min-pxcor reports the pxcor of the leftmost patches in a model. This primitive, and its siblings max-pxcor, max-pycor, min-pycor, are very useful in modeling the agent behavior that involves the boundaries of an environment. For example, if we wanted to build a model where we had a wall at the edges, we would write the following code:

ask patches [
    if pxcor = max-pxcor or
       pxcor = min-pxcor or
       pycor = max-pycor or
       pycor = min-pycor [
            set pcolor gray
       ]
]

Or if we wanted turtles to not walk beyond the world's borders, we would write the following code:

ask turtles [
    if [pxcor] of patch-ahead 1 < min-pxcor [
        forward 1
    ] 
]

Things to keep in mind when using min-pxcor:

• max-pxcor,min-pxcor,max-pycor, and min-pycor are not variables; they are constant reporters. That is, a code such as set min-pxcor 30 would show an error message.
• You can change your model's size through the Settings button in the Interface Tab or using the resize-world primitive.

In the model example below, we use min-pxcor and its siblings to create walls that represent a container. The balls inside the container bounce off of the green wall but they stick to the red wall.