Need help understanding LINQ combinations

Question

I have this code from internet that finds all combinations of a list with LINQ. But I'm having hard time understanding in what order these LINQ methods execute and what is the result of each method call. Could someone please talk through the steps if not too much to ask.

public static class Combination
{
    public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<T> elements, int k)
    {
        return k == 0 ?                              // if k = 0
            new[] { new T[0] } :                    // return empty Type array
            elements.SelectMany((element, i) => elements.Skip(i + 1)
            .Combinations(k - 1)
            .Select(combo => (new[] { element })
            .Concat(combo)));
    }
}

Thanks,

Answer 1

Determine via the ternary operator ? if index is 0, if so return value on the left of : else start with flattening elements via SelectMany which leads to Skipping the parameter index which is from a recursive call to Combinations extension then adding combo to the array.

Below I reformatted and changed variable names, the order should be clearer now via code indenting also.

public static class Combination  
{  
    public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<T> elements, int index)  
    {  
        return index == 0 ?   
            new[] { new T[0] } :   
            elements.SelectMany((element, indexer) =>   
                elements.Skip(indexer + 1).Combinations(index - 1)  
                    .Select(combo => (new[] { element })  
                        .Concat(combo)));  
    }  
}  

Using the debugger to step into code can assist understanding code too.