Looping through 2 Lists at once

Question

I have two lists that are of the same length, is it possible to loop through these two lists at once?

I am looking for the correct syntax to do the below

foreach itemA, itemB in ListA, ListB
{
  Console.WriteLine(itemA.ToString()+","+itemB.ToString());
}

do you think this is possible in C#? And if it is, what is the lambda expression equivalent of this?

Answer 1

[edit]: to clarify; this is useful in the generic LINQ / IEnumerable<T> context, where you can't use an indexer, because a: it doesn't exist on an enumerable, and b: you can't guarantee that you can read the data more than once. Since the OP mentions lambdas, it occurs that LINQ might not be too far away (and yes, I do realise that LINQ and lambdas are not quite the same thing).

It sounds like you need the missing Zip operator; you can spoof it:

static void Main()
{
    int[] left = { 1, 2, 3, 4, 5 };
    string[] right = { "abc", "def", "ghi", "jkl", "mno" };

    // using KeyValuePair<,> approach
    foreach (var item in left.Zip(right))
    {
        Console.WriteLine("{0}/{1}", item.Key, item.Value);
    }

    // using projection approach
    foreach (string item in left.Zip(right,
        (x,y) => string.Format("{0}/{1}", x, y)))
    {
        Console.WriteLine(item);
    }
}

// library code; written once and stuffed away in a util assembly...

// returns each pais as a KeyValuePair<,>
static IEnumerable<KeyValuePair<TLeft,TRight>> Zip<TLeft, TRight>(
    this IEnumerable<TLeft> left, IEnumerable<TRight> right)
{
    return Zip(left, right, (x, y) => new KeyValuePair<TLeft, TRight>(x, y));
}

// accepts a projection from the caller for each pair
static IEnumerable<TResult> Zip<TLeft, TRight, TResult>(
    this IEnumerable<TLeft> left, IEnumerable<TRight> right,
    Func<TLeft, TRight, TResult> selector)
{
    using(IEnumerator<TLeft> leftE = left.GetEnumerator())
    using (IEnumerator<TRight> rightE = right.GetEnumerator())
    {
        while (leftE.MoveNext() && rightE.MoveNext())
        {
            yield return selector(leftE.Current, rightE.Current);
        }
    }
}

Answer 2

It'll be much simpler to just do it in a plain old for loop instead...

for(int i=0; i<ListA.Length; i++)
{
    Console.WriteLine(ListA[i].ToString() + ", " + ListB[i].ToString());
}

Answer 3

You can do it explicit.

IEnumerator ListAEnum = ListA.GetEnumerator();
IEnumerator ListBEnum = ListB.GetEnumerator();

ListBEnum.MoveNext();
while(ListAEnum.MoveNext()==true)
{
  itemA=ListAEnum.getCurrent();
  itemB=ListBEnum.getCurrent();
  Console.WriteLine(itemA.ToString()+","+itemB.ToString());
}

At least this (or something like this) is what the compiler does for a foreach-loop. I haven't tested it though and I guess some template parameters are missing for the enumerators.

Just look up GetEnumerator() from List and the IEnumerator-Interface.

Answer 4

I recommend using plain old for loop, but you should consider different array lengths. So

for(int i=0; i<ListA.Length; i++)
{
    Console.WriteLine(ListA[i].ToString() + ", " + ListB[i].ToString());
}

can turn into

for(int i = 0; i < Math.Min(ListA.Length, ListB.Lenght); i++)
{
    Console.WriteLine(ListA[i].ToString() + ", " + ListB[i].ToString());
}

or even into

    for(int i = 0; i < Math.Max(ListA.Length, ListB.Lenght); i++)
    {
        string valueA = i < ListA.Length ? listA[i].ToString() : "";
        string valueB = i < ListB.Length ? listB[i].ToString() : "";

        Console.WriteLine(valueA+ ", " + valueB);
    }