Replace string in the list of string

Question

I want to replace the string "$id" with the next list item string. For example if the list is

var list = new List<string>()
{
    "test",
    "$id",
    "central",
};

then the list will become

"test"
"central"
"central"

Similarly if the list has only 1 item "$id" or "$id" is the last item in the list then it will not be replaced with anything.

I have created the following logic

 int index = list.FindIndex(x => x == "$id");
 if (index < list.Count()-1)
 {
    var newList = list.Select(x => x.Replace(list[index], list[index+1])).ToList();
    list = newList;
 }

Is this the right way of doing it or there is more efficient way of doing it.

Any help or suggestion would be appreciated.

Answer 1

This works for me (provided that the list has at least one element):

list =
    list
        .Skip(1)
        .Zip(list, (x1, x0) => x0 == "$id" ? x1 : x0)
        .Append(list.Last())
        .ToList();

Answer 2

For collections with indexers(Array or List), you can do it in one loop

for(var i = 0; i < values.Length; i++)
{
    if (i > 0 && values[i - 1] == "$id")
    {
        values[i - 1] = values[i];
    }
}

For any type of collection you can use enumerator to "loop" the collection only once and have access to current and previous element.

Approach below supports multiple occurrences of "$id" as well.

public static IEnumerable<string> ReplaceTemplateWithNextValue(
    this IEnumerable<string> source, 
    string template
)
{
    using (var iterator = source.GetEnumerator())
    {
        var previous = default(string);
        var replaceQty = 0;
        while (iterator.MoveNext())
        {
            if (iterator.Current == "$id") replaceQty++;

            if (previous == "$id" && iterator.Current != "$id")
            {
                for (var i = 0; i < replaceQty; i++) yield return iterator.Current;
                replaceQty = 0;
            }

            if (iterator.Current != "$id") yield return iterator.Current;

            previous = iterator.Current;
        }

        if (previous == $"$id")
        {
            for (var i = 0; i < replaceQty; i++) yield return previous;
        }
    }
}    

Usage

var list = new List<string>() { "test", "$id", "central" };

var replaced = list.ReplaceTemplateWithNextValue("$id");
// => { "test", "central", "central" }

Supported cases:

[Fact]
public void TestReplace()
{
    ReplaceId(Enumerable.Empty<string>()).Should().BeEmpty(); // Pass
    ReplaceId(new[] { "one", "two" })
        .Should().BeEquivalentTo(new[] { "one", "two" }); // Pass
    ReplaceId(new[] { "$id", "two" })
        .Should().BeEquivalentTo(new[] { "two", "two" }); // Pass
    ReplaceId(new[] { "one", "$id", "two" })
        .Should().BeEquivalentTo(new[] { "one", "two", "two" }); // Pass
    ReplaceId(new[] { "one", "two", "$id" })
        .Should().BeEquivalentTo(new[] { "one", "two", "$id" }); // Pass
    Replace(new[] { "one", "$id", "$id" })
        .Should().BeEquivalentTo(new[] { "one", "$id", "$id" }); // Pass
}

Answer 3

This seems simpler to me, but maybe doesn't work, or is slow?

int index = list.IndexOf("$id");
list.RemoveAt(index);
list.Insert(index, list[index]); //list[index] was list[index+1] before RemoveAt

// With error checks:
int index = list.IndexOf("$id");
if (index == -1) return; // or return error;
list.RemoveAt(index);
if (index >= list.Count-1) return;  // or return error;
list.Insert(index, list[index+1]);