input: 
  def find_index(arr, value):
      """
      Performs a linear search on an array for a given value and returns its index.
      If the value is not found, the function returns -1.
      
      >>> find_index([5, 3, 7, 2], 7)
      2
      >>> find_index([5, 3, 7, 2], 1)
      -1
      """
  
output:     for i in range(len(arr)):
          if arr[i] == value:
              return i
      return -1