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How to modify each element of a list into a new list?

I would like to learn how to do the following code using a => or ForEach or something else. It basically gets all elements of List1, modifies it, and then stores it into a new List2. List1 is not altered.
//List1 exists from before
List<string> List2 = new List<string>();
foreach (element in List1)
{
List2.Add(element + " concact string");
}
Is there an easy way to write this code in a more concise/readable way?

Using the System.Linq extension method Select, it's simple to select each item from List1, modify it, and return the results as a new List<string>:
List<string> List2 = List1.Select(element => element + " concat string").ToList();

var newList = List1.Select(x => x.Concat(" concact string")).ToList();

How to replace nested loops with LINQ - in a clean, manageable manner

Codewise, what it the cleanest way to do this using linq? Below, I have a crude example where I want to find a matching class instance based on name.
class item
{
string name {get;set;}
int identifier {get;set;}
}
void DoSomething()
{
List<item> List1 = GetSampleItems();
List<item> List2 = GetOtherSampleItems();
for(int a=0;a<List1.count;a++)
{
for(int b=0;b<List2.count;b++)
{
if(List1[a].identifier == List2[b].identifier)
{
List1[a].name = List2[b].name;
}
}
}
}

Linq is for querying, not updating, so you'll still need to loop through the results to make the changes, but you can join to match up the two lists like so:
var query = from l1 in List1
join l2 in List2
on l1.identifier equals l2.identifier
select new {l1, l2};
Now loop through the query to update the l1 items:
foreach(var item in query)
item.l1.name = item.l2.name;
As a side note, there's nothing wrong with the way you're doing it (other than you could break out of the inner loop if a match is found). If you understand how it works and the performance is acceptable, there's no compelling reason to change it.

This should work:
var query = from l1 in List1
join l2 in List2 on l1.identifier equals l2.identifier
select new
{
l1values = l1,
l2Name = l2.name
};
foreach(var item in query)
item.l1Values.name = item.l2Name;

A better way is using a Dictionary<TK,TV>:
Dictionary<int,item> l2dic = List2.ToDictionary(x => x.identifier);
item itm;
List1.ForEach(x => {
if(l2dic.TryGetValue(x.identifier,out itm)) {
x.name = itm.name;
}
});
Or as #Rawling says, use a foreach loop instead:
Dictionary<int,item> l2dic = List2.ToDictionary(x => x.identifier);
item itm;
foreach(item x in List1) {
if(l2dic.TryGetValue(x.identifier,out itm)) {
x.name = itm.name;
}
}
Ideone demo (with slight modifications to your item class).
This runs on average in linear time whereas your approach runs in quadratic time.
The assumption is however that the identifiers are unique: no two elements in the same list can have the same identifier.
A concluding note is that variables in general start with a lowercase character so list1 and list2 whereas classes and properties start with a capital one (this Item, Identifier and Name).

How to get values out of IGrouping?

I have applied IGrouping<> over a list - here's what it looks like:
IEnumerable<IGrouping<TierRequest,PingtreeNode>> Tiers
{
get { return ActiveNodes.GroupBy(x => new TierRequest(x.TierID, x.TierTimeout, x.TierMaxRequests)); }
}
Later in my code I iterate over Tiers. Its simple to get the key data using the Key element, but how do I get the IEnumerable<PingtreeNode> that forms the value part?
Thanks in advance

Tiers.Select(group => group.Select(element => ...));

in foreach you can get values like this
foreach(var group in tiers)
{
TierRequest key = group.Key;
PingtreeNode[] values = group.ToArray();
}

The group itself implements IEnumerable<T> and can be iterated over, or used with linq methods.
var firstGroup = Tiers.First();
foreach(var item in firstGroup)
{
item.DoSomething();
}
// or using linq:
firstGroup.Select(item => item.ToString());
// or if you want to iterate over all items at once (kind of unwinds
// the grouping):
var itemNames = Tiers.SelectMany(g => g.ToString()).ToList();

How to get first object out from List<Object> using Linq

I have below code in c# 4.0.
//Dictionary object with Key as string and Value as List of Component type object
Dictionary<String, List<Component>> dic = new Dictionary<String, List<Component>>();
//Here I am trying to do the loping for List<Component>
foreach (List<Component> lstComp in dic.Values.ToList())
{
// Below I am trying to get first component from the lstComp object.
// Can we achieve same thing using LINQ?
// Which one will give more performance as well as good object handling?
Component depCountry = lstComp[0].ComponentValue("Dep");
}

Try:
var firstElement = lstComp.First();
You can also use FirstOrDefault() just in case lstComp does not contain any items.
http://msdn.microsoft.com/en-gb/library/bb340482(v=vs.100).aspx
Edit:
To get the Component Value:
var firstElement = lstComp.First().ComponentValue("Dep");
This would assume there is an element in lstComp. An alternative and safer way would be...
var firstOrDefault = lstComp.FirstOrDefault();
if (firstOrDefault != null)
{
var firstComponentValue = firstOrDefault.ComponentValue("Dep");
}

[0] or .First() will give you the same performance whatever happens.
But your Dictionary could contains IEnumerable<Component> instead of List<Component>, and then you cant use the [] operator. That is where the difference is huge.
So for your example, it doesn't really matters, but for this code, you have no choice to use First():
var dic = new Dictionary<String, IEnumerable<Component>>();
foreach (var components in dic.Values)
{
// you can't use [0] because components is an IEnumerable<Component>
var firstComponent = components.First(); // be aware that it will throw an exception if components is empty.
var depCountry = firstComponent.ComponentValue("Dep");
}

You also can use this:
var firstOrDefault = lstComp.FirstOrDefault();
if(firstOrDefault != null)
{
//doSmth
}

for the linq expression you can use like this :
List<int> list = new List<int>() {1,2,3 };
var result = (from l in list
select l).FirstOrDefault();
for the lambda expression you can use like this
List list = new List() { 1, 2, 3 };
int x = list.FirstOrDefault();

You can do
Component depCountry = lstComp
.Select(x => x.ComponentValue("Dep"))
.FirstOrDefault();
Alternatively if you are wanting this for the entire dictionary of values, you can even tie it back to the key
var newDictionary = dic.Select(x => new
{
Key = x.Key,
Value = x.Value.Select( y =>
{
depCountry = y.ComponentValue("Dep")
}).FirstOrDefault()
}
.Where(x => x.Value != null)
.ToDictionary(x => x.Key, x => x.Value());
This will give you a new dictionary. You can access the values
var myTest = newDictionary[key1].depCountry

Try this to get all the list at first, then your desired element (say the First in your case):
var desiredElementCompoundValueList = new List<YourType>();
dic.Values.ToList().ForEach( elem =>
{
desiredElementCompoundValue.Add(elem.ComponentValue("Dep"));
});
var x = desiredElementCompoundValueList.FirstOrDefault();
To get directly the first element value without a lot of foreach iteration and variable assignment:
var desiredCompoundValue = dic.Values.ToList().Select( elem => elem.CompoundValue("Dep")).FirstOrDefault();
See the difference between the two approaches: in the first one you get the list through a ForEach, then your element. In the second you can get your value in a straight way.
Same result, different computation ;)

There are a bunch of such methods:
.First .FirstOrDefault .Single .SingleOrDefault
Choose which suits you best.

var firstObjectsOfValues = (from d in dic select d.Value[0].ComponentValue("Dep"));

I would to it like this:
//Dictionary object with Key as string and Value as List of Component type object
Dictionary<String, List<Component>> dic = new Dictionary<String, List<Component>>();
//from each element of the dictionary select first component if any
IEnumerable<Component> components = dic.Where(kvp => kvp.Value.Any()).Select(kvp => (kvp.Value.First() as Component).ComponentValue("Dep"));
but only if it is sure that list contains only objects of Component class or children

Using lambda expressions to get a subset where array elements are equal

I have an interesting problem, and I can't seem to figure out the lambda expression to make this work.
I have the following code:
List<string[]> list = GetSomeData(); // Returns large number of string[]'s
List<string[]> list2 = GetSomeData2(); // similar data, but smaller subset
List<string[]> newList = list.FindAll(predicate(string[] line){
return (???);
});
I want to return only those records in list in which element 0 of each string[] is equal to one of the element 0's in list2.
list contains data like this:
"000", "Data", "more data", "etc..."
list2 contains data like this:
"000", "different data", "even more different data"
Fundamentally, i could write this code like this:
List<string[]> newList = new List<string[]>();
foreach(var e in list)
{
foreach(var e2 in list2)
{
if (e[0] == e2[0])
newList.Add(e);
}
}
return newList;
But, i'm trying to use generics and lambda's more, so i'm looking for a nice clean solution. This one is frustrating me though.. maybe a Find inside of a Find?
EDIT:
Marc's answer below lead me to experiment with a varation that looks like this:
var z = list.Where(x => list2.Select(y => y[0]).Contains(x[0])).ToList();
I'm not sure how efficent this is, but it works and is sufficiently succinct. Anyone else have any suggestions?

You could join? I'd use two steps myself, though:
var keys = new HashSet<string>(list2.Select(x => x[0]));
var data = list.Where(x => keys.Contains(x[0]));
If you only have .NET 2.0, then either install LINQBridge and use the above (or similar with a Dictionary<> if LINQBridge doesn't include HashSet<>), or perhaps use nested Find:
var data = list.FindAll(arr => list2.Find(arr2 => arr2[0] == arr[0]) != null);
note though that the Find approach is O(n*m), where-as the HashSet<> approach is O(n+m)...

You could use the Intersect extension method in System.Linq, but you would need to provide an IEqualityComparer to do the work.
static void Main(string[] args)
{
List<string[]> data1 = new List<string[]>();
List<string[]> data2 = new List<string[]>();
var result = data1.Intersect(data2, new Comparer());
}
class Comparer : IEqualityComparer<string[]>
{
#region IEqualityComparer<string[]> Members
bool IEqualityComparer<string[]>.Equals(string[] x, string[] y)
{
return x[0] == y[0];
}
int IEqualityComparer<string[]>.GetHashCode(string[] obj)
{
return obj.GetHashCode();
}
#endregion
}

Intersect may work for you.
Intersect finds all the items that are in both lists.
Ok re-read the question. Intersect doesn't take the order into account.
I have written a slightly more complex linq expression that will return a list of items that are in the same position (index) with the same value.
List<String> list1 = new List<String>() {"000","33", "22", "11", "111"};
List<String> list2 = new List<String>() {"000", "22", "33", "11"};
List<String> subList = list1.Select ((value, index) => new { Value = value, Index = index})
.Where(w => list2.Skip(w.Index).FirstOrDefault() == w.Value )
.Select (s => s.Value).ToList();
Result: {"000", "11"}
Explanation of the query:
Select a set of values and position of that value.
Filter that set where the item in the same position in the second list has the same value.
Select just the value (not the index as well).
Note I used:
list2.Skip(w.Index).FirstOrDefault()
//instead of
list2[w.Index]
So that it will handle lists of different lengths.
If you know the lists will be the same length or list1 will always be shorter then list2[w.Index] would probably a bit faster.