How to get Next and Prevous values?

Question

I've got this piece of code to get the next value using LINQ namespace.

public static class Extension
{
    public static T Next<T>(this IEnumerable<T> source, Func<T, bool> predicate)
    {
        bool flag = false;

        using (var enumerator = source.GetEnumerator())
        {
            while (enumerator.MoveNext())
            {
                if (flag) return enumerator.Current;

                if(predicate(enumerator.Current))
                {
                    flag = true;
                }
            }
        }
        return default(T);
    }
}

But now, I need also to get the previous value, but I here I do not know what to do.

score 2 · Answer 1 · answered Feb 23 '12 at 23:16

public static T Previous<T>(this IEnumerable<T> source, Func<T, bool> predicate)
    {       
        T t = default(T);
        using (var enumerator = source.GetEnumerator())
        {
            while (enumerator.MoveNext())
            {   
                if(predicate(enumerator.Current))
                {
                    return t;
                }
                t = enumerator.Current;
            }
        }
        return default(T);
    }

Lukazoid · Accepted Answer · 2012-02-28T19:54:50.127

I may be missing something here, but what's the reasoning behind directly accessing IEnumerable<T>.GetEnumerator(), IEnumerator<T>.MoveNext() and IEnumerator<T>.Current? Seems like you're directly replicating what the foreach loop does.

public static T Next<T>(this IEnumerable<T> source, Func<T, bool> predicate)
{
    bool shouldReturn = false;
    foreach (var item in source)
    {
        if(shouldReturn)
            return item;

        if (predicate(item))
            shouldReturn = true;
    }
    return default(T);
}

public static T Previous<T>(this IEnumerable<T> source, Func<T, bool> predicate)
{
    var previous = default(T);
    foreach (var item in source)
    {
        if (predicate(item))
            return previous;
        previous = item;
    }
    return default(T);
}

score 1 · Answer 3 · edited May 23 '17 at 12:20

1

If you only need the immediately preceding value, why not just hang on to each "previous" as you iterate through the collection. If you need a more complex iterator, check out this SO post.

edited May 23 '17 at 12:20

Community

1
1

answered Feb 23 '12 at 23:18

Adam S

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How to get Next and Prevous values?

3 Answers3