Access last 3 elements of the array in c# and perform action in a single statement-CodePudding

For example, I have n number of integer array elements in c#. Can we access the last 3 elements of the array and modify these elements for instance multiple each element by 4. Can we achieve this in a single statement instead of using foreach/for loop(can we use the regular expression)?

Before operation

arr[0] = 3

.. ..

arr[n-3] = 1

arr[n-2] = 5

arr[n-1] = 6

After operation

arr[0] = 3

.. ..

arr[n-3] = 4

arr[n-2] = 20

arr[n-1] = 24

CodePudding user response：

You can do this:

var arr = new int[] {1, 2, 3, 4, 5};
        
var result = arr.TakeLast(3).Select(x => x * 4).ToArray();

p.s. this is done in .NET 6

CodePudding user response：

With C# 8 (i.e. dotnet core 3) and later you can also use Ranges to write as a one-liner:

int[] a = { 3, 4, 5, 6, 7, 8, 9, 1, 5, 6 };

var result = a[..^3].Concat(a[^3..].Select(e => e * 4));

a[..^3] does return all elements except the last 3.

a[^3..] does return the last 3 elements, which are then multiplied by 4.

The two arrays are then concatenated.

If you want to use result as an array, add ToArray().

CodePudding user response：

If you do not have a formula that applies to all of them you could use tuples:

int []x = new[] {10, 20, 30, 40, 50, 60};

var len =x.Length;

(x[len-3], x[len-2], x[len-1]) = (6, 7659, 854);

Of course, this only works if the array has at least 3 elements. If you have a formula and need to select a dynamic number of items for update, then @Jim de Vries' answer works best.

CodePudding user response：

You can use arr.Length and sub by 3, 2, 1 Like:

int[] arr = new int[] { 1, 2, 3, 4, 5, 6 };
int arrLength = arr.Length;
Console.WriteLine("Before Operation: {0}", string.Join(", ", arr));

arr[arrLength - 3] *= 4;
arr[arrLength - 2] *= 4;
arr[arrLength - 1] *= 4;
Console.WriteLine("After operation: {0}", string.Join(", ", arr));

CodePudding user response：

You can use LINQ for same. Below is the sample code.

int[] arr = new[] { 1, 3, 3, 3, 5 };
int elementsCount = 3;
int multiplier = 4;
var result = arr.Take(arr.Length - elementsCount).Concat(arr.Skip(arr.Length - elementsCount).Select(a => a * multiplier)).ToArray();