meet it

An iterator can go on forever, safely.

Iterators are : they do no work until someone asks for the next item, and they stop the moment nobody asks. That makes a strange thing possible: an . while (true) inside an iterator is fine, because each yield return pauses it.

The caller decides how much to take. Take(5) asks for five items, then stops asking. TakeWhile(n => n < 60) keeps asking while a test passes. Skip(2) throws the first two away. Never call Count() or ToList() on an infinite sequence: they would wait forever.

using System;
using System.Collections.Generic;
using System.Linq;

var firstFive = Squares().Take(5);
Console.WriteLine(string.Join(" ", firstFive));
var small = Squares().Skip(2).TakeWhile(n => n < 60);
Console.WriteLine(string.Join(" ", small));

IEnumerable<int> Squares()
{
    int n = 1;
    while (true)
    {
        yield return n * n;
        n++;
    }
}

Each query calls Squares() again, so both start from 1.

guess first

An endless sequence, filtered. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

using System;
using System.Collections.Generic;
using System.Linq;

var evens = Fibonacci().Where(n => n % 2 == 0).Take(3);
Console.WriteLine(string.Join(" ", evens));

IEnumerable<int> Fibonacci()
{
    int a = 1, b = 1;
    while (true)
    {
        yield return a;
        int next = a + b;
        a = b;
        b = next;
    }
}

your turn

Your turn. Number the invoices.

On line 5, choose the first invoice number and how many invoices to make. The iterator never ends, so Take decides when to stop.

Your code

output

press run to see what happens

fix it

Page 2 of the results is empty. Can you fix it?

Broken code to fix

output

press run to see what happens

Page 2 comes out empty. Read line 6 from left to right: what happens first?
Take(3) keeps the squares 1, 4 and 9, then Skip(3) throws all three away.
Swap them on line 6 to .Skip(pageSize).Take(pageSize), then run.

nailed it

you can tame sequences that never end. :D

Iterators are lazy: they only work when asked.
Take and TakeWhile stop asking an endless sequence.
Skip first, then take, to read one page.

new words in your collection

lazy
infinite sequence

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