C#

BillWoodruff wrote:

because a source IEnumerable can be "used" any number of times.

BillWoodruff wrote:

whether multiple evaluations of the IEnumerable source are very expensive ... in terms of memory, time.

PIEBALDconsult wrote:

Queue and Stack implement IEnumerable, but they can be consumed only once

BillWoodruff wrote:

Perhaps it is the case that transforming the IEnumerable to a List<T;> is a good thing to do, if it needs to be evaluated more than once.

BillWoodruff wrote:

Perhaps it is the case that transforming the IEnumerable to a List<T;> is a good thing to do, if it needs to be evaluated more than once.

PIEBALDconsult wrote:

it is clear that you (Bill) do not want an IEnumerable at all for the method presented.

Jörgen Andersson wrote:

would consume

public static IEnumerable<KeyValuePair<T1, List<T1>>> ToChunkedKvPList<T1>(this IEnumerable<T1> source, int chunksz)
{
    if (source == null) throw new ArgumentNullException("source");
    if (chunksz <= 0) throw new ArgumentOutOfRangeException("chunksz", "The chunk size must be greater than 0.");
    
    // The iterator method should be separate from the argument validation,
    // so that the exceptions are throw when the method is called,
    // rather than when the returned sequence is iterated.
    
    return ToChunkedKvPListIterator(source, chunksz);
}

private static IEnumerable<KeyValuePair<T1, List<T1>>> ToChunkedKvPListIterator<T1>(IEnumerable<T1> source, int chunksz)
{
    int currentChunkSize = 0;
    T1 currentHeader = default(T1);
    var currentChunk = new List<T1>(chunksz);
    
    foreach (T1 item in source)
    {
        if (currentChunkSize == 0)
        {
            currentHeader = item;
        }
        else
        {
            currentChunk.Add(item);
        }
        
        currentChunkSize++;
        
        if (currentChunkSize == chunksz)
        {
            yield return new KeyValuePair<T1, List<T1>>(currentHeader, currentChunk);
            
            currentChunkSize = 0;
            currentHeader = default(T1);
            currentChunk = new List<T1>(chunksz);
        }
    }
    
    if (currentChunkSize != 0)
    {
        // We have a partial chunk left over:
        yield return new KeyValuePair<T1, List<T1>>(currentHeader, currentChunk);
    }
}

IEnumerable<int> AskDeepThought()
{
    while (true)
    {
        yield return 42;
    }
}

var answers = AskDeepThought().ToChunkedKvPList().Take(42);
// If ToChunkedKvPList uses Count(), this will never return.

BillWoodruff wrote:

pending eye-surgery