input: def linearSearch(arr, value):
      """ This function performs a linear search on an array to find the index of the specified value. If the value is found, it returns the index; otherwise, it returns -1.
      >>> linearSearch([5, 3, 7, 2], 7)
      2
      >>> linearSearch([5, 3, 7, 2], 9)
      -1
      """
output:     for i in range(len(arr)):
          if arr[i] == value:
              return i
      return -1