How to pause/play a sketch in processing with the same button?

I'm trying to implement a pause/play for a sketch with the same key, for example if I press p the sketch should stop and if I press p again, the sketch should start again. So far I used the noLoop()/loop() to do this but with two different keys (p for pause, r for start). It does work if I use keyPressed() and keyReleased() but this means to hold down the key but this doesn't answer my question. Also in the pause mode I used redraw() for a single step while noLoop() and works good. Here is some code I tried so far with two different keys:

public void draw(){
    background(random(255));
}

public void keyPressed(){
    if ( key == 'p' )
        noLoop();
    if ( key == 'r' )
        loop();
    if ( key == 's' )
        redraw();
}

And this is the code with the same key:

public void draw(){
    background(random(255));
}

public void keyPressed(){
    if ( key == 'p' )
        noLoop();
    if ( key == 'p' )
        loop();
    if ( key == 's' )
        redraw();
}

In this case when I press key it doesn't have any effect. And the last one I tried is this:

public void draw(){
    background(random(255));
}
public void keyPressed(){
    if ( key == 'p' )
        noLoop();
    else
        loop();
    if ( key == 's' )
        redraw();
}

In this case when I press 'p' it stops the sketch but is doesn't play again. Because of the 'else' it plays again when I press any key including 's' which suppose to be just for a single step. Any help is more than welcome. Thanks!