C# equivalent of Python's range with step?

Question

Is there a C# equivalent of Python's range with step?

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Documentation:

For a positive step, the contents of a range r are determined by the formula r[i] = start + step*i where i >= 0 and r[i] < stop.

For a negative step, the contents of the range are still determined by the formula r[i] = start + step*i, but the constraints are i >= 0 and r[i] > stop.

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Example:

>>> list(range(0, 10, 3))
[0, 3, 6, 9]
>>> list(range(0, -10, -1))
[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]

Answer 1

I would go with two methods implementation. First one for parameters validation and providing defaults:

public static IEnumerable<int> Range(int start, int stop, int step = 1)
{
    if (step == 0)
        throw new ArgumentException(nameof(step));

    return RangeIterator(start, stop, step);
}

That is required for iterators with deferred execution. Otherwise, you will not validate arguments until iterator will be executed. Which might happen a long time after you get an iterator reference. And iterator itself (actually with C# 7 you can use a local function instead of creating separate method):

private static IEnumerable<int> RangeIterator(int start, int stop, int step)
{
    int x = start;

    do
    {
        yield return x;
        x += step;
        if (step < 0 && x <= stop || 0 < step && stop <= x)
            break;
    }
    while (true);
}

To implement Python's range behavior we need one more method which accepts only stop parameter. We can simplify code with C# 6 expression-bodied member:

public static IEnumerable<int> Range(int stop) => RangeIterator(0, stop, 1);

You also can make static methods available in global scope with C# 6. Assume class with Range method description is named PythonUtils:

using static YourNamespace.PythonUtils;

And usage in code will look like

foreach(var i in Range(0, 10, 3))
   Print(i);

You can also use default values

Range(0, 10, 3)     // [0,3,6,9]
Range(4, -3, -1)    // [4,3,2,1,0,-1,-2]
Range(5)            // [0,1,2,3,4]
Range(2, 5)         // [2,3,4]

Looks like Pascal-case Python :)

Answer 2

We can implement a static utility class to handle this.

For completeness, this solution mimics Python's range behavior for one parameter (stop), two parameters (start, stop), and three parameters (start, stop, step):

using System;
using System.Collections.Generic;

public static class EnumerableUtilities
{
    public static IEnumerable<int> RangePython(int start, int stop, int step = 1)
    {
        if (step == 0)
            throw new ArgumentException("Parameter step cannot equal zero.");

        if (start < stop && step > 0)
        {
            for (var i = start; i < stop; i += step)
            {
                yield return i;
            }
        }
        else if (start > stop && step < 0)
        {
            for (var i = start; i > stop; i += step)
            {
                yield return i;
            }
        }
    }

    public static IEnumerable<int> RangePython(int stop)
    {
        return RangePython(0, stop);
    }
}

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Example Usage with Step:

foreach (var i in EnumerableUtilities.RangePython(0, 10, 3))
{
    Console.WriteLine(i);
}

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Output:

0
3
6
9