C# – Property with Enumerable or list

c++enumerablelinqlist

I'm playing around with LINQ and related subjects and was wondering about the following.

I've 2 methods of getting a Fibonacci sequence.
I started with:

public static IEnumerable<int> Fibonacci
    {
        get
        {
            int i = 0;
            int j = 1;
            int temp = 0;

            while (true)
            {
                yield return i;

                temp = i;
                i = j;
                j = temp + i;
            }
        }
    }

But it got me thinking, why would I choose this over:

public static IList<int> Fibonacci(long to)
    {            
        IList<int> fibList = new List<int>();
        int i = 0;
        int j = 1;
        int temp, index = 0;

        while (index < to)
        {
            fibList.Add(i);

            temp = i;
            i = j;
            j = temp + i;

            index++;
        }
        return fibList;
    }

IList is an Enumerable as well + I might want to add some parametrization to it.
I'm not really looking for optimisations or stuff or thing like use < long > because the number gets big fast, its merely a quick example. Just some arguments pro and con each method. Why and when should I use which method?

Best Solution

An important difference between the two is that with your second version you have to know in advance when you want to stop, but in the first version you can start iterating and later decide when to stop. You don't have to know in advance.

You also don't have to store the entire list in memory at once with the first version. You can handle the data in a streaming manner.

An advantage of the second is that returning a list allows you to index into the array rather than handling elements one by one from the beginning. You could use the second version if you do know how many elements you want, and you know that the list is small enough to fit in memory.

Note that these differences have nothing to do with whether you use a property or a function call. You could rewrite the first to be a function call that takes no parameters.

If you have only the first version available, you could easily emulate the second version by using Fibinocci().Take(20).ToList().

Caveats follow

Note that while this is perhaps the cleanest way to answer the question as asked, index is a rather weak component of the list API, and I can't remember the last time I used it in anger. It's been pointed out to me in the comments that because this answer is heavily referenced, it should be made more complete. Some caveats about list.index follow. It is probably worth initially taking a look at the documentation for it:

list.index(x[, start[, end]])
Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError if there is no such item.

The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.

Linear time-complexity in list length

An index call checks every element of the list in order, until it finds a match. If your list is long, and you don't know roughly where in the list it occurs, this search could become a bottleneck. In that case, you should consider a different data structure. Note that if you know roughly where to find the match, you can give index a hint. For instance, in this snippet, l.index(999_999, 999_990, 1_000_000) is roughly five orders of magnitude faster than straight l.index(999_999), because the former only has to search 10 entries, while the latter searches a million:

>>> import timeit
>>> timeit.timeit('l.index(999_999)', setup='l = list(range(0, 1_000_000))', number=1000)
9.356267921015387
>>> timeit.timeit('l.index(999_999, 999_990, 1_000_000)', setup='l = list(range(0, 1_000_000))', number=1000)
0.0004404920036904514

Only returns the index of the first match to its argument

A call to index searches through the list in order until it finds a match, and stops there. If you expect to need indices of more matches, you should use a list comprehension, or generator expression.

>>> [1, 1].index(1)
0
>>> [i for i, e in enumerate([1, 2, 1]) if e == 1]
[0, 2]
>>> g = (i for i, e in enumerate([1, 2, 1]) if e == 1)
>>> next(g)
0
>>> next(g)
2

Most places where I once would have used index, I now use a list comprehension or generator expression because they're more generalizable. So if you're considering reaching for index, take a look at these excellent Python features.

Throws if element not present in list

A call to index results in a ValueError if the item's not present.

>>> [1, 1].index(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: 2 is not in list

If the item might not be present in the list, you should either

Check for it first with item in my_list (clean, readable approach), or
Wrap the index call in a try/except block which catches ValueError (probably faster, at least when the list to search is long, and the item is usually present.)

Best Solution

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Python – Finding the index of an item in a list

Caveats follow

Linear time-complexity in list length

Only returns the index of the first match to its argument

Throws if element not present in list

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