How to change each element of a List?

Question

I have a List of different DayTime (Ticks). I try to get a list of the time remaining from now to each time element.

List<long> diffliste = new List<long>(m_DummyAtTime);

// 864000000000 ≙ 24h
diffliste.ForEach(item => { item -= now; if (item < 0) item += 864000000000; }); 

// test, does also not work
// diffliste.ForEach(item => { item -= 500; });

However, the list is not changed. Do I miss something? (now is DateTime.Now.TimeOfDay.Ticks)

vc 74 · Accepted Answer · 2011-11-24T10:25:56.623

2

var times = diffliste.Select(ticks => new DateTime(ticks) - DateTime.Now);

Will return a collection of TimeSpans between now and each time.

Without using Linq:

List<TimeSpan> spans = diffliste.ConvertAll(ticks => new DateTime(ticks) - DateTime.Now);

(modified as suggested by Marc)

edited Nov 24 '11 at 10:25

answered Nov 24 '11 at 10:06

vc 74

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This **does not** update the existing list, which is what the OP is asking for. It also **does not** return a collection (it returns a deferred *sequence*) – Marc Gravell Nov 24 '11 at 10:09
@Marc, I know but I suspected that was what the OP needed though he did not explicitly request it – vc 74 Nov 24 '11 at 10:12
This looks like a nice solution. Does it require LINQ or a special .NET-Framework? For compatibility reasons, I couldn't use it then. – Nikodemus RIP Nov 24 '11 at 10:14
@Nikodemus, yes it uses Linq to objects, I guess you can ignore my post if you don't have access to .net >= 3.5 – vc 74 Nov 24 '11 at 10:15
@Nikodemus, I've updated my post with a solution that does not use linq – vc 74 Nov 24 '11 at 10:20
Your updated answer would be cleaner and more efficient as: `List spans = diffliste.ConvertAll(ticks => ...);` – Marc Gravell Nov 24 '11 at 10:22
Ah, ok. That's what I was hoping for. Thank you. – Nikodemus RIP Nov 24 '11 at 10:23

score 1 · Answer 2 · answered Nov 24 '11 at 10:05

You are changing a standalone copy in a local variable (well, parameter actually), not the actual value in the list. To do that, perhaps:

for(int i = 0 ; i < diffliste.Count ; i++) {
    long val = diffliste[i]; // copy the value out from the list
    ... change it
    diffliste[i] = val; // update the value in the list
}

Ultimately, your current code is semantically similar to:

long firstVal = diffliste[0];
firstVal = 42;

which also does not change the first value in the list to 42 (it only changes the local variable).

score 1 · Answer 3 · answered Nov 24 '11 at 10:05

You cannot change the value of an item inside a foreach cycle.
You can do it using a classic for cycle or creating and assigning items to a new list.

for (int i = 0 ; i < diffliste.Count; i++) 
{
    long value = diffliste[i];
    // Do here what you need
    diffliste[i] = value;
}

score 0 · Answer 4 · edited May 23 '17 at 10:08

0

The iteration var in a foreach cycle is immutable, so you cannot change it. You either have to create a new list or use a for cycle... see also here.

edited May 23 '17 at 10:08

Community

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answered Nov 24 '11 at 10:07

mamoo

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The OP is not using `foreach`, they are using `ForEach` - while similar, this exposes the value as a parameter to a delegate, which *is* mutable... it just doesn't change the underlying value in the list in any way – Marc Gravell Nov 24 '11 at 10:10

How to change each element of a List?

4 Answers4