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rzachariah's solution

to Grains in the C# Track

Published at Oct 13 2019 · 3 comments
Instructions
Test suite
Solution

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board, with the number of grains doubling on each successive square.

There are 64 squares on a chessboard (where square 1 has one grain, square 2 has two grains, and so on).

Write code that shows:

  • how many grains were on a given square, and
  • the total number of grains on the chessboard

For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

  • Optimize for speed.
  • Optimize for readability.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

Running the tests

To run the tests, run the command dotnet test from within the exercise directory.

Initially, only the first test will be enabled. This is to encourage you to solve the exercise one step at a time. Once you get the first test passing, remove the Skip property from the next test and work on getting that test passing. Once none of the tests are skipped and they are all passing, you can submit your solution using exercism submit Grains.cs

Further information

For more detailed information about the C# track, including how to get help if you're having trouble, please visit the exercism.io C# language page.

Source

JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

GrainsTest.cs

// This file was auto-generated based on version 1.2.0 of the canonical data.

using System;
using Xunit;

public class GrainsTest
{
    [Fact]
    public void Number_1()
    {
        Assert.Equal(1UL, Grains.Square(1));
    }

    [Fact(Skip = "Remove to run test")]
    public void Number_2()
    {
        Assert.Equal(2UL, Grains.Square(2));
    }

    [Fact(Skip = "Remove to run test")]
    public void Number_3()
    {
        Assert.Equal(4UL, Grains.Square(3));
    }

    [Fact(Skip = "Remove to run test")]
    public void Number_4()
    {
        Assert.Equal(8UL, Grains.Square(4));
    }

    [Fact(Skip = "Remove to run test")]
    public void Number_16()
    {
        Assert.Equal(32768UL, Grains.Square(16));
    }

    [Fact(Skip = "Remove to run test")]
    public void Number_32()
    {
        Assert.Equal(2147483648UL, Grains.Square(32));
    }

    [Fact(Skip = "Remove to run test")]
    public void Number_64()
    {
        Assert.Equal(9223372036854775808UL, Grains.Square(64));
    }

    [Fact(Skip = "Remove to run test")]
    public void Square_0_raises_an_exception()
    {
        Assert.Throws<ArgumentOutOfRangeException>(() => Grains.Square(0));
    }

    [Fact(Skip = "Remove to run test")]
    public void Negative_square_raises_an_exception()
    {
        Assert.Throws<ArgumentOutOfRangeException>(() => Grains.Square(-1));
    }

    [Fact(Skip = "Remove to run test")]
    public void Square_greater_than_64_raises_an_exception()
    {
        Assert.Throws<ArgumentOutOfRangeException>(() => Grains.Square(65));
    }

    [Fact(Skip = "Remove to run test")]
    public void Returns_the_total_number_of_grains_on_the_board()
    {
        Assert.Equal(18446744073709551615UL, Grains.Total());
    }
}
using System;
using System.Linq;

public static class Grains
{
    public static ulong Square(int n)
    {
        if (n < 1 || n > 64) throw new ArgumentOutOfRangeException(nameof(n));
        return (ulong)Math.Pow(2, n - 1);
    }

    public static ulong Total()
    {
        return Enumerable.Range(1, 64)
            .Select(Square)
            .Aggregate((cur, x) => cur + x);
    }
}

Community comments

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Avatar of beldebala

Hello rzachariah's,

Can you please help me understand How Aggregate works.

Thanks, Bala

Avatar of rzachariah
rzachariah
Solution Author
commented 22 days ago

Hi Bala,

Aggregate works like an inductive proof. You define a base state (default is 0) and then you define how to get to the next state given the current state and the next input. ie, you write an accumulator function.

next = f(cur, x)

cur = current state x = next input next = next state

In this, the accumulator function is simply summing the current state with the next input to get the next state.

Hope that helps! Ranj

Avatar of beldebala

It helps :-) Thanks rzachariah

rzachariah's Reflection

I love LINQ.