How can I iterate over two IEnumerables simultaneously in .NET 2

Question

EDIT: I split this to two questions:

Iterate over two lists ending at the shorter one
Iterate over several lists until the last element of the longest list has been reached

Assume I have two IEnumerable with many elements. Every IEnumerable has another type T.

IEnumerable<int> ints=getManyInts();
IEnumerable<string> strings=getSomeStrings();

What I want to do is to iterate over both lists, and get an item containing one int and one string for each step, until the end of the shortest list has been reached.

for(Item<int,string> item in Foo.Combine<int,string>(ints, strings))
{
    int i=item.Val1;
    string s=item.Val2;
}

You can also give me a hint how to do this in .NET 4.

score 5 · Answer 1 · answered Feb 03 '11 at 15:47

5

Hint for .NET 4.0:

var result = ints.Zip(strings, (i, s) => new { Key = i, Value = s });
foreach(var item in result)
{
    int key = item.Key;
    string value = item.Value;
    // Do something with the kvp
}

answered Feb 03 '11 at 15:47

Darin Dimitrov

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Just answering the short (.NET 4) part is considered lazy ;-) – TToni Feb 03 '11 at 15:48
1

@TToni, I am developer and by definition lazy :-) – Darin Dimitrov Feb 03 '11 at 15:50
Thanks. I knew it must be easy in .NET 4. But unfortunately I also have to maintain some older projects. – matthias.lukaszek Feb 03 '11 at 15:50
I think it is a safe bet to just give the .net 4 code as most other people seem to be giving just .net 2 code. – Scott Chamberlain Feb 03 '11 at 15:51

Lucero · Answer 2 · 2011-02-08T18:08:17.960

As non-LINQ-apprach (.NET 2) this can be done like this top iterate until one of the enumeration is finished:

using (IEnumerator<int> intItem = GetManyInts().GetEnumerator())
using (IEnumerator<string> stringItem = GetSomeStrings().GetEnumerator()) {
  while (intItem.MoveNext() && stringItem.MoveNext()) {
    int i=intItem.Current;
    string s=stringItem.Current;
  }
}

Edit (for your new condition), to iterate with default values until the last enumeration is done:

using (IEnumerator<int> intItem = GetManyInts().GetEnumerator())
using (IEnumerator<string> stringItem = GetSomeStrings().GetEnumerator()) {
  bool hasInt;
  bool hasString;
  while ((hasInt = intItem.MoveNext()) | (hasString = stringItem.MoveNext())) {
    int i=hasInt ? intItem.Current : default(int);
    string s=hasString ? stringItem.Current : default(string);
  }
}

There is a logical error in your answer: The || operator will skip the second MoveNext — matthias.lukaszek, Feb 03 '11 at 16:33

Scott Chamberlain · Accepted Answer · 2011-02-03T16:02:27.583

This can be done by manually doing what the for-each loop is doing internally

IEnumerable<int> ints=getManyInts();
IEnumerable<string> strings=getSomeStrings();
using (IEnumerator<int> intsEnum = ints.GetEnumerator())
using (IEnumerator<string> stringsEnum = strings.GetEnumerator())
{
    while (intsEnum.MoveNext() && stringsEnum.MoveNext())
    {
        int i = intsEnum.Current;
        string s = stringsEnum.Current;
    }
}

EDIT for longest list:

IEnumerable<int> ints=getManyInts();
IEnumerable<string> strings=getSomeStrings();
using (IEnumerator<int> intsEnum = ints.GetEnumerator())
using (IEnumerator<string> stringsEnum = strings.GetEnumerator())
{
    bool intIsValid = intsEnum.MoveNext()
    bool stringIsValid = stringsEnum.MoveNext()
    while (intIsValid || stringIsValid)
    {
        int i = default(int)
        string s = default(string)
        if(intIsValid)
        {
           i = intsEnum.Current;
           intIsValid = intsEnum.MoveNext();
        }
        if(stringIsValid)
        {
           s = stringsEnum.Current;
           stringIsValid = stringsEnum.MoveNext();
        }
        //code goes here
    }
}

score 1 · Answer 4 · edited May 23 '17 at 12:21

1

Answered here: Iterate two Lists or Arrays with one ForEach statement in C#

edited May 23 '17 at 12:21

Community

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1

answered Feb 03 '11 at 15:57

blueberryfields

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score 0 · Answer 5 · answered Feb 03 '11 at 15:56

I'm not sure that i understand the question / context.

Would using Dictionary work for you?

internal static class Foo
    {
        internal static Dictionary<TKey, TValue> Combine<TKey, TValue>
            ( IList<TKey> keys, IList<TValue> values )
        {
            var dictionary = new Dictionary<TKey, TValue>();
            // write your own 'combination' code...
            for (int i = 0; i < keys.Count && i<values.Count; i++)
            {
                dictionary.Add(keys[i], values[i]);
            }
            return dictionary;
        }
    }

    private static void Main(string[] args)
    {
        // collection initialization and var: .NET 3.0 or higher
        var keys = new List<int> {1, 2, 3, 5, 6, 7, 8, 10, 15, 99};
        var values = new List<string> {"one", "two", "tree", "four"};

        var combinded = Foo.Combine<int, string>(keys, values);
        foreach (KeyValuePair<int, string> keyValuePair in combinded)
        {
            int key = keyValuePair.Key;
            string value = keyValuePair.Value;
        }
    }

The dictionary will mess with the order and fail with duplicates of the key. — Lucero, Feb 03 '11 at 15:58
No. The lists are extreme long (generated or from DB). I don't want to hold every item in memory. — matthias.lukaszek, Feb 03 '11 at 15:59

How can I iterate over two IEnumerables simultaneously in .NET 2

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