Don't compare floats

Floating-point numbers are tricky; one of the first things a programmer needs to remember when working with them is to never check floats for equality. (I believe ReSharper warns you if you do that; I don't know if plain Visual Studio does because I never use it without ReSharper.)

If precise representations of decimal numbers is needed, like when manipulating currencies, use decimals instead (if using C#); if a similar primitive type does not exist in your language, write a separate package / module / library to "fake it" by using integer values and scale them down by two or four digits, depending on your needs. (For example, the number 123456 can represent either 1,234.56 or 12.3456, depending on your application.)

Here's a simple code example to show the difference between floats and decimals in C#:

float f1 = 0.1f;
float f2 = f1 * 10.0f;

float f3 = 0.0f;
for (var i = 1; i <= 10; i++)
    f3 += f1;

Console.WriteLine(f2);
Console.WriteLine(f3);
Console.WriteLine(f2 == f3);

decimal d1 = 0.1m;
decimal d2 = d1 * 10.0m;

decimal d3 = 0.0m;
for (var i = 1; i <= 10; i++)
    d3 += d1;

Console.WriteLine(d2);
Console.WriteLine(d3);
Console.WriteLine(d2 == d3);

Running this in LinqPad prints out:

1
1
False
1.00
1.0
True

(A peculiarity of the decimal type in C# is that it keeps track of the number of decimal places that have been used, hence the 1.00 vs 1.0 in the second part of the results.)

Comments

Post a Comment

Popular posts from this blog

Posting dynamic Master / Detail forms with Knockout

Image
I have the following dynamic form in my GitHub Inventory project : I created the form with KnockoutJS , using some ideas from this article . The detail view looks like this: <table id="acquisition_items" class="jtable"> <thead> <tr> <th>Product Name</th> <th>Quantity</th> <th>Price</th> <th>Value</th> <th><button data-bind="click: addItem">[+]</button></th> </tr> </thead> <tbody data-bind="foreach: items"> <tr> <td> <input data-bind="value: ProductName" type="text" value="" /> </td> <td> <input data-bind="value: Quantity" type="text" value="" /> </td> <td> ...
Read more

Comparing Excel files, take two

Image
I already wrote this project once, on GitHub , but I can't say that I like the result. I wrote that code without TDD, mostly to see if I can still work in the "normal" fashion. I can, but I had a bug that took me a while to even discover and then was difficult to write tests for. I also hate the temporal dependency in that design (if you don't call the SortBy method first, the ExcludeRecords method will return incorrect results). Anyway; here's attempt number two, writing that code using TDD. Structure I start by creating my usual structure for projects of this type - a library for the logic, a console application for the main program and a test project for tests. I also use the Enable NuGet Package Restore option (right-click on the solution node) to add the three NuGet files that will allow someone to automatically download all the packages I'm referencing without wasting space in the source code repository. (Right now I've only added the Moq pa...
Read more

EF Code First: seeding with foreign keys

This is just a quick tip, more for future reference for myself than anything else. When using Entity Framework Code First, one of the things you might want to do while developing the application is to seed your database with some test data. This can be something as easy as: protected override void Seed(InventoryDB context) { context.Products.AddOrUpdate( p => p.Name, new Product { Name = "Hammer", SalePrice = 11.99m, }, new Product { Name = "Nail Pack x100", SalePrice = 0.05m, }, new Product { Name = "Saw", SalePrice = 19.99m, }, new Product { Name = "Toolkit", SalePrice = 39.99m, } ); //... } (I am using the AddOrUpdate method so the code detects whether those products already exist and doesn't add them again if they do.) The Product class is quite simple and – most importantly – doesn't depend on any other: public class Product { public int Id { g...
Read more