Reputation: 2789
I'm not sure if this is the right place to ask this, I'm sorry if not.
using the same code as in:
string[] digits = { "zero", "one", "two", "three", "four", "five",
"six", "seven", "eight", "nine" };
var shortDigits = digits.Where((digit, index) => digit.Length < index);
foreach (var sD in shortDigits)
{
Console.WriteLine(sD);
}
// Output:
// five
// six
// seven
// eight
// nine
How does the system know that, digit
are the items in digits, and index
the position within the array?
Upvotes: 3
Views: 171
Reputation: 57220
The overload of Where
that you are using is implemented like this:
public static IEnumerable<T> Where<T>(
this IEnumerable<T> list, Func<T,int,bool> predicate)
{
int num = -1;
foreach (T current in list)
{
num++;
if (predicate(current, num))
{
yield return current;
}
}
yield break;
}
as you can see, on each iteration, the predicate (i.e. the passed lambda expression) is invoked passing the current element of the list and the current index.
In this way, the code in the lambda expression know both the element and its index.
Upvotes: 1
Reputation: 67135
This is just a naming thing, you could name digit anything, and index anything. This is just a lambda expression for an anonymous function...it could be rewritten:
(string digit, int index) => digit.Length < index
so, the names are the same as any parameter you normally set up in a method. There is nothing truly special about the names being recognized by the C# engine.
(string index, int digit) => index.Length < digit
The above would work also...it would be confusing, but it would work. It is just to show that the name can be whatever you want
If you are referring how to the signature itself then it is due to an overload of the Where function
public static IEnumerable<TSource> Where<TSource>(
this IEnumerable<TSource> source,
Func<TSource, int, bool> predicate
)
So, the TSource in this case is string
, making the Func become Func<string, int, bool>
. Meaning that the lambda must take a string
parameter followed by an int
param, and return a bool
value
Upvotes: 2
Reputation: 839234
That particular overload of the Where
extension method is defined to accept a Func
where the first parameter is the element and the second parameter is the index of that element. This is mentioned explicitly in the documentation:
predicate
Type:
System.Func<TSource, Int32, Boolean>
A function to test each source element for a condition; the second parameter of the function represents the index of the source element.
Emphasis mine.
Upvotes: 6
Reputation: 33272
Because there is an overload of where
taking as a predicate a function like this:
Func<TSource, int, bool>
the semantic of this predicate ios defined so the second parameter is the index. So if you pass a lambda with two args that version of where is called.
Upvotes: 0