Control pingpong expression with slider controler

What are you trying to do? loopOut("pingpong") can create a pendulum or metronome animation or make a ball bounce up and down forever. Do you want the animation to accelerate, decelerate and repeat? Starting and stopping the animation is easiest to accomplish by just splitting the layer and turning off the expression (Shift + Ctrl/Cmnd + d). There is a great temptation to overcomplicate things by doing everything on one layer.

Controlling the speed and even the direction of an animation with an expression and a slider is explained very well by Dan Ebberts on his Expression Speed and Frequency Control - MotionScript.com website. 

If you explain your design goal, we can point you to the most efficient workflow. Directly controlling loopOut("ping-pong") with a slider will be a very complex expression. Pre-composing and time remapping is going to be an efficient way to slow down, stop, then speed up or even time-reverse any animation.

Try something like this:

• Add a slider control to your layer
• Enable Time Remapping
• In the expressions area, type: 

• linear(effect("Slider Control")("Slider"),0,100,0,3)

• 0 = the bottom value of the slider
• 100 = the top value
• 0 = the start of the video, e.g. 0 seconds
• 3 = equals the length, in this case three seconds