Obstacle Avoiding Robot with L293D

I want to write a code with Ardiuno.

it's OK - you don't have to ask permission.

I had changed the code which i wrote before.

And . . ?
What happened?

motors always will run because sensor is not see the any obstacles which is behind the sensor.

I can't think of many directional sensors that will detect objects placed behind them

(what about the code tags?)