Indices and ranges

Daily Knowledge Drop

C#8, first introduced with .NET Core 3, added support for indices and ranges, which provide a succinct syntax for accessing single elements or ranges in a sequence.

Two new operators were introduced to support this functionality:

• ^: The index from end operator
• ..: The range operator

^ operator

The new ^ operator is an index from end operator, which specifies that an index is relative to the end of the sequence.

var words = new string[]
{
    "This", "is", "a", "sequence", "of", 
    "word", "to", "demo", "indices", "and", "ranges"
};

Console.WriteLine(words[^1]); // last word (ranges)
Console.WriteLine(words[^2]); // 2nd last word (and)
Console.WriteLine(words[^3]); // 3rd last word (indices)
Console.WriteLine(words[^4]); // (demo)
Console.WriteLine(words[^5]); // (to)

The output is:

    ranges
    and
    indices
    demo
    to

^1 indicates 1 index from the end, in other words the last item.
^2 indicates 2 indexes from the end, the second last item, etc.

^0 is used to represent the length of the sequence, and is equivalent to sequence.Length.

.. operator

The new .. operator is a range operator, which specifies the start and end of the range as its operands.

Constant values

Constant int values can be used with the .. operator:

var words = new string[]
{
    "This", "is", "a", "sequence", "of", 
    "word", "to", "demo", "indices", "and", "ranges"
};

// get elements 0,1,2,3 and 4 (not 5) from the sequence
var snippet = words[0..5];
// join the items in "snippet" and separate them with a space
Console.WriteLine(string.Join(" ", snippet));

The output is:

    This is a sequence of

0..5 indicates a range of items 0 to 4. words[5] is not included in the range.

'Index from end' values

The range (..) operator can also be used in conjunction with the new ^ operator:

var words = new string[]
{
    "This", "is", "a", "sequence", "of", 
    "word", "to", "demo", "indices", "and", "ranges"
};

// get the 3rd, 2nd and last words from the sequence
var lastThreeWord = words[^3..^0];
// join the items in "lastThreeWord" and separate them with a space
Console.WriteLine(string.Join(" ", lastThreeWord));

The output is:

    indices and ranges

As the last item specified by a Range, is not included in the range, the ^0 is used, to indicate the last item in the sequence when used in a Range.

Range variable

A Range can also be declared as a variable, which has a value set at runtime, then used:

var words = new string[]
{
    "This", "is", "a", "sequence", "of", 
    "word", "to", "demo", "indices", "and", "ranges"
};

Range GetRange(int start, int end)
{
    return start..end;
}

var dynamicWords = words[GetRange(2, 8)];
Console.WriteLine(string.Join(" ", dynamicWords));

The output is:

    a sequence of word to demo

String example

The new operators are not only supported on arrays, but also can also be used on string, as well as Span<T> and ReadOnlySpan<T>

A string example:

string alwaysDeveloping = "alwaysdeveloping.net";

// get the last 4 characters
Console.WriteLine(alwaysDeveloping[^4..^0]);

// get the last character
Console.WriteLine(alwaysDeveloping[^1]);
Console.WriteLine(alwaysDeveloping[alwaysDeveloping.Length - 1]);

The output is:

    .net
    t
    t

As you can see from the output, alwaysDeveloping[^1] is equilveilant to alwaysDeveloping[alwaysDeveloping.Length - 1] - just a lot more concise and succinct.

Notes

While maybe not for everyday use, especially if not dealing with a lot of arrays (and other supported types) - the new operators can prove very useful when they are required, the resulting code being more succinct and less verbose.

References

Indices and ranges

Daily Drop 37: 24-03-2022

At the start of 2022 I set myself the goal of learning one new coding related piece of knowledge a day.
It could be anything - some.NET / C# functionality I wasn't aware of, a design practice, a cool new coding technique, or just something I find interesting. It could be something I knew at one point but had forgotten, or something completely new, which I may or may never actually use.

The Daily Drop is a record of these pieces of knowledge - writing about and summarizing them helps re-enforce the information for myself, as well as potentially helps others learn something new as well.