**Percentages** are often useful in programs.

With the numbers one*and two,**we can get a percentage of 50%.* We display and process percentages with doubles. We solve an annoying rounding problem.

First, we see some code that uses string.Format to display two numbers or ratio as a percentage. The following code shows four methods. The first is Main. And the last three are custom methods for displaying percentages.

string.FormatProgram that calculates percents: C#using System; class Program { static void Main() {// Display percentage of visits that resulted in purchases.int purchases = 10; int visitors = 120; DisplayPercentage((double)purchases / visitors);// Display 50 percent with overloaded method.DisplayPercentage(1, 2);// Write percentage string of nine tenths.Console.WriteLine(GetPercentageString((double)9 / 10)); }/// <summary> /// This method writes the percentage form of a double to the console. /// </summary>static voidDisplayPercentage(double ratio) { string percentage = string.Format("Percentage is {0:0.0%}", ratio); Console.WriteLine(percentage); }/// <summary> /// This method writes the percentage of the top number to the bottom number. /// </summary>static voidDisplayPercentage(int top, int bottom) { DisplayPercentage((double)top / bottom); }/// <summary> /// This method returns the percentage-formatted string. /// </summary>static stringGetPercentageString(double ratio) { return ratio.ToString("0.0%"); } }OutputPercentage is 8.3% Percentage is 50.0% 90.0%

**DisplayPercentage** accepts a double that is a ratio, usually between 0 and 1. It uses a custom formatting string to display the percentage to the Console. The {0:0.0%} substitution indicates you want a percentage with one decimal place.

**Also:**The second DisplayPercentage accepts two parameters and then passes the ratio of them to the other method. It casts to double.

**Finally:**GetPercentageString accepts a double containing a ratio and returns a percentage string using ToString().

Here we convert two integers into a percentage manually with division and multiplication. Sometimes you can need raw percentages when you have percentages in the database stored in different formats.

Program that converts ratios: C#using System; class Program { static void Main() {// We want to have 92.9% from these two numbers.int valid = 92; int total = 99;// First multiply top by 100 then divide.double percent = (double)(valid * 100) / total;// <-- Use cast// This is the percent number.Console.WriteLine(percent); Console.WriteLine(Math.Floor(percent)); Console.WriteLine(Math.Ceiling(percent)); Console.WriteLine(Math.Round(percent, 1)); } }Output92.9292929292929 92 93 92.9

**Casting to double.** The double must be assigned to a value cast to double.

If you omit the cast,*your value will be rounded**and probably useless.* When casting to double, you do not need to surround the entire expression with parentheses.

**The final four statements** in the program display different forms of the percentage. Math.Floor rounds down to the nearest integer. Math.Ceiling rounds up to the nearest integer. And Math.Round rounds to a single decimal place.

The percentage sign in the C# language has a use as the modulo operator. This forms an expression that will return the remainder of a division of the two operands. You can find more about the modulo operator.

Modulo**Tip:**With modulo division, we can run an operation every N times. This has uses in many programs.

We saw two examples of using percentages in the C# language. First we saw how to format ratios as percentages with three different methods. Second, we saw how to get a percentage value directly with math, and then round it.