Who knows Enumerable.Range(...)?

Introduction

I quite often see constructs like:

for(int n = 0; n < len; ++n)
{
   //... some more or less complex conditional control flow...
   n = 5; // causes the loop to never terminate if len is greater than 5
   //... some more code ...
}

This is a legal language construct, but it introduces the danger of non-determinism.

This tip aims to show some deterministic alternatives to that plain for loop.

Using the Code

If you need to loop over some range of integer, consider using the Enumerable.Range[^] based foreach loop instead:

using System.Linq;
...
foreach (int n in Enumerable.Range(0, len))
{
    ...
}

This guarantees that it iterates over all elements in strict sequence. Another benefit is that you cannot assign any other value to the loop variable n. E.g.

foreach (int n in Enumerable.Range(0, len))
{
    n = 5; // Compiler error!
}

Note: Enumerable.Range(...) is not from, to, but from, count:

//
// Summary:
//     Generates a sequence of integral numbers within a specified range.
//
// Parameters:
//   start:
//     The value of the first integer in the sequence.
//
//   count:
//     The number of sequential integers to generate.
//
// Returns:
//     An IEnumerable<Int32> in C# or IEnumerable(Of Int32) in Visual Basic that
//     contains a range of sequential integral numbers.
//
// Exceptions:
//   System.ArgumentOutOfRangeException:
//     count is less than 0.-or-start + count -1 is larger than System.Int32.MaxValue.
public static IEnumerable<int> Range(int start, int count);

Alternatives

One could also build his own iterator functions (see also C# Iterator Pattern demystified[^]). E.g.

public static IEnumerable<int> CountUp(int n, int count)
{
    while (count-- > 0) yield return n++;
}
public static IEnumerable<int> CountDown(int n, int count)
{
    while (count-- > 0) yield return n--;
}

When used like this...

foreach (int n in CountUp(0, 5)) Console.WriteLine(n);
foreach (int n in CountDown(100, 5)) Console.WriteLine(n);

...results in:

0
1
2
3
4
100
99
98
97
96

You can define any complexity of traversing sequence in that function and let the foreach-loop terminate deterministically, based on that sequence.

Advise

Try to avoid plain for (...) loops and replace by some deterministic loop alternative like:

• foreach( ... Range(...))
• foreach( ... CountUp(...))
• etc.

Other alternatives are Linq iterations like Enumerable.Aggregate[^] etc. But these are a bit more advanced.

History

• 2012-04-18 First version
• 2012-04-19 Added hand-crafted CountUp/CountDown functions
• 2013-04-14 Fixed some broken HTML markup in C# code generics