What is the best way to read input from keyboard using SDL? - input

Question

I'm running an update() method n times per second to "update" the keyboard input from the user so I can read it later in the logic part of the program. So I find two ways of implementing this in the SDL Docs and I'm not sure which one should I use.
1; Loop for all events using SDL_PollEvent searching for key down/up events and saving the key states in a map so I can check for each key state in the logic of the program.
Note: Alternatively, I can also use SDL_PeepEvents instead of SDL_PollEvent to take only the event types that matter; so, it would not "thrown away" the events on the queue.
std::map<int, bool> keyboard; // Saves the state(true=pressed; false=released) of each SDL_Key.
void update()
{
SDL_Event event;
while(SDL_PollEvent(&event))
{
switch(event.type)
{
case SDL_KEYDOWN:
keyboard[event.key.keysym.sym] = false;
break;
case SDL_KEYUP:
keyboard[event.key.keysym.sym] = true;
break;
}
}
}
2; Taking a snapshot from the keyboard each frame so I can read it easily.
Uint8* keyboard;
void update()
{
SDL_PumpEvents();
keyboard = SDL_GetKeyState(NULL);
}
With any of above implementations I can read keyboard just like this:
if (key_map[SDLK_Return]) printf("Return has been pressed.");
Also, is there another way to do so?

Answer 1

I prefer to do a variation of 1, where I fill three arrays, indicating not only the current state, but also which keys just went down and which keys just went up. This allows me to easily check for those events in code (without comparing to the previous snapshot), but, most importantly, it won't miss events that last less than a frame. For example, if your game is running at 10 fps due to a slow machine, the user might press and release an important key between two calls of your update routine, and then your system will never register it. This is extremely frustrating.
SDL also sends key events when the key is held down, which allow you to have multiple key down events for each key up. I find this particularly useful when implementing keyboard scrolling through a list of items, e.g. a keyboard-controlled menu.

Answer 2

You should use solution 2.
Why? As SDL_GetKeyState() docs point out, before using it you are expected to call SDL_PumpEvents() to update the state array.
When you are calling SDL_PollEvent(), it implicitly calls SDL_PumpEvents(). So, it basically updates the array for SDL_GetKeyState() anyway. By parsing these events manually, you just create a second array (well, actually a much slower map) holding the same information which SDL already collected for you.
So, I would dare say that first solution means doing the same thing twice. And if you ever decide to support things such as repeated keystrokes (SDL_EnableKeyRepeat()), you'll be reimplementing even a larger part of SDL.

Answer 3

I realize this question is quite old, but my answer could benefit someone. Personally, I use two arrays with SDL_GetKeyState. I store one array holding the current frame's keyboard state, and one array holding that last frame's keyboard state. (With some memcpy commands, it's really easy to update them.) Along with those two arrays, I have a map that converts strings like "A" to the SDL scancode values, but that is optional.
Then, when you need to check if something is released or pressed, you can combine the two arrays to check. (I made this a function.) For example, if you know that the key is pressed this frame, but wasn't pressed last frame, it was clearly just pressed this frame. if (currentFrame["A"] == true && lastFrame["A"] == false) {/*just pressed*/}
You would then do the opposite for the released. I find that method super easy to implement and use.