使用foreach循环为c＃中的两个列表创建列表值

Question

List<String> listA = new List<string> { "A1", "A2" };
List<String> listB = new List<string> { "B1", "B2" };

for(int i = 0; i < listA.Count; i++)
{
    text += listA[i] + " and " + listB[i];
}

How can I do this using a foreach loop ?

Answer 1

You can use Linq and the Zip method:

List<String> listA = new List<string> { "A1", "A2" };
List<String> listB = new List<string> { "B1", "B2" };

foreach (var pair in listA.Zip(listB, (a,b) => new {A = a, B = b}))
{
    text += pair.A + " and " + pair.B;
}

Answer 2

You cannot do this with a foreach any better than you are already doing it with for -- foreach is really only designed to work well when there is only one sequence to enumerate.

However, you can do it very conveniently with LINQ:

text = string.Join("", listA.Zip(listB, (a, b) => a + " and " + b));

This requires .NET 4 both for Zip and for the specific overload of string.Join .

Answer 3

Another way of doing this is doing that with simple Enumerator:

IEnumerator<string>  enumerator = listB.GetEnumerator(); 
enumerator.MoveNext();
foreach(var stra in listA) {
    text += stra + " and " + enumerator.Current.ToString() + ", ";
    enumerator.MoveNext();
}

Answer 4

使用LINQ

string text = listA.Zip(listB, (a, b) => new {A = a, B = b}).Aggregate("", (current, pair) => current + (pair.A + " and " + pair.B));

Answer 5

And if you do not want to use LINQ and you want them to iterate parallel you have few options - with new classes etc. like below OR you can use foreach , but only for one of the lists, like this:

List<String> listA = new List<string> { "A1", "A2" };
List<String> listB = new List<string> { "B1", "B2" };
string text = "";
int i = 0;
foreach (string s in listA) {
   text += s + " and " + listB [i++] + "\n";
}
Console.WriteLine (text);

or make it a bit nicer using GetEnumerator:

List<String> listA = new List<string> { "A1", "A2" };
List<String> listB = new List<string> { "B1", "B2" };
string text = "";       
List<string>.Enumerator e = listB.GetEnumerator ();
foreach (string s in listA) {
   e.MoveNext ();
   text += s + " and " + e.Current + "\n";
}
Console.WriteLine (text);

also you can create yourself an Enumberable metacollection which will return out of that always a simple string array - for that you will need to create an Enumerator and a class which is derieves from IEnumerable:

First the Enumerator:

private class DoubleStringEnumerator : IEnumerator
{
    private DoubleString _elemList;
    private int _index;
    public DoubleStringEnumerator(DoubleString doubleStringObjt)
    {
        _elemList = doubleStringObjt;
        _index = -1;
    }
    public void Reset()
    {
        _index = -1;
    }
    public object Current {
        get {
            return _elemList.getNext();
        }
    }
    public bool MoveNext ()
    {
        _index++;
        if (_index >= _elemList.Length)
            return false;
        else
            return true;
    }
}

The Current method does not really reflects it's name in the given example - but it is for learning purposes.

Now the class:

public class DoubleString : IEnumerable
{
    public int Length;
    List<String> listA;
    List<String> listB;
    List<string>.Enumerator eA,eB;
    public DoubleString(List<String> newA,List<String> newB)
    {
        if(newA.Count != newB.Count) {
            throw new Exception("Lists lengths must be the same");    
        }
        listA = newA;
        listB = newB;
        eA = listA.GetEnumerator ();
        eB = listB.GetEnumerator ();
        Length = listA.Count;
    }
    IEnumerator IEnumerable.GetEnumerator ()
    {
        return (IEnumerator)new DoubleStringEnumerator(this);
    }
    public string[] getNext ()
    {
        eA.MoveNext ();
        eB.MoveNext ();
        return new string[] {eA.Current ,eB.Current };
    }
}

And the code itself:

List<String> listA = new List<string> { "A1", "A2" };
List<String> listB = new List<string> { "B1", "B2" };               
DoubleString newDoubleString = new DoubleString (listA, listB);             
string text = "";
foreach (string[] s in newDoubleString) {
    text += s[0] + " and " + s[1] + "\n";
}
Console.WriteLine (text);

Of course still better to use LINQ. The code is not optimsied, but I had no compiler with me so writting from my head - hope it will clarify few things. Feel free to ask questions.

Answer 6

List<int> = new [] { 1, 2, 3, 4 };
List<String> words = new [] { "one", "two", "three" };

var numbersAndWords = numbers.Zip(words, (n, w) => new { Number = n, Word = w });
foreach(var nw in numbersAndWords)
{
    Console.WriteLine(nw.Number + nw.Word);
}

Answer 7

Here is the solution using foreach:

string text = null;
int cnt = 0;

List<String> listA = new List<string> { "A1", "A2" };
List<String> listB = new List<string> { "B1", "B2" };

foreach (string i in listA)
  {
         text += listA[cnt] + " and " + listB[cnt];
         cnt++;
  }

Thanks & Regards,
Subhankar