C# Security: Computing File Hashes

Hello again! :)

We're celebrating! :D Today, Programmer's Ranch turned one year old, and although I've turned most of my attention to an interesting spare-time project for the time being, I wanted to mark this occasion with a new article. And some cake.

Right, and today's article is about hashing. We've seen in "C# Security: Securing Passwords by Salting and Hashing" that a hash function transforms an input string into a totally different piece of data (a hash):

If you make even a slight change to the input, such as changing the first character from uppercase to lowercase, you get a totally different output:

Also, if you use a decent hash function (i.e. not MD5), it is normally not possible to get the input string from the hash.

In today's article, we're going to use hashes for something much simpler than securing passwords. We're going to hash the content of files, and then use that hash to check whether the file changed. Since I haven't been very impressed with SharpDevelop 5 Beta, I'm going to ditch it and use Visual Studio 2013 instead. You can use whatever you like - SharpDevelop, Visual Studio Express for Desktop, or maybe even MonoDevelop.

Create a new Console Application, and add the following at the top:

using System.Security.Cryptography;

This will allow you to use a variety of hash functions, which all derive from the HashAlgorithm class.

We'll also need a little helper function to convert our hashes from a byte array to a string, so that they may be displayed in hex in the command line. We'll use the following, which is a modified version of the Hash() method from "C# Security: Securing Passwords by Salting and Hashing":

        public static string ToHexString(byte[] bytes)

        {

            StringBuilder sb = new StringBuilder();

            foreach (byte b in bytes)

            sb.Append(b.ToString("x2").ToLower());

            return sb.ToString();

        }

Now, let's create a text file in the same folder as our .sln file and name it "test.txt", and put the following lyrics from the Eagles' "Hotel California" in it:

So I called up the Captain,

"Please bring me my wine"

He said, "We haven't had that spirit here since nineteen sixty nine"

And still those voices are calling from far away,

Wake you up in the middle of the night

Just to hear them say...

Let's read that file into memory. First, we need to add the following:

using System.IO;

We can now read the contents of the file into a string:

            string fileContents = File.ReadAllText(@"../../../test.txt");

...and quite easily compute the hash of those contents:

            using (HashAlgorithm hashAlgorithm = SHA256.Create())

            {

                byte[] plainText = Encoding.UTF8.GetBytes(fileContents);

                byte[] hash = hashAlgorithm.ComputeHash(plainText);

                Console.WriteLine(ToHexString(hash));

            }

            Console.ReadLine();

Note that I'm using SHA256 as the hash function this time - it's a lot more robust than MD5. If you check the documentation for the HashAlgorithm class, you can find a bunch of different hash algorithms you can use. As it is, we get the following output:

Now, let's see what happens if your little toddler manages to climb onto your keyboard and modify the file. Let's remove the first character in the file (the initial "S") - that might be within a toddler's ability - and save the file. When we rerun the program, the output is quite different:

And here we have already seen how hashing gives us a mean to verify a file's integrity, or in other words, check whether it has been tampered with. In fact, popular Linux distributions such as Ubuntu distribute MD5 hashes for the files they release, so that the people who can download them can check that they are really downloading the file they wanted, and not some weird video of goats yelling like humans:

So let's actually see this in action. After downloading an Ubuntu distribution, let's change the filename to that of the Ubuntu file we downloaded, and the hash algorithm to MD5:

            string fileContents = File.ReadAllText(@"../../../../ubuntu-14.04-desktop-amd64.iso");

            using (HashAlgorithm hashAlgorithm = MD5.Create())

Now, let's try to compute a hash of the Ubuntu file:

Oops! We tried to read a ~1GB file into memory, and that's a pretty stupid thing to do. Unless you've got a pretty awesome computer, you'll see the memory usage spike until you get an OutOfMemoryException, as above. And even if you do have a pretty awesome computer, you shouldn't load an entire massive file just to perform an operation on its contents.

In one of my first articles here, "C#: Working with Streams", I explained how you could read a file bit by bit (e.g. line by line) and work on those parts without having to have the entire file in memory at any one time. And quite conveniently, the hash algorithms have a variant of the ComputeHash() method that takes a stream as a parameter.

So let's change our code as follows:

        static void Main(string[] args)

        {

            using (FileStream fs = File.OpenRead(@"../../../../ubuntu-14.04-desktop-amd64.iso"))

            using (HashAlgorithm hashAlgorithm = MD5.Create())

            {

                byte[] hash = hashAlgorithm.ComputeHash(fs);

                Console.WriteLine(ToHexString(hash));

            }

Console.ReadLine();

        }

And let's run it:

There are a few things to note from the output:

• It computes pretty quickly, despite the fact that it's going through a ~1GB file.
• Memory levels remain at a pretty decent level (in fact the memory used by the program is negligible).
• The output matches the first hash in the list of hashes on the Ubuntu webpage (in the background of the above screenshot).

Wonderful! :) In this first anniversary article, we revisited the concept of hashing, and learned the following:

• There are several different hash algorithms provided by .NET that you can use, including MD5, SHA256, and others.
• A hash gives you a way to verify whether a file has been tampered with.
• Streaming provides the ability to process large files quickly and with very little memory overhead.

Thank you so much for reading, and please check back for more interesting articles here at Programmer's Ranch! :)

C# AppSettings: Upgrading Settings between Assembly Versions

Hullo! :)

In "C# AppSettings: Saving User Preferences in a WPF Browser application", we learned how to define and use user settings in a .NET application. In this article we'll deal with a slight complication when it comes to maintaining settings between different application versions. But first, let's build a very simple application to get the point across.

After creating a new Console Application in your favourite IDE, go Add -> New Item... and add a Settings file called... well... Settings.settings! In the designer, add a setting called Name which will be a String in User scope:

In our Main() method, let us now write some code that does something with this setting.

        public static void Main(string[] args)
        {
            string name = Settings.Default.Name;
           
            if (string.IsNullOrEmpty(name))
            {
                Console.WriteLine("Hello! Who might you be?");
                Console.Write("You say: ");
                name = Console.ReadLine();
                Console.WriteLine("Pleased to meet you, {0}!", name);
               
                Settings.Default.Name = name;
                Settings.Default.Save();
            }
            else
            {
                Console.WriteLine("Hi {0}! Nice to see you again!", name);
            }
           
            Console.ReadKey(true);
        }

We're not doing anything fancy here. If the Name setting contains something, then we just output a one-liner; otherwise we ask the user for his name and then save it in the Name setting. Here's what it looks like when you run it for the first time:

And when we run it again:

Now you see, these settings are tied to the version of the program. Each project you compile has something called an assembly version. This is an internal version number assigned to each .exe and .dll file. In SharpDevelop, by default this is set to 1.0.*, which generates something like 1.0.5218.35490 (the actual value is different each time you build, because of the '*' bit). You can find the AssemblyVersion within the AssemblyInfo.cs file that is automatically created when you create a new project:

Now, since a different assembly version is generated with each build, you'll be quick to notice a problem. If you change the code (even just add a space) and build and run again, the setting you had earlier is somehow lost, and you're back to the name prompt:

The problem here is that the settings are tied not only to program and user, but also to the assembly version. Each time the assembly version changes, a new set of settings are created for the application.

We can use a fixed assembly version:

[assembly: AssemblyVersion("1.0.0.0")]

...and now even after changing code, the settings are remembered:

However, the problem remains. At some point we'll want to change that assembly version to, say, 1.0.0.1, and we don't want to lose our settings.

There is actually a solution to this problem, described in this StackOverflow answer. In a nutshell, you need to import the settings from your old application version by upgrading them.

To do this, first add a boolean setting called UpgradeRequired in your settings file, and make sure to set it to True by default:

Before we actually use this, let's change the assembly version to see that the settings are not loaded:

[assembly: AssemblyVersion("1.0.0.1")]

And sure enough, you are asked for your name once again when running the program:

Then, at the beginning of Main(), add the following code to upgrade the existing settings:

            if (Settings.Default.UpgradeRequired)
            {
                Settings.Default.Upgrade();
                Settings.Default.UpgradeRequired = false;
                Settings.Default.Save();
            }

If you now run the application, the old settings are retrieved and saved into the new settings. Note that the UpgradeRequired setting is set to false as part of the upgrade operation, so that it is done only the first time.

It works pretty nicely.

In this article, we learned about the assembly version, an internal version associated with each .dll and .exe file produced when compiling a .NET project. We also learned about the role it plays in .NET user settings, and how to upgrade them to make them survive changes in the assembly version.

SDL2: Animations with Sprite Sheets

Hi people! :)

[Update 2015-11-14: This article is out of date. Check out the latest version at Gigi Labs.]

We've worked with images in several of the SDL2 tutorials published so far. In today's article, we're going to use a very simple technique to animate our images and make them feel more alive.

Before we start, we first need to set up a new project as follows:

1. Follow the steps in "SDL2: Setting up SDL2 in Visual Studio (2013 or any other)" to set up an SDL2 project.
2. Follow the steps at the beginning of "SDL2: Loading Images with SDL_image" to use SDL_image in your project.
3. Configure your project's working directory as described in "SDL2: Displaying text with SDL_ttf", i.e. in your project's Properties -> Configuration Properties -> Debugging, set Working Directory to $(SolutionDir)$(Configuration)\ .
4. Start off with the following code, adapted from "SDL2: Loading Images with SDL_image":

#include <SDL.h>
#include <SDL_image.h>

int main(int argc, char ** argv)
{
    bool quit = false;
    SDL_Event event;

    SDL_Init(SDL_INIT_VIDEO);
    IMG_Init(IMG_INIT_PNG);

    SDL_Window * window = SDL_CreateWindow("SDL2 Sprite Sheets",
        SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 640,
        480, 0);
    SDL_Renderer * renderer = SDL_CreateRenderer(window, -1, 0);
    SDL_Surface * image = IMG_Load("spritesheet.png");
    SDL_Texture * texture = SDL_CreateTextureFromSurface(renderer,
        image);

    while (!quit)
    {
        SDL_WaitEvent(&event);

        switch (event.type)
        {
        case SDL_QUIT:
            quit = true;
            break;
        }

        SDL_RenderCopy(renderer, texture, NULL, NULL);
        SDL_RenderPresent(renderer);
    }

    SDL_DestroyTexture(texture);
    SDL_FreeSurface(image);
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    IMG_Quit();
    SDL_Quit();

    return 0;
}

Before we do anything, we need a sprite sheet. A sprite sheet is really just a cartoon. Like this:

This image is 128 pixels wide and 64 pixels high. It consists of 4 sub-images (called sprites or frames), each 32 pixels wide. If we can rapidly render each image in quick succession, just like a cartoon, then we have an animation! :D

Now, those ugly borders in the image above are just for demonstration purposes. Here's the same image, without borders and with transparency:

If we now try to draw the above on the default black background, we're not going to see anything, are we? Fortunately, it's easy to change the background colour, and we've done it before in "SDL2: Keyboard and Mouse Movement (Events)". Just add the following two lines before the while loop:

    SDL_SetRenderDrawColor(renderer, 168, 230, 255, 255);
    SDL_RenderClear(renderer);

Now we get an early peek at what the output is going to look like. Press Ctrl+Shift+B to build the project, and then copy SDL2.dll, all the SDL_image DLLs, and the spritesheet into the Debug folder where the executable is generated.

Once that is done, hit F5:

So at this point, there are two issues we want to address. First, we don't want our image to take up the whole window, as it's doing above. Secondly, we only want to draw one sprite at a time. Both of these are pretty easy to solve if you remember SDL_RenderCopy()'s last two parameters: a source rectangle (to draw only a portion of the image) and a destination rectangle (to draw the image only to a portion of the screen).

So let's add the following at the beginning of the while loop:

        SDL_Rect srcrect = { 0, 0, 32, 64 };
        SDL_Rect dstrect = { 10, 10, 32, 64 };

...and then update our SDL_RenderCopy() call as follows:

        SDL_RenderCopy(renderer, texture, &srcrect, &dstrect);

Note that the syntax we're using to initialise our SDL_Rects is just shorthand to set all of the x, y, w (width) and h (height) members all at once.

Let's run the program again and see what it looks like:

Okay, so like this we are just rendering the first sprite to a part of the window. Now, let's work on actually animating this. At the beginning of the while loop, add the following:

        Uint32 ticks = SDL_GetTicks();

SDL_GetTicks() gives us the number of milliseconds that passed since the program started. Thanks to this, we can use the current time when calculating which sprite to use. We can then simply divide by 1000 to convert milliseconds to seconds:

        Uint32 seconds = ticks / 1000;

We then divide the seconds by the number of sprites in our spritesheet, in this case 4. Using the modulus operator ensures that the sprite number wraps around, so it is never greater than 3 (remember that counting is always zero-based, so our sprites are numbered 0 to 3).

        Uint32 sprite = seconds % 4;

Finally, we replace our srcrect declaration by the following:

        SDL_Rect srcrect = { sprite * 32, 0, 32, 64 };

Instead of using an x value of zero, as we did before, we're passing in the sprite value (between 0 and 3, based on the current time) multiplied by 32 (the width of a single sprite). So with each second that passes, the sprite will be extracted from the image at x=0, then x=32, then x=64, then x=96, back to x=0, and so on.

Let's run this again:

You'll notice two problems at this stage. First, the animation is very irregular, in fact it doesn't animate at all unless you move the mouse or something. Second, the sprites seem to be dumped onto one another, as shown by the messy image above.

Fortunately, both of these problems can be solved with code we've already used in "SDL2: Keyboard and Mouse Movement (Events)". The first issue is because we're using SDL_WaitEvent(), so the program doesn't do anything unless some event occurs. Thus, we need to replace our call to SDL_WaitEvent() with a call to SDL_PollEvent():

        SDL_PollEvent(&event);

The second problem is because we are drawing sprites without clearing the ones we drew before. All we need to do is add a call to SDL_RenderClear() before we call SDL_RenderCopy():

        SDL_RenderClear(renderer);

Great! You can now admire our little character shuffling at one frame per second:

It's good, but it's a bit slow. We can make it faster by replacing the animation code before the srcrect declaration with the following (10 frames per second):

        Uint32 ticks = SDL_GetTicks();
        Uint32 sprite = (ticks / 100) % 4;

Woohoo! :D Look at that little guy dance!

So in this article, we learned how to animate simple characters using sprite sheets, which are really just a digital version of a cartoon. We used SDL_RenderCopy()'s srcrect parameter to draw just a single sprite from the sheet at a time, and selected that sprite using the current time based on SDL_GetTicks(). The full code is below.

Thanks for reading! Stay tuned for more articles. :)

#include <SDL.h>
#include <SDL_image.h>

int main(int argc, char ** argv)
{
    bool quit = false;
    SDL_Event event;

    SDL_Init(SDL_INIT_VIDEO);
    IMG_Init(IMG_INIT_PNG);

    SDL_Window * window = SDL_CreateWindow("SDL2 Sprite Sheets",
        SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 640,
        480, 0);
    SDL_Renderer * renderer = SDL_CreateRenderer(window, -1, 0);
    SDL_Surface * image = IMG_Load("spritesheet.png");
    SDL_Texture * texture = SDL_CreateTextureFromSurface(renderer,
        image);

    SDL_SetRenderDrawColor(renderer, 168, 230, 255, 255);
    SDL_RenderClear(renderer);

    while (!quit)
    {
        Uint32 ticks = SDL_GetTicks();
        Uint32 sprite = (ticks / 100) % 4;
        SDL_Rect srcrect = { sprite * 32, 0, 32, 64 };
        SDL_Rect dstrect = { 10, 10, 32, 64 };

        SDL_PollEvent(&event);

        switch (event.type)
        {
        case SDL_QUIT:
            quit = true;
            break;
        }

        SDL_RenderClear(renderer);
        SDL_RenderCopy(renderer, texture, &srcrect, &dstrect);
        SDL_RenderPresent(renderer);
    }

    SDL_DestroyTexture(texture);
    SDL_FreeSurface(image);
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    IMG_Quit();
    SDL_Quit();

    return 0;
}

C# AppSettings: Launching Programs and Storing AppSettings

Hi all! :)

Today we're going to write a little program that can start other programs. For example, it might launch MS Word, or your favourite browser. From this we will learn how to actually start other programs using C#, and also how to add settings into an application configuration file so that they can easily be changed.

Start off by creating a new Console Application using your favourite IDE. Throw out any code already in Main(), and add this initial code:

            Console.Title = "Launcher";
          
            // TODO code goes here
          
            Console.ReadLine();

Now, to start a program from our code, we need to use the Process class. To be able to use it, we need to add the following to our usings:

using System.Diagnostics;

Starting a program is really easy. All we need to do is replace our "TODO" comment with the following:

            string filePath = @"C:\tools\notepad++\notepad++.exe";
            Process.Start(filePath);

And sure enough, when we run our program, Notepad++ is opened as well:

Let us now take this a step further. We can allow the user to type in the name of an application, and the corresponding application will be launched. We could do this using a switch statement as in "C# Basics: Command Interpreter Using Methods". Or even better, we can use a Dictionary to store the relationship between application names and their path. First, add the following using at the top if it's not there already (e.g. if you're using SharpDevelop):

using System.Collections.Generic;

Next, our Dictionary, using the collection initialiser syntax as in "C# Basics: Morse Code Converter Using Dictionaries":

            Dictionary<string, string> programs = new Dictionary<string, string>()
            {
                { "notepad++", @"C:\tools\notepad++\notepad++.exe" },
                { "firefox", @"C:\tools\firefox\firefox.exe" }
            };

We can now accept user input and launch the appropriate program. Replace the two lines we wrote earlier (which include the call to Process.Start()) with this:

            Console.Write("Enter program to launch: ");
            string programName = Console.ReadLine();
            if (programs.ContainsKey(programName))
            {
                string path = programs[programName];
                Process.Start(path);
                Console.WriteLine("{0} launched from {1}", programName, path);
            }
            else
                Console.WriteLine("Unknown program");

The user gives us the name of the program he wants to launch. If that name exists in our dictionary, we get the corresponding path, and then launch it as we did before. If we try this now, it works pretty nicely:

Although this works pretty well, it isn't very flexible to have the program names hardcoded in your program code. Imagine you give this program to a friend. It wouldn't be very useful if he could only launch Firefox and Notepad++. What if he wanted to be able to launch SharpDevelop as well? And maybe in future he might want to add VLC as well. It obviously isn't convenient for you to have to change the code, recompile, and give him an updated program each time.

One option could be to store these program settings in a file and read it, but .NET provides us with a simpler option that exists specifically for this kind of scenario. .NET applications can optionally come with an application configuration file, which is usually called App.config. In fact, if, like me, you're using SharpDevelop 5 Beta 2, you should have one already (otherwise just add an Application Configuration File to your project):

When you build the application, App.config gets renamed to the name of the executable, plus a ".config", and is stored in the same folder:

In this App.config file, under the <configuration> node, we can add an <appSettings> node and store our settings. The settings have a key, which is a name that identifies them, and a value which is the value of the setting. Ours will be like this:

    <appSettings>
        <add key="notepad++" value="C:\tools\notepad++\notepad++.exe" />
        <add key="firefox" value="C:\tools\firefox\firefox.exe" />
    </appSettings>

With this done, we can now rewrite our program to use these settings. First, we need to add a reference to System.Configuration. To do this, right click on the name of the project in Solution Explorer, and select "Add Reference":

In the "Add Reference" window, locate "System.Configuration" and then click "OK".

Next, add the following using statement at the top:

using System.Configuration;

We can now use ConfigurationManager to obtain our app settings, like this:

        public static void Main(string[] args)
        {
            Console.Title = "Launcher";
           
            string firefoxPath = ConfigurationManager.AppSettings["firefox"];
            Console.WriteLine(firefoxPath);
           
            Console.ReadLine();
        }

...And here's what we see when we run this:

So now, all we need to do is let the user tell us which program he wants to launch, and find it in the AppSettings:

        public static void Main(string[] args)
        {
            Console.Title = "Launcher";
           
            Console.Write("Enter program to launch: ");
            string programName = Console.ReadLine();
           
            string path = ConfigurationManager.AppSettings[programName];
            if (path == null)
                Console.WriteLine("Unknown program.");
            else
            {
                Process.Start(path);
                Console.WriteLine("{0} launched from {1}", programName, path);
            }
           
            Console.ReadLine();
        }

The only thing worth noting here is that if the specified key does not exist in the AppSettings, retrieving that key via ConfigurationManager.AppSettings[] returns null, allowing us to take appropriate action in that case.

So now, when you give this program to a friend, he can just add the following line in the CsAppSettings.exe.config file that conveniently comes with the program:

         <add key="fiddler" value="C:\tools\fiddler2\Fiddler.exe" />

...and without further ado, he can now launch Fiddler as well:

Wonderful! :)

In this article, we learned how to start another program from our code, and we also learned how to work with application settings in an application configuration file.

Storing your application settings in an App.config file is convenient because they can be tweaked as needed, without having to recompile your program. These kinds of settings are best suited for settings that don't change frequently, so you just set them when you need them, and forget about them. In the next article, we'll learn about another kind of settings that are great for storing stuff like user preferences which can potentially change a lot.

C: Rock, Scissors, Paper (using random numbers)

Hello everyone, and welcome back to the Ranch! :)

In my last C article, "C: Calculating the Average of Several Numbers", we learned how to work with numbers in C. This included using integer variables, input/output of integers, and arithmetic operations.

Today, we're going to take this further and learn how to generate random numbers in C. We'll use this to write a little Rock, Scissors, Paper game.

Pseudo-random numbers in C

So let's start off with a pretty empty program:

#include <stdio.h>

int main(int argc, char ** argv)
{
    return 0;
}

We can generate a random integer using the rand() function, which is defined in stdlib.h. This gives us a random number between 0 and a constant integer called RAND_MAX. We can output the value of RAND_MAX as we would any other integer, although its value is not guaranteed to be the same across different computers:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char ** argv)
{
    int a = rand();
    int b = rand();
    int c = rand();

    printf("RAND_MAX is: %d\n", RAND_MAX);
    printf("a is: %d\n", a);
    printf("b is: %d\n", b);
    printf("c is: %d\n", c);

    return 0;
}

Let us now run this program a few times and see what happens:

On my computer, RAND_MAX is defined as 2147483647, so each time I call rand(), it will give me a random number between 0 and 2147483647.

Another thing you'll see from the output is that each time I run the program, I get the same values for a, b and c. That isn't quite random, is it?

The reason why this is happening is that these random numbers aren't really random.  They are actually generated from a mathematical sequence, that takes a very, very long time to begin repeating itself, but which isn't truly random. These are called pseudo-random numbers. Imagine this sequence:

4, 29, 55, 12, 4, 7, 3, 97, 84, ...

This sequence seems pretty random; there isn't any pattern in it. So each time we get a random number with rand(), we'll get a different number: first 4, then 29, then 55, etc.

Our problem is that each time we run the program, we're starting from the beginning of the sequence, and this gives us the same values. So what we want to do is start from a different position each time.

We do this by seeding the pseudo-random number generator using the srand() function. Let's just add this line at the beginning of main():

    srand(100);

Let's run the program again:

You'll notice the numbers have changed, since we're using a different seed. However, each time we run the program, the same numbers are repeated. In fact we are still making the same mistake as before: we are using the same seed, and starting the pseudo-random number sequence from the same point each time.

In order to use a different seed each time we run the program, we can use the current time as the seed. We can use the time() function, defined in time.h, to give us the current time. So let's include that header file at the top...

#include <time.h>

...and replace our call to srand() with this:

srand(time(NULL));

When we run the program, things work as we expect, and we get different pseudorandom numbers each time we run:

Scaling random numbers

Great. Now, the numbers we are generating are a little bit big. For example, for Rock, Scissors, Paper, we don't need a number between 0 and 2147483647; all we need is a number between 1 and 3 (both inclusive), to represent either rock, scissors, or paper. We can limit the range of a pseudorandom number by dividing it by the maximum we want, and extracting the remainder. This is done using the modulus (%) operator, which gives us the remainder of a division:

    int a = rand() % 10;
    int b = rand() % 10;
    int c = rand() % 10;

Let's say rand() gives us 55. If we divide 55 by 10, then the remainder can be at most 9. So what we're doing above is limiting each random number to the range 0 to 9 (both inclusive). We'll need to add 1 to the number by which we're dividing, in order to make it inclusive (i.e. between 0 and 10 in this case).

What if we need a minimum value that is not zero? In short, we declare a general-purpose function that returns random integers between two integers (both inclusive) like this:

// returns a random number between min and max, both inclusive)
int randomInt(int min, int max)
{
    return min + (rand() % (max - min + 1));
}

We can then use this function from within our main() like this, to give us random numbers between 1 and 10:

    int a = randomInt(1, 10);
    int b = randomInt(1, 10);
    int c = randomInt(1, 10);

And voilà:

Rock, Scissors, Paper in C

OK, so now that we know how to generate pseudorandom numbers, we can write out game of Rock, Scissors, Paper. In this game, two people independently choose one of rock, scissors or paper, and state their choice at the same time. The outcome is then as follows:

• Rock beats scissors (breaks it).
• Scissors beats paper (cuts it).
• Paper beats rock (wraps it).

To start off, let's throw away all our code in main(), but keep our randomInt() function:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

// returns a random number between min and max, both inclusive)
int randomInt(int min, int max)
{
    return min + (rand() % (max - min + 1));
}

int main(int argc, char ** argv)
{
    // code goes here

    return 0;
}

First, we add code to accept user input:

    int input = -1;

    while (input != 0)
    {
        printf("Choose...\n");
        printf("   1 for rock\n");
        printf("   2 for scissors\n");
        printf("   3 for paper\n");
        printf("or 0 to exit\n");

        scanf("%d", &input);
    }

The user will choose a number between 1 and 3 to represent rock, scissors, or paper respectively. A while loop repeats the game until the user enters 0 to exit the game. We have already used for loops in C: Calculating the Average of Several Numbers, but a while loop will simply keep executing the code within curly brackets as long as the condition within round brackets is true, in this case as long as input is not zero:

Once the human player has chosen, the computer player makes his choice at random:

       int computerChoice = randomInt(1, 3);

We can then display the computer's choice:

       switch(computerChoice)
       {
           case 1: printf("Computer chooses rock\n"); break;
           case 2: printf("Computer chooses scissors\n"); break;
           case 3: printf("Computer chooses paper\n"); break;
       }

A switch statement allows us to specify separate code to be executed depending on the value of a variable. In this case, if computerChoice is 1, we write that the computer has chosen rock, and similarly for the other choices. Note that each case in a switch statement should always end in a break statement; otherwise unexpected things may happen. :)

Finally, we can add the game logic to check who won:

       if (input == computerChoice)
       {
           printf("Game ends in a draw.\n");
       }
       else if (input == 1 && computerChoice == 2)
       {
          printf("Rock beats scissors! You win!\n");
       }
       else if (input == 2 && computerChoice == 3)
       {
          printf("Scissors beats paper! You win!\n");
       }
       else if (input == 3 && computerChoice == 1)
       {
          printf("Paper beats rock! You win!\n");
       }
       else if (computerChoice == 1 && input == 2)
       {
          printf("Rock beats scissors! Computer wins!\n");
       }
       else if (computerChoice == 2 && input == 3)
       {
          printf("Scissors beats paper! Computer wins!\n");
       }
       else if (computerChoice == 3 && input == 1)
       {
          printf("Paper beats rock! Computer wins!\n");
       }

Using a cascade of if statements, we compare the user input with the computer's random choice and see who won. The == operator allows us to compare two integers and determine whether they are equal. The && operator, on the other hand, is a boolean AND operator. If both conditions around the && operator are true, then the printf() statement within the curly brackets of that if statement is executed.

We can now actually play our game:

Summary

Great! :) We have a fully working Rock, Scissors, Paper game here. And in creating it, we learned about pseudo-random numbers, while loops, if statements, switch statements, and also a few operators including == (is equal to), != (is not equal to) and && (boolean AND). Below is the full code.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

// returns a random number between min and max, both inclusive)
int randomInt(int min, int max)
{
    return min + (rand() % (max - min + 1));
}

int main(int argc, char ** argv)
{
    int input = -1;

    while (input != 0)
    {
        printf("Choose...\n");
        printf("   1 for rock\n");
        printf("   2 for scissors\n");
        printf("   3 for paper\n");
        printf("or 0 to exit\n");

        scanf("%d", &input);

       int computerChoice = randomInt(1, 3);

       switch(computerChoice)
       {
           case 1: printf("Computer chooses rock\n"); break;
           case 2: printf("Computer chooses scissors\n"); break;
           case 3: printf("Computer chooses paper\n"); break;
       }

       if (input == computerChoice)
       {
           printf("Game ends in a draw.\n");
       }
       else if (input == 1 && computerChoice == 2)
       {
          printf("Rock beats scissors! You win!\n");
       }
       else if (input == 2 && computerChoice == 3)
       {
          printf("Scissors beats paper! You win!\n");
       }
       else if (input == 3 && computerChoice == 1)
       {
          printf("Paper beats rock! You win!\n");
       }
       else if (computerChoice == 1 && input == 2)
       {
          printf("Rock beats scissors! Computer wins!\n");
       }
       else if (computerChoice == 2 && input == 3)
       {
          printf("Scissors beats paper! Computer wins!\n");
       }
       else if (computerChoice == 3 && input == 1)
       {
          printf("Paper beats rock! Computer wins!\n");
       }
    }

    return 0;
}