foreach with index

Question

Is there a C# equivalent of Python's enumerate() and Ruby's each_with_index?

Answer 1

I keep this extension method around for this:

public static void Each<T>(this IEnumerable<T> ie, Action<T, int> action)
{
    var i = 0;
    foreach (var e in ie) action(e, i++);
}

And use it like so:

var strings = new List<string>();
strings.Each((str, n) =>
{
    // hooray
});

Or to allow for break-like behaviour:

public static bool Each<T>(this IEnumerable<T> ie, Func<T, int, bool> action)
{
    int i = 0;
    foreach (T e in ie) if (!action(e, i++)) return false;
    return true;
}

var strings = new List<string>() { "a", "b", "c" };

bool iteratedAll = strings.Each ((str, n)) =>
{
    if (str == "b") return false;
    return true;
});

Answer 2

You can do the following

foreach (var it in someCollection.Select((x, i) => new { Value = x, Index = i }) )
{
   if (it.Index > SomeNumber) //      
}

This will create an anonymous type value for every entry in the collection. It will have two properties

• Value: with the original value in the collection
• Index: with the index within the collection

Answer 3

The C# foreach doesn't have a built in index. You'll need to add an integer outside the foreach loop and increment it each time.

int i = -1;
foreach (Widget w in widgets)
{
   i++;
   // do something
}

Alternatively, you could use a standard for loop as follows:

for (int i = 0; i < widgets.Length; i++)
{
   w = widgets[i];
   // do something
}

Answer 4

I like being able to use foreach, so I made an extension method and a structure:

public struct EnumeratedInstance<T>
{
    public long cnt;
    public T item;
}

public static IEnumerable<EnumeratedInstance<T>> Enumerate<T>(this IEnumerable<T> collection)
{
    long counter = 0;
    foreach (var item in collection)
    {
        yield return new EnumeratedInstance<T>
        {
            cnt = counter,
            item = item
        };
        counter++;
    }
}

and an example use:

foreach (var ii in new string[] { "a", "b", "c" }.Enumerate())
{
    Console.WriteLine(ii.item + ii.cnt);
}

One nice thing is that if you are used to the Python syntax, you can still use it:

foreach (var ii in Enumerate(new string[] { "a", "b", "c" }))

Answer 5

Aside from the LINQ answers already given, I have a "SmartEnumerable" class which allows you to get the index and the "first/last"-ness. It's a bit ugly in terms of syntax, but you may find it useful.

We can probably improve the type inference using a static method in a nongeneric type, and implicit typing will help too.

Answer 6

My solution involves a simple Pair class I created for general utility, and which is operationally essentially the same as the framework class KeyValuePair. Then I created a couple extension functions for IEnumerable called Ordinate (from the set theory term "ordinal").

These functions will return for each item a Pair object containing the index, and the item itself.

public static IEnumerable<Pair<Int32, X>> Ordinate<X>(this IEnumerable<X> lhs)
{
    return lhs.Ordinate(0);
}

public static IEnumerable<Pair<Int32, X>> Ordinate<X>(this IEnumerable<X> lhs, Int32 initial)
{
    Int32 index = initial - 1;

    return lhs.Select(x => new Pair<Int32, X>(++index, x));
}

Answer 7

No, there is not.

As other people have shown, there are ways to simulate Ruby's behavior. But it is possible to have a type that implements IEnumerable that does not expose an index.

Answer 8

This is your collection

var values = new[] {6, 2, 8, 45, 9, 3, 0};

Make a range of indexes for this collection

var indexes = Enumerable.Range(0, values.Length).ToList();

Use the range to iterate with index

indexes.ForEach(i => values[i] += i);
indexes.ForEach(i => Console.Write("[{0}] = {1}", i, values[i]));

Answer 9

I just figured out interesting solution:

public class DepthAware<T> : IEnumerable<T>
{
    private readonly IEnumerable<T> source;

    public DepthAware(IEnumerable<T> source)
    {
        this.source = source;
        this.Depth = 0;
    }

    public int Depth { get; private set; }

    private IEnumerable<T> GetItems()
    {
        foreach (var item in source)
        {
            yield return item;
            ++this.Depth;
        }
    }

    public IEnumerator<T> GetEnumerator()
    {
        return GetItems().GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

// Generic type leverage and extension invoking
public static class DepthAware
{
    public static DepthAware<T> AsDepthAware<T>(this IEnumerable<T> source)
    {
        return new DepthAware<T>(source);
    }

    public static DepthAware<T> New<T>(IEnumerable<T> source)
    {
        return new DepthAware<T>(source);
    }
}

Usage:

var chars = new[] {'a', 'b', 'c', 'd', 'e', 'f', 'g'}.AsDepthAware();

foreach (var item in chars)
{
    Console.WriteLine("Char: {0}, depth: {1}", item, chars.Depth);
}

Answer 10

It depends on the class you are using.

Dictionary<(Of <(TKey, TValue>)>) Class For Example Support This

The Dictionary<(Of <(TKey, TValue>)>) generic class provides a mapping from a set of keys to a set of values.

For purposes of enumeration, each item in the dictionary is treated as a KeyValuePair<(Of <(TKey, TValue>)>) structure representing a value and its key. The order in which the items are returned is undefined.

foreach (KeyValuePair kvp in myDictionary) {...}