C#列表<string>将索引添加到列表控制台 output</string>

Question

In the bellow sample scenario i like to add the index of the item to the output console. And i just dont get it to work.

        public static void Main()
        {
            List<string> myList = new List<string> { "This", "is", "a", "test" };
            foreach (var item in myList)
                Debug.WriteLine(item);
        }

Something like this

Debug.WriteLine(item.index + " : : + item);

Answer 1

You can use Select to get an items with their index and print it out

List<string> myList = new List<string> { "This", "is", "a", "test" };
foreach (var item in myList.Select((value, index) => new { value, index }))
    Debug.WriteLine($"{item.value}:{item.index}");

You can also use IndexOf method, bit it'll return a first index and can cause a problem, if list has a duplicated items

foreach (var item in myList)
    Debug.WriteLine($"{item}:{myList.IndexOf(item)}");

Using a regular for loop also might be an option

for (int i = 0; i < myList.Count; i++) 
    Debug.WriteLine($"{myList[i]}:{i}");

Answer 2

public static void Main()
{
    int index = 0;
    List<string> myList = new List<string> { "This", "is", "a", "test" };
    foreach (var item in myList)
    {
        Debug.WriteLine(item + " " + index);
        index++;
    }
}

Remember that sometimes the simplest approach is the best, especially if you are a beginner. While Pavel's answer is also correct and preferable, if you need to ask this question you are just starting with programming, I would advice you to stick to general solutions (incrementing an index variable) which work in all programing languages. Once you are comfortable with those, move to language specific sugars

Answer 3

You can declare index variable:

List<string> myList = new List<string> { "This", "is", "a", "test" };
var index = 0;
foreach (var item in myList)
    Console.WriteLine($"{item} {index++}");

Or using simple for loop where i is alread declared and your loop more flexible - you can iterate from end to start and from start to end of List<T> :

List<string> myList = new List<string> { "This", "is", "a", "test" };
for (int i = 0; i < myList.Count; i++)
{
     Console.WriteLine($"{myList[i]} {i}");
}

Answer 4

The C# foreach does not have an inbuilt index. You can go with two approaches.

int index=-1;
foreach (var item in myList){
     index++;
     Debug.WriteLine(index + ":" + item);
}

or you can use the forloop

for (int i = 0; i < myList.Length; i++)
{
   var item = myList[i];
   Debug.WriteLine(i+ ":" + item);
}

Answer 5

You can use

myList.IndexOf(item)

to access item's index so your code should be:

public static void Main()
{
    List<string> myList = new List<string> { "This", "is", "a", "test" };
    foreach (var item in myList)
        Debug.WriteLine(item + ": " + myList.IndexOf(item));
}