Question

Here’s a simple problem. I have an application that takes a phone number like ‘13335557777’, and needs to reverse it and insert a dot between each number, like this:

‘7.7.7.7.5.5.5.3.3.3.1.’

I know I can do this with a StringBuilder and a for-loop to reverse the string and insert the dots, but is there a clever way to do this in LINQ (or some other way)?

Note: for this, I’m not really concerned with performance or memory allocation or whatever, just curious to see how this would be done in LINQ.

Answer 1

Try this

var source = GetTheString(); var reversed = source.Reverse().Select(x => x.ToString()).Aggregate((x,y) => x + '.' + y); 

EDIT

This solution is definitely aimed at the ‘clever’ end. It’s likely much more performant to use a StringBuilder to build up the string. This solution creates many intermediate strings.

EDIT2

There was some debate about the relative speed of the ‘clever’ solution vs. the StringBuilder approach. I wrote up a quick benchmark to measure the approach. As expected, StringBuilder is faster.

• Normal Aggregate (100 elements): 00:00:00.0418640
• WithStringBuilder (100 elements): 00:00:00.0040099
• Normal Aggregate (1000 elements): 00:00:00.3062040
• WithStringBuilder (1000 elements): 00:00:00.0405955
• Normal Aggregate (10000 elements): 00:00:03.0270392
• WithStringBuilder (10000 elements): 00:00:00.4149977

However, whether or not the speed difference is signficant is highly dependent upon where it is actually used in your application.

Code for the benchmark.

public static class AggregateUnchanged {     public static string Run(string input) {         return input             .Reverse()             .Select(x => x.ToString())             .Aggregate((x, y) => x + '.' + y);     } }  public static class WithStringBuilder {     public static string Run(string input) {         var builder = new StringBuilder();         foreach (var cur in input.Reverse()) {             builder.Append(cur);             builder.Append('.');         }          if (builder.Length > 0) {             builder.Length = builder.Length - 1;         }          return builder.ToString();     } }  class Program {     public static void RunAndPrint(string name, List<string> inputs, Func<string, string> worker) {          // Test case. JIT the code and verify it actually works          var test = worker('123456');         if (test != '6.5.4.3.2.1') {             throw new InvalidOperationException('Bad algorithm');         }          var watch = new Stopwatch();         watch.Start();         foreach (var cur in inputs) {             var result = worker(cur);         }         watch.Stop();         Console.WriteLine('{0} ({2} elements): {1}', name, watch.Elapsed, inputs.Count);     }      public static string NextInput(Random r) {         var len = r.Next(1, 1000);         var builder = new StringBuilder();         for (int i = 0; i < len; i++) {             builder.Append(r.Next(0, 9));         }         return builder.ToString();     }      public static void RunAll(List<string> input) {         RunAndPrint('Normal Aggregate', input, AggregateUnchanged.Run);         RunAndPrint('WithStringBuilder', input, WithStringBuilder.Run);     }      static void Main(string[] args) {         var random = new Random((int)DateTime.Now.Ticks);         RunAll(Enumerable.Range(0, 100).Select(_ => NextInput(random)).ToList());         RunAll(Enumerable.Range(0, 1000).Select(_ => NextInput(random)).ToList());         RunAll(Enumerable.Range(0, 10000).Select(_ => NextInput(random)).ToList());     } }