# How to get multiple keys input at once in c++

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Hello,

I have been programming with c++ and SDL 2.

I have run into a problem where I want multiple keys input to be read at once but it will only use one key at a time and stop the instructions for each key after another has been hit even though it is still being pressed. (for example using the W key and the A key to move diagonally).  Here is part of the code

		/*switch(e.key.keysym.sym)
{

case SDLK_s:

position.y += vely;

break;

case SDLK_a:

position.x -= velx;

break;

case SDLK_d:

position.x += velx;

break;

case SDLK_w:

position.y -= vely;

break;

default:
break;
}

//here is a second try:

if(e.key.keysym.sym == SDLK_w )
{
position.y -= vely;

if ( e.key.keysym.sym == SDLK_a  )
{
position.y -= vely;
position.x -= velx;
}



It will interrupt the past input and will not accept two key values but only one.  when I hit another key it will stop the past inputs instructions even though it is still being presses.  I know this issue is probably very noobish but any help would be greatly appreciated.

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What Alvaro linked has the problem written rather clearly.

Keyboard events only take place when a keys state changes from being unpressed to pressed, and vice versa

That's how the operating system sends messages in general, it tells when something happens, the most it does after that is send periodic press events again if it is set up for key repeat. So you'll have to adjust your thinking of how to handle key presses to more of a boolean state way of thinking rather than "down" being a continuous event.

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This is how i habdle it in allegro 5, which lets you use both state based and event based input

if(al_get_key(&keyState, ALLEGRO_KEY_A && ALLEGRO_KEY_B))
{
......
}

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There are 2 ways to get input, one is using events (some callback function is called for each single key event, or you get the events stored somewhere) and the other is checking the keyboard state manually (you get the state of the keyboard keys you want and check if they are pressed or not). Here's the reference to use the second method: http://wiki.libsdl.org/SDL_GetKeyboardState

If you use the second the solution it's easy, you have all the information at the same time, but you must check for input periodically even if nothing happened.

Using the first method you can get something similar, you can keep track of the state of a particular key (say LShift) in a boolean. If the key was pressed set it to true, if it was released set it to false. Now, when another key is pressed (say Up) you check for that boolean variable inside the "case Up" block, and walk or run depending on it.

Edit: In you sample of W and A to move diagonaly, you can have some booleans for each direction (W, A, S, D), set those booleans inside the switch and update the x and y position in some update function checking those booleans.

Edited by DiegoSLTS

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Like the others have mentioned you need to figure out what information is being generated and when it is generated. Then map that to what should happen when you press a key, when you release a key and everything else in between.

Here is some (not very good) code I have for basic free-camera movement. It's using GLFW but you should get the idea.

void GraphicsEngine::KeyboardEventHandler( int key, int scancode, int action, int mods )
{
static const float walkingSpeed = 2.0f;
static const float turningSpeed = 35.0f;
auto cameraVelocity = m_camera.GetVelocity();
auto angularCameraVelocity = m_camera.GetAngularVelocity();

switch( key )
{
case GLFW_KEY_W:
if( action == GLFW_PRESS ) cameraVelocity.z += walkingSpeed;
if( action == GLFW_RELEASE ) cameraVelocity.z -= walkingSpeed;
break;
case GLFW_KEY_S:
if( action == GLFW_PRESS ) cameraVelocity.z += -walkingSpeed;
if( action == GLFW_RELEASE ) cameraVelocity.z -= -walkingSpeed;
break;
case GLFW_KEY_A:
if( action == GLFW_PRESS ) cameraVelocity.x += -walkingSpeed;
if( action == GLFW_RELEASE ) cameraVelocity.x -= -walkingSpeed;
break;
case GLFW_KEY_D:
if( action == GLFW_PRESS ) cameraVelocity.x += walkingSpeed;
if( action == GLFW_RELEASE ) cameraVelocity.x -= walkingSpeed;
break;
case GLFW_KEY_E:
if( action == GLFW_PRESS ) cameraVelocity.y += walkingSpeed;
if( action == GLFW_RELEASE ) cameraVelocity.y -= walkingSpeed;
break;
case GLFW_KEY_Q:
if( action == GLFW_PRESS ) cameraVelocity.y += -walkingSpeed;
if( action == GLFW_RELEASE ) cameraVelocity.y -= -walkingSpeed;
break;
case GLFW_KEY_Z:
if( action == GLFW_PRESS ) angularCameraVelocity.y += turningSpeed;
if( action == GLFW_RELEASE ) angularCameraVelocity.y -= turningSpeed;
break;
case GLFW_KEY_C:
if( action == GLFW_PRESS ) angularCameraVelocity.y += -turningSpeed;
if( action == GLFW_RELEASE ) angularCameraVelocity.y -= -turningSpeed;
break;
}

m_camera.SetVelocity( cameraVelocity );
m_camera.SetAngularVelocity( angularCameraVelocity );
}

Edited by Lantre

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Thank you guys very much, all of your input has helped me and I have solved this problem.

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I just want to jump in here and mention, you should be aware some keyboards can NEVER transmit some keys together. This article has more information: http://www.microsoft.com/appliedsciences/antighostingexplained.mspx

Just something to be aware of in general.

Edited by Promit