Write a function to find the diameter of a circle.
def diameter_circle(r):
  diameter=2*r
  return diameter
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a function to find the diameter of a circle.
def diameter_circle(r):
  diameter=2*r
  return diameter[/INST]assert diameter_circle(10)==20
assert diameter_circle(40)==80
assert diameter_circle(15)==30</s>