yoyo

; 创建 turtles 和 patches
create-turtles 50 [
  set color blue
  setxy random-xcor random-ycor
]

ask turtles [
  ; 每只 turtle 随机移动一步
  fd 1
]

ask turtles [
  ; 如果 turtle 到达边界，就改变颜色并反弹
  if xcor >= max-pxcor or xcor <= min-pxcor [
    set color red
    set heading (- heading)
  ]
  if ycor >= max-pycor or ycor <= min-pycor [
    set color green
    set heading (180 - heading)
  ]
]

; 在 patches 上绘制一个简单的图案
ask patches [
  if pycor = 0 [
    set pcolor yellow
  ]
]

; 打印 turtles 的数量
print count turtles