Sometimes, you have to find the index of an element that matches a condition within in a collection. This is done like this:

int firstMatchingIndex;
for (int i = 0; i < collection.Count; ++i)
{
  if (!predicate(collection[i]) continue;
  firstMatchingIndex = i; break;
}

This takes a minimum of 6 lines of code that take some time to decode their purpose.

List<T> has a FindIndex method that shortens this syntax to a single line:

int firstMatchingIndex = collection.FindIndex(e => { /* predicate */ });

But this method is only available to List<T> and does not support finding the last matching index.

This package provides FirstIndex and LastIndex to all IList<T> types to find the first and last index of an element matching a condition, respectively. Due to using manual index counting in its implementation, FirstIndex is also available to all IEnumerable<T> collections.

Example:

using CLSS;

var numbers = new int[] { 6, 4, -3, 15, -18, 9, 23 };
var firstNegativeIdx = numbers.FirstIndex(n => n < 0); // 2
var lastSingleDigitIdx = numbers.LastIndex(n => n > -10 && n < 10); // 5

Note: FirstIndex and LastIndex work on all types implementing the IList<T> interface, including raw C# array.