Infecting\transmitting value to the adjacent items of list, in C#

Question

There is a list of binary values:

List<bool> myList = new List<bool>(){true, true, false, false, true, false, false, false, true, true, true, false, false};

my algorithm aims to convert any false item to true, if they are adjacent to a true value:

result = {true, true, true, true, true, true, false, true, true, true, true, true, false}

My solution works, as you will see. I can do it through two different loops, and then zip two lists:

List<bool> firstList = new List<bool>();
List<bool> secondList = new List<bool>();
for(int i=0; i<myList.Count()-1; i++){
  if(myList[i]==true){
    firstList[i]=true;
    firstList[i+1]=true;
  }
}

for(int i=1; i<myList.Count(); i++){
  if(myList[i]==true){
    secondList[i]=true;
    secondList[i-1]=true;
  }
}

List<bool> finalList = firstList.Zip(secondList, (a,b)=>a||b).ToList();

However, it doesn't seem to be the best solution since the problem looks very easy. Any idea to do it through one loop or preferably using linq?

Answer 1

Here is a Linq approach

Basically it has the same behaviour as your approach - the element self x, the previous .ElementAtOrDefault(i - 1) or the next .ElementAtOrDefault(i + 1) element has to be true.

List<bool> result = myList.Select((x, i) => x || myList.ElementAtOrDefault(i - 1) || myList.ElementAtOrDefault(i + 1)).ToList();

Answer 2

You can do it in one loop:

List<bool> result = myList.Select((b, index) => 
                  b || 
                  (index > 0 && myList[index-1]) || 
                  (index < (myList.Count - 1) && myList[index+1])).ToList();

This takes every b in your myList and checks (via index) if this itself or the adjacting values are true. Of course we have to check index for the list boundaries.

Answer 3

I don't think this is particularly readable, but:

var indexesToChange = 
    Enumerable.Range(0, myList.Count)
    .Where(n => myList[n]
        || (n-1 >= 0 && myList[n-1])
        || (n+1 < myList.Count && myList[n+1]))
    .ToList();

foreach (var i in indexesToChange)
{
    myList[i] = true;
}

This will update the old list. You could copy to a new list in the foreach loop if you don't want to change the old one.