CODEWITHSUNDEEP
c#design-patterns
phil soady
phil soady

Reputation: 11318

Why should iteration be used instead of tail recursion?

What is the design smell, poor practice in recursion ? once I saw resharper suggesting improvements I quickly looked around on google. Saw numerous comments around re-factoring tail recursion to iterations and referring to it as a design smell. 

public static void DebugOutput2(Exception ex) {
    if (ex == null) {
        return;
    }
    Debug.WriteLine(ex.Message);
    if (ex.InnerException != null) {
        DebugOutput2(ex.InnerException);
    }
}

// WAS REFACTORED TO

public static void DebugOutput(Exception ex) {
    if (ex == null) {
        return;
    }
    while (true) {
        Debug.WriteLine(ex.Message);
        if (ex.InnerException != null) {
            ex = ex.InnerException;
            continue;
        }
        break;
    }
}

EDIT: BAsed on C# compiler treatment comment. Looks like it is recursive now
Target .net 4.5 . C# 5.0

ILDASM Output for tail recursion version: Shows recursive call and not iteration

.method public hidebysig static void  DebugOutput(class [mscorlib]System.Exception ex) cil managed
{
  // Code size       54 (0x36)
  .maxstack  2
  .locals init ([0] bool CS$4$0000)
  IL_0000:  nop
  IL_0001:  ldarg.0
  IL_0002:  ldnull
  IL_0003:  ceq
  IL_0005:  ldc.i4.0
  IL_0006:  ceq
  IL_0008:  stloc.0
  IL_0009:  ldloc.0
  IL_000a:  brtrue.s   IL_000e
  IL_000c:  br.s       IL_0035
  IL_000e:  ldarg.0
  IL_000f:  callvirt   instance string [mscorlib]System.Exception::get_Message()
  IL_0014:  call       void [System]System.Diagnostics.Debug::WriteLine(string)
  IL_0019:  nop
  IL_001a:  ldarg.0
  IL_001b:  callvirt   instance class [mscorlib]System.Exception [mscorlib]System.Exception::get_InnerException()
  IL_0020:  ldnull
  IL_0021:  ceq
  IL_0023:  stloc.0
  IL_0024:  ldloc.0
  IL_0025:  brtrue.s   IL_0035
  IL_0027:  nop
  IL_0028:  ldarg.0
  IL_0029:  callvirt   instance class [mscorlib]System.Exception [mscorlib]System.Exception::get_InnerException()
  IL_002e:  call       void ca1.Program::DebugOutput(class [mscorlib]System.Exception)
  IL_0033:  nop
  IL_0034:  nop
  IL_0035:  ret

} // end of method Program::DebugOutput

Upvotes: 19

Views: 3321

Answers (2)

stan0
stan0

Reputation: 11807

The recursion makes additional (recursive) function call, which also means a stack allocation, for every level (every object you handle).

Generally the iteration is better because it doesn't make that additional call/allocation.

Of course, the difference would be visible in case there are many objects to handle, which I suppose is not the case in your example. So for you it's not an issue and I guess the intention is to teach you a better practice in general.

Upvotes: 8

ILMTitan
ILMTitan

Reputation: 11017

Because people mistakenly care more about micro-optimizations than clear and readable code.

Upvotes: 23

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