HashSet in C#

A collection that answers one question fast: have I seen this before?

var seen = new HashSet<string> { "read", "eval" };
Console.WriteLine(seen.Add("print"));   // true: it was new
Console.WriteLine(seen.Add("read"));    // false: already there

var other = new HashSet<string> { "eval", "loop" };
seen.IntersectWith(other);
Console.WriteLine(string.Join(",", seen));
Console.WriteLine(seen.Contains("loop"));

How it works

1. Add returns false instead of storing a duplicate.
2. IntersectWith mutates the set down to the overlap.
3. Contains is the constant-time membership test the set exists for.

Keywords and builtins used here

• new
• string
• var

The run, in numbers

Lines

8

Characters to type

339

Tokens

87

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 55 seconds.

Type this snippet

Step 4 of 4 in Collections, step 14 of 29 in Language basics.

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HashSet in other languages

• Rust