Two algorithms

Two small code fragments for problems I found interesting.

First, I had a list of criteria that were being used to decide which files in a file list to mark as selected. Some of the criteria were simple, like "none" or "all", but some of them were more complex, like "modified in the last 15 minutes". Since I strongly disliked the idea of a series of ifs (or the equivalent switch statement), I came up with the following:

    var filters = new Dictionary<string, Predicate<FileObject>>
    {
        {"None", file => false},
        {"All (*.*)", file => true},
        {"Invert Selection", file => !file.Selected},
        {"Modified Last 15min", file => file.LastWriteTime.AddMinutes(15) >= DateTime.Now},
        {"Modified Today", file => file.LastWriteTime.Date == DateTime.Now.Date},
        {"*.dll (not Telerik)", file => !file.Name.StartsWith("Telerik") && file.Name.EndsWith(".dll")},
    };

    foreach (var file in fileList)
        file.Selected = filter(file);

Of course, this is rather specific to the exact problem I had, but the idea can be used in the more general case: create one of

    Dictionary<string, Predicate<T>>
    Dictionary<string, Func<T, TResult>>
    Dictionary<string, Action<T>>

and then iterate through the list and get the result / apply the action for each item. This is much easier to understand and modify than a complicated series of ifs, if only because the tendency when modifying the ifs is to add ad-hoc code to solve the particular problem you have right now instead of trying to preserve the code structure.

The second problem I had had to do with formatting a file size to the closest approximation in bytes, KB, MB or GB. For example, a 345 byte file would return "345 bytes", while a 345,000 byte file would say "336.91 KB". (1 KB = 1024 bytes.)

This is again a problem that would normally be solved with a succession of ifs, so once again I looked for something else. (One of the things I've read and that stuck with me was: if possible, move logic out of code and into data.) Here's what I came up with:

    public static string Prettify(this long size)
    {
      var formats = new List<string>
      {
        "#,##0 bytes",
        "#,##0.## KB",
        "#,##0.## MB",
        "#,##0.## GB",
      };

      var limit = 1024L;
      var index = 0;
      while (size >= limit)
      {
        limit *= 1024L;
        index++;
      }

      var format = formats[index];
      return ((double) size / limit * 1024.0).ToString(format);
    }

The reason for the multiplication with 1024 at the end is due to the fact that the limit has already been multiplied once; what I'm doing is basically dividing it back down: size / (limit / 1024), I've just opened the parenthesis.

So, here they are: alternative algorithms for problems normally solved with multiple ifs.

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