Key States

Last Updated 2/24/06

You already know how to do event driven and time driven programming, now it's time to learn state driven programming.

This tutorial will teach you to check if the key is pressed, rather than monitoring key events.

//While the user hasn't quit while( quit == false ) { //While there's events to handle while( SDL_PollEvent( &event ) ) { //If the user has Xed out the window if( event.type == SDL_QUIT ) { //Quit the program quit = true; } }

As you can see, despite the fact that we're showing the messages based on key presses we don't check for any key events.

//Get the keystates Uint8 *keystates = SDL_GetKeyState( NULL );

Instead what we do is use SDL_GetKeyState().

What SDL_GetKeyState() does is give us the key state array. The key state array is a list of every key and whether the key is pressed or not, kind of like this:


Now we can tell which key is down.

Just for information's sake, the argument we give SDL_GetKeyState() gets the number of keys available. Since we don't care about how many keys there are, we just set it to NULL.

//If up is pressed if( keystates[ SDLK_UP ] ) { apply_surface( ( SCREEN_WIDTH - up->w ) / 2, ( SCREEN_HEIGHT / 2 - up->h ) / 2, up, screen ); } //If down is pressed if( keystates[ SDLK_DOWN ] ) { apply_surface( ( SCREEN_WIDTH - down->w ) / 2, ( SCREEN_HEIGHT / 2 - down->h ) / 2 + ( SCREEN_HEIGHT / 2 ), down, screen ); } //If left is pressed if( keystates[ SDLK_LEFT ] ) { apply_surface( ( SCREEN_WIDTH / 2 - left->w ) / 2, ( SCREEN_HEIGHT - left->h ) / 2, left, screen ); } //If right is pressed if( keystates[ SDLK_RIGHT ] ) { apply_surface( ( SCREEN_WIDTH / 2 - right->w ) / 2 + ( SCREEN_WIDTH / 2 ), ( SCREEN_HEIGHT - right->h ) / 2, right, screen ); } //Update the screen if( SDL_Flip( screen ) == -1 ) { return 1; }

Here's a basic if up is pressed show the up message, if down is pressed show the down message, etc.

Had this program been done with events, the code would be a good size longer.

SDL_GetKeyState() and other state functions like SDL_GetModState(), SDL_GetMouseState(), SDL_JoystickGetAxis() and others can be incredibly useful. Learn more about them in the SDL API reference, which you should have handy on your computer.

Download the media and source code for this tutorial .