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C# Math.Round ExamplesUse the Math.Round method to the nearest desired value. Examine the MidpointRounding enum type.
Math.Round. This C# method rounds numbers to the nearest value. It receives the desired number of significant digits. It is part of the System namespace.
Method details. This Math.Round static method provides an accurate way to round double and decimal types. It reduces the risk of bugs.
Math
Using System
Simple example. To begin, consider this simple example of calling Math.Round. The fractional part of 1.234 is removed, as it is closer to 1 than 2, and we are left with a result of 1.
using System; class Program { public static void Main() { double value = 1.234; Console.WriteLine("ROUND: {0}", Math.Round(value)); } }
ROUND: 1
Complex example. Math.Round has several overloads and 2 rounding modes defined on the MidpointRounding enum. Here we round an example double and decimal type.
Return We see what Math.Round will return with different arguments. We use 1, 2 and 3 arguments.
Overload
Double
Decimal
Argument 1 The first argument is the number you are rounding. This can be a double or decimal type.
Argument 2 The second argument is the number of digits after the decimal place you want to keep.
Argument 3 And the third argument is an optional rounding mode enumerated constant.
Enum
using System; class Program { static void Main() { // // Round double type in three ways. // double before1 = 123.45; double after1 = Math.Round(before1, 1, MidpointRounding.AwayFromZero); // Rounds "up" double after2 = Math.Round(before1, 1, MidpointRounding.ToEven); // Rounds to even double after3 = Math.Round(before1); Console.WriteLine(before1); // Original Console.WriteLine(after1); Console.WriteLine(after2); Console.WriteLine(after3); // // Round decimal type. // decimal before2 = 125.101M; decimal after4 = Math.Round(before2); decimal after5 = Math.Round(before2, 1); Console.WriteLine(before2); // Original Console.WriteLine(after4); Console.WriteLine(after5); } }
123.45 123.5 <-- MidpointRounding.AwayFromZero 123.4 <-- MidpointRounding.ToEven 123 125.101 125 125.1
MidpointRounding. What is the difference between MidpointRounding.ToEven and MidpointRounding.AwayFromZero? My testing indicates that the difference is found when rounding the number 0.5.
Info MidpointRounding.ToEven will round 0.5 to 0—this is because zero is even.
And MidpointRounding.AwayFromZero will round 0.5 to 1—this is because one is away from zero.
using System; class Program { static void Main() { for (double i = 0.1; i < 0.99; i += 0.1) { Console.WriteLine("{0}=({1},{2})", i, Math.Round(i, MidpointRounding.ToEven), Math.Round(i, MidpointRounding.AwayFromZero)); } } }
0.1=(0,0) 0.2=(0,0) 0.3=(0,0) 0.4=(0,0) 0.5=(0,1) 0.6=(1,1) 0.7=(1,1) 0.8=(1,1) 0.9=(1,1)
Performance. How does Math.Round affect performance? I benchmarked Math.Round to get a general idea. Sometimes using a local cache can speed up this logic.
Version 1 We call Math.Round with a second argument of 0. The result is tested against in an if-statement.
Version 2 This version of the code calls Math.Round again, but with a second argument of 1.
Result The Math.Round calls are slower than a local variable load, but not slow enough to be a big concern.
Important In .NET 5, calling Math.Round in this program was made several times faster—the results are impressive.
using System; using System.Diagnostics; class Program { const int _max = 100000000; static void Main() { // Version 1: use Math.Round with argument of 0. var s1 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { double d = Math.Round(1.3665, 0); if (d == 1.5) { throw new Exception(); } } s1.Stop(); // Version 2: use Math.Round with argument of 1. var s2 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { double d = Math.Round(1.3665, 1); if (d == 1.5) { throw new Exception(); } } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); } }
5.52 ns Math.Round, 0 4.91 ns Math.Round, 1
Rounding notes. Certain computations could be optimized by avoiding Math.Round, or by caching its results. An if-statement would evaluate faster.
If
Note This article had an error in its description of rounding behaviors on scientific data. Thanks to Gus Gustafson for a correction.
A summary. We looked at the Math.Round method. We saw an example of the MidpointRounding enumerated type and how you can round numbers "away from zero" and to the nearest even number.
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