Question

a quick, simple question from me about for-loops.

Situation
I’m currently writing some high-performance code when I suddenly was wondering how the for-loop actually behaves. I know I’ve stumbled across this before, but can’t for the life of me find this info again :/

Still, my main concern was with the limiter. Say we have:

for(int i = 0; i < something.awesome; i++)
{
// Do cool stuff
}

Question
Is something.awesome stored as an internal variable or is the loop constantly retrieving something.awesome to do the logic-check? Why I’m asking is of course because I need to loop through a lot of indexed stuff and I really don’t want the extra function-call overhead for each pass.

However if something.awesome is only called once, then I’m going back under my happy rock! 🙂

Answer 1

You can use a simple sample program to check the behaviour:

using System;

class Program
{
    static int GetUpperBound()
    {
        Console.WriteLine("GetUpperBound called.");
        return 5;
    }

    static void Main(string[] args)
    {
        for (int i = 0; i < GetUpperBound(); i++)
        {
            Console.WriteLine("Loop iteration {0}.", i);
        }
    }
}

The output is the following:

GetUpperBound called. 
Loop iteration 0. 
GetUpperBound called. 
Loop iteration 1. 
GetUpperBound called. 
Loop iteration 2. 
GetUpperBound called. 
Loop iteration 3. 
GetUpperBound called. 
Loop iteration 4. 
GetUpperBound called.

The details of this behaviour are described in the C# 4.0 Language Specification, section 8.3.3 (You will find the spec inside C:\Program Files\Microsoft Visual Studio 10.0\VC#\Specifications\1033):

A for statement is executed as
follows:

• If a for-initializer is present, the variable initializers or statement
  expressions are executed in the order
  they are written. This step is only
  performed once.

• If a for-condition is present, it is evaluated.

• If the for-condition is not present or if the evaluation yields true,
  control is transferred to the embedded
  statement. When and if control reaches
  the end point of the embedded
  statement (possibly from execution of
  a continue statement), the expressions
  of the for-iterator, if any, are
  evaluated in sequence, and then
  another iteration is performed,
  starting with evaluation of the
  for-condition in the step above.

• If the for-condition is present and the evaluation yields false, control
  is transferred to the end point of the
  for statement.