18. Generating Random Numbers

Learn how to generate random numbers.

18.1 Generating Random Integers

• Random(a, b) randomly generates integers from a to b inclusive
  • a < b is required
• The pattern of generated random numbers is different each time

Output 10 random numbers from 1 to 6

# include <Siv3D.hpp>

void Main()
{
	Scene::SetBackground(ColorF{ 0.6, 0.8, 0.7 });

	for (int32 i = 0; i < 10; ++i)
	{
		// Output random numbers from 1 to 6
		Print << Random(1, 6);
	}

	while (System::Update())
	{

	}
}

18.2 Fortune Telling Program

• By displaying different results based on random values, you can create a fortune telling app
• The following program randomly displays one of four emojis each time you click the mouse

Display random emoji from 4 options each click

# include <Siv3D.hpp>

void Main()
{
	Scene::SetBackground(ColorF{ 0.6, 0.8, 0.7 });

	const Texture emoji0{ U"😄"_emoji };
	const Texture emoji1{ U"😵‍💫"_emoji };
	const Texture emoji2{ U"😭"_emoji };
	const Texture emoji3{ U"😋"_emoji };

	// Emoji number
	int32 emojiIndex = 0;

	while (System::Update())
	{
		if (MouseL.down())
		{
			// Randomly select new emoji number
			emojiIndex = Random(0, 3);

			// Output emoji number
			Print << emojiIndex;
		}

		if (emojiIndex == 0)
		{
			emoji0.drawAt(400, 300);
		}
		else if (emojiIndex == 1)
		{
			emoji1.drawAt(400, 300);
		}
		else if (emojiIndex == 2)
		{
			emoji2.drawAt(400, 300);
		}
		else
		{
			emoji3.drawAt(400, 300);
		}
	}
}

18.3 Generating Random Floating Point Numbers

• When a and b in Random(a, b) are floating point numbers, it randomly generates floating point numbers from a inclusive to b exclusive
  • a < b is required
  • This differs from integers where it's "from a to b inclusive"
• The following code randomly changes the emoji's scale each time you click

Randomly change emoji scale each click

# include <Siv3D.hpp>

void Main()
{
	Scene::SetBackground(ColorF{ 0.6, 0.8, 0.7 });

	const Texture emoji{ U"🎁"_emoji };

	// Scale factor
	double scale = 1.0;

	while (System::Update())
	{
		if (MouseL.down())
		{
			// Generate random value from 0.5 inclusive to 2.0 exclusive
			scale = Random(0.5, 2.0);

			// Output scale factor
			Print << scale;
		}

		emoji.scaled(scale).drawAt(400, 300);
	}
}

18.4 Moving to Random Coordinates

• Determine X and Y coordinates with random numbers to move the emoji to a random location

Emoji moves to random location each click

# include <Siv3D.hpp>

void Main()
{
	Scene::SetBackground(ColorF{ 0.6, 0.8, 0.7 });

	const Texture emoji{ U"🛸"_emoji };

	Vec2 pos{ 400, 300 };

	while (System::Update())
	{
		if (MouseL.down())
		{
			pos = Vec2{ Random(100, 700), Random(100, 500) };
		}

		emoji.drawAt(pos);
	}
}

🧩 Practice

Try creating the following programs.

Practice ① Rolling Dice

• Stop the rotating (numbers changing) dice by clicking
• Click again to start rotating again

Hint

# include <Siv3D.hpp>

void Main()
{
	Scene::SetBackground(ColorF{ 0.6, 0.8, 0.7 });

	const Font font{ FontMethod::MSDF, 48, Typeface::Bold };

	// Dice square area
	const Rect diceRect{ Arg::center(400, 300), 200 };

	// Dice result
	int32 result = 1;

	while (System::Update())
	{
		// If mouse cursor is over the dice
		if (diceRect.mouseOver())
		{
			// Change mouse cursor to hand icon
			Cursor::RequestStyle(CursorStyle::Hand);
		}

		// Draw dice square
		diceRect.draw();

		// Draw dice number
		font(U"{}"_fmt(result)).drawAt(120, Vec2{ 400, 300 }, ColorF{ 0.1 });
	}
}

Sample Solution

• not isRolling has the same meaning as !isRolling. Siv3D adopts the style of writing ! as not for visibility

# include <Siv3D.hpp>

void Main()
{
	Scene::SetBackground(ColorF{ 0.6, 0.8, 0.7 });

	const Font font{ FontMethod::MSDF, 48, Typeface::Bold };

	// Dice square area
	const Rect diceRect{ Arg::center(400, 300), 200 };

	// Dice result
	int32 result = 1;

	// Whether rolling
	bool isRolling = true;

	while (System::Update())
	{
		// If rolling
		if (isRolling)
		{
			// Change dice result to random value
			result = Random(1, 6);
		}

		// If mouse cursor is over the dice
		if (diceRect.mouseOver())
		{
			// Change mouse cursor to hand icon
			Cursor::RequestStyle(CursorStyle::Hand);
		}

		// If dice is left-clicked
		if (diceRect.leftClicked())
		{
			// Toggle rolling state
			isRolling = (not isRolling);
		}

		// Draw dice square
		diceRect.draw();

		// Draw dice number
		font(U"{}"_fmt(result)).drawAt(120, Vec2{ 400, 300 }, ColorF{ 0.1 });
	}
}

Review Checklist

• Learned that Random(a, b) randomly generates integers from a to b inclusive
• Learned that when a and b in Random(a, b) are floating point numbers, it randomly generates floating point numbers from a inclusive to b exclusive