CODEWITHSUNDEEP
c#linqforeach
Johnny
Johnny

Reputation: 195

How alter List<int> with LINQ ForEach

I have a List<int> myInts and want to multiply all with 10. I want to use linq (not foreach loop).I tryed this but nothing happend:

List<int> myInts = new List<int>() { 1, 2, 3 };
myInts .ForEach(act => act=act*10);

Of what do I have to take care in the .ForEach(...) part? And yes, I want to use ForEach if it is somehow possible.

Probably its simple, but I cant see it, I apoligize. Thank you all!

Upvotes: 0

Views: 2387

Answers (8)

Johnny
Johnny

Reputation: 195

To use a .Select or .ConvertAll are good solutions.

But my intention was to let "ForEach" return an alterd list. I found out, over msdn documentation, that this isn´t possible because ForEach is a void type and has no returntype.

This kind of action works if I would have objects in my List instead of ints. Then I would be able to use the "void" Method to change the properties of my objects.

Upvotes: 1

user2864740
user2864740

Reputation: 61865

"Nothing happens" because reassigning to the local variable (act) has no effect in the caller (ForEach) - C# is Call By Value (except for ref/out parameters).

To modify the list in place, simply use a standard for-each over the indices (which I find readable and upfront of the side-effect intent):

var myInts = new List<int>() { 1, 2, 3 };
for (var i = 0; i < myInts.Count; i++) {
    myInts[i] = myInts[i] * 10;
}

To perform the operation and create a new list/sequence (which can be re-assigned to the same variable), see IEnumerable.Select which is a map transformation.

Upvotes: 3

ojlovecd
ojlovecd

Reputation: 4892

Another and simpler solution:

list = list.ConvertAll(i => i * 10);

Upvotes: 3

ojlovecd
ojlovecd

Reputation: 4892

try this:

Enumerable.Range(0, myInts.Count).ToList().ForEach(i => myInts[i] = myInts[i] * 10);

Upvotes: 0

StuartLC
StuartLC

Reputation: 107237

What is happening is that you are getting a value copy of the int to your the lambda, which so you won't be able to change the 'external' int.

How about projecting a new list?

List<int> myInts = new List<int>() { 1, 2, 3 };
myInts = myInts.Select(act => act*10).ToList();

Upvotes: 1

zey
zey

Reputation: 6105

Do you mean like this ?

     List<int> _tempList = new List<int>();
     myInts.ToList().ForEach(x => _tempList.Add(x * 10));

Upvotes: 0

Luc Morin
Luc Morin

Reputation: 5380

From MSDN documentation:

Modifying the underlying collection in the body of the Action<T> delegate 
is not supported and causes undefined behavior.

So, you need to project your exisistin List into a new one, or you need to use a for loop if you must modify the List "in place"

Regards

Upvotes: 2

David
David

Reputation: 623

This creates a new instance of List.

myInts = myInts.Select(p=>p*10).ToList();

Upvotes: 5

Related Questions