Module 4: Every foreach Starts Over

Practice Tasks

Complete these tasks to reinforce what you learned in this module.

Task 1: Count the runs

Write the iterator so that the counter records how many times the sequence was enumerated. Main enumerates it twice and prints the counter.

Expected Output:

The number 2.

Hint: Increment the counter as the very first statement of the iterator body — it runs once per enumeration, when the first value is requested.

Starter Code:

task.cs

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

class Program
{
    static int _calls;

    static IEnumerable<int> Source()
    {
        // Record that an enumeration has begun, then hand back 1, 2, 3
        yield break;
    }

    static void Main()
    {
        var seq = Source();

        seq.Count();
        seq.Sum();

        Console.WriteLine(_calls);
    }
}

Task 2: Run it once

Same sequence, same two operations — but the source must run exactly once. Change only Main.

Expected Output:

The number 1.

Hint: Materialise the sequence with ToList() before using it.

Starter Code:

task.cs

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

class Program
{
    static int _calls;

    static IEnumerable<int> Source()
    {
        _calls++;
        yield return 1;
        yield return 2;
        yield return 3;
    }

    static void Main()
    {
        var seq = Source();

        seq.Count();
        seq.Sum();

        Console.WriteLine(_calls);
    }
}

Task 3: Report the count and the items, reading once

Print how many even numbers there are, then each of them — while the source runs only once. Its producing message must appear exactly one time.

Expected Output:

`producing`, then 3, then 2, 4 and 6.

Hint: Take the sequence into a List<int> first, then use it for both the count and the loop.

Starter Code:

task.cs

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

class Program
{
    static IEnumerable<int> Evens()
    {
        Console.WriteLine("producing");
        yield return 2;
        yield return 4;
        yield return 6;
    }

    static void Main()
    {
        // Print the count, then every item — reading the source only once
    }
}

using System; using System.Collections.Generic; using System.Linq; class Program { static int _calls; static IEnumerable<int> Source() { // Record that an enumeration has begun, then hand back 1, 2, 3 yield break; } static void Main() { var seq = Source(); seq.Count(); seq.Sum(); Console.WriteLine(_calls); } }

using System; using System.Collections.Generic; using System.Linq; class Program { static int _calls; static IEnumerable<int> Source() { _calls++; yield return 1; yield return 2; yield return 3; } static void Main() { var seq = Source(); seq.Count(); seq.Sum(); Console.WriteLine(_calls); } }

using System; using System.Collections.Generic; using System.Linq; class Program { static IEnumerable<int> Evens() { Console.WriteLine("producing"); yield return 2; yield return 4; yield return 6; } static void Main() { // Print the count, then every item — reading the source only once } }