previous).ToString("p");
}

Selfpreservation