在matlab中反转粒子的方向

i have a problem with an algorithm, i have a space with x,y coordinates and a low and upper bound. I have also some particles within this space free to move. I want that any particle remains into my space and if one of those are moving out the bounders i need to change its direction. If there is no way to bring back the particle i can create a particle in a randomic point within my space. I was thinking with a for loop that check each particles and when one moves out to bring it back to the opposit direction, but i don' t know how to write it.i have a problem with an algorithm, i have a sp