Create list with 2 columns using foreach

Question

I have a list that is just 1 row initially as such:

    One
    Two
    Three
    Four
    Five 
    Six
    Seven

I would then have the following in a list - note how I have 2 columns - first column is for odd number, second column is for even number:

    One     Two
    Three   Four
    Five    Six
    Seven

I am trying the following:

foreach(var item in mod)
{
    int i = 0;

    i = i + 1;
    if (i % 2 == 0) 
    {                  
        //add to list here for even number 
    }

    if (i % 2 != 0)
    {
        // add to list here for odd number
    } 
}

You can replace the 2nd if statement with an else statement. — Jason Down
– Jason Down, Commented Jan 18, 2012 at 22:40

tzaman · Accepted Answer · 2012-01-18 22:43:33Z

2

I'd suggest LINQ:

var odds = mod.Where((item, index) => index % 2 == 0).ToList();
var evens = mod.Where((item, index) => index % 2 == 1).ToList();

answered Jan 18, 2012 at 22:43

tzaman

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3 Comments

ChrisF Over a year ago

But this is two passes through the input list rather one. While not a big thing for this small data set it would have an impact on performance if the input list were large.

Only Bolivian Here Over a year ago

Aren't you iterating through the collection twice with this?

tzaman Over a year ago

True, but until we know that the size is large enough to warrant it, I prefer code simplicity over speed.

Brandon · Accepted Answer · 2012-01-18 22:45:14Z

2

You're redeclaring i each time. Move the declaration outside of the foreach.

List<int> even = new List<int>();
List<int> odd = new List<int>();

int i = 0;
foreach (var item in mod)
{
    i = i + 1;
    if (i % 2 == 0)
    {
        even.Add(i);
    }
    else
    {
        odd.Add(i);
    }
}

answered Jan 18, 2012 at 22:45

Brandon

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Comments

D-Mac · Accepted Answer · 2012-01-18 22:47:36Z

Previous answer had an error:

static void Main(string[] args)
    {
        var mod = new string[] { "One", "Two", "Three", "Four", "Five", "Six", "Seven" };

        var OddNumbers = new List<string>();
        var EvenNumbers = new List<string>();
        int i = 0;
        foreach (var item in mod)
        {

            i = i + 1;
            if (i % 2 == 0)
            {
                EvenNumbers.Add(item);
            }
            else
            {
                OddNumbers.Add(item);
            }
        }
}

        // but when you use an index in your loop I find it more readable to use this
        for (var j = 0; j < mod.Length; ++j)
        {
            if (j % 2 == 0)
                 OddNumbers.Add(mod[j]);
            else
                EvenNumbers.Add(mod[j]);
        }

Only Bolivian Here · Accepted Answer · 2012-01-18 22:41:43Z

1

How about:

int i = 0;
foreach(var item in mod)  //I assume 'mod' is a collection of ints?
{             
    if (i % 2 == 0)
    {
        //Do something with 'item'.
    }
    else 
    {
        //Do something else with 'item'.
    }

    i++;
}

answered Jan 18, 2012 at 22:41

Only Bolivian Here

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Comments

Brissles · Accepted Answer · 2012-01-18 22:41:52Z

1

Based on your last question, I'm guessing you want to do this:

int i = 0; 
foreach(var item in mod) 
{ 
     ..
     // rest of foreach loop here
}

answered Jan 18, 2012 at 22:41

Brissles

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Comments

Xtian Macedo · Accepted Answer · 2012-01-18 22:44:06Z

1

You can create two lists and store the numbers depending on their type:

List<int> OddNumbers = new List<int>();
List<int> EvenNumbers = new List<int>();

then do the following:

         foreach(var item in mod) 
         { 
             if (item % 2 == 0)  
             {                   
                EvenNumbers.Add(item);
             } 
             else 
             {
                OddNumbers.Add(item);
             }
         }

answered Jan 18, 2012 at 22:44

Xtian Macedo

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1 Comment

Edward Thomson Over a year ago

Pretty sure that all the numbers are going to be odd in your code sample.

legrandviking · Accepted Answer · 2012-01-18 22:46:44Z

1

Here is a corrected version of your algoryth that fixes the fact that you'll alway be comparing a value of i = 1 because your declaration of int i was in the loop

    int i = 0; 
     foreach(var item in mod)
     {
         i++;
         if (i % 2 == 0) 
         {                  
            //add to list here for even number 
         }
         else
         {
          // add to list here for odd number
         } 
     }

answered Jan 18, 2012 at 22:46

legrandviking

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Comments

Igby Largeman · Accepted Answer · 2012-01-18 22:49:39Z

1

A simple bool will suffice.

var odds = new List<string>();
var evens = new List<string>();

bool odd = true;
foreach (var item in new[] {"one", "two", "three", "four"})
{
    if (odd)
        odds.Add(item);
    else
        evens.Add(item);

    odd = !odd;
}

answered Jan 18, 2012 at 22:49

Igby Largeman

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Comments

Standage · Accepted Answer · 2012-01-18 23:31:46Z

If you want a list with two columns you could use tuples like this.

            List<Tuple<List<int>, List<int>>> listTest = new List<Tuple<List<int>, List<int>>>();

            List<int> evenNumber = new List<int>();
            List<int> oddNumber = new List<int>();

            int i = 0;

            foreach (var item in mod)
            {

                if (i % 2 == 0)
                {
                   //add to list here for even number 
                    evenNumber.Add(i);                      
                }

                if (i % 2 != 0)
                {
                    // add to list here for odd number
                    oddNumber.Add(i);  
                }

                i++;
            }
            listTest.Add(Tuple.Create(oddNumber, evenNumber));

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