# Practice Diagnostic Solutions

For most problems there are several correct answers. Important: We stronly encourage you to attempt all of the problems before looking at the solutions. It is much easier to recognize a correct answer than to come up with one.

## 1. Debugging & Tracing (8 points)

#### Part A:

The divide_and_round function correctly divides n by 2 and rounds up to the nearest whole number, but does not return or print n. Therefore, the value of n in main’s print statement is still 42, and so 42 gets printed.

#### Part B:

"""
Part B: Here is the fixed program
"""
def divide_and_round(n):
"""
Divides an integer n by 2 and rounds
up to the nearest whole number
"""
if n % 2 == 0:
n = n / 2
else:
n = (n + 1) / 2
return n

def main():
n = 42
n = divide_and_round(n)
print(n)


## 2. Short Programs (12 points)

#### Odd numbers

There are several ways of solving this problem. Here are a few

def main():
# this approach loops 100 times
for i in range(100):
print(2 * i + 1)   # each time i goes up by 1, this term goes up by 2

#-----------------------------------------------------------#
def main():
# this approach loops 200 times and looks for odd numbers
for i in range(200):
if i % 2 == 1:     # is i odd? If you divide by 2 is 1 left over?
print(i)         # if i is odd, print it!

#-----------------------------------------------------------#
def main():
# this approach uses a while loop
value = 1           # this number keeps track of the current oddnum
while value < 200:  # keep going while its less than 200
print(value)      # dont forget to actually print it
value += 2        # change the value to increase by 2


### Can I Ride Rollercoaster

"""
Write your answer for odd numbers below here:
"""

def main():
height = float(input("Enter height in meters: "))
if height < 1 or height > 2:
print("You can't ride the roller coaster")
else:
print("You can ride the roller coaster ")


## 3. Karel Problem (14 points)

Errata: in Karel problems we generally allow and, or and not. We should have stated this clearly in the practice midterm! We updated the wording on Oct 4th.
def main():
while left_is_clear():
farm_row()
change_row()
farm_row()

def farm_row():
"""
Pick up all the beepers in one row
Pre: Karel is facing East at the start of a row
Post: Karel is facing East at the end of a farmed row
"""
move_to_next_beeper_on_row()
while beepers_present():
farm_beeper()
move_to_next_beeper_on_row()

def change_row():
""" Reposition for the next row """
turn_around()
move_to_wall()
turn_right()
move()
turn_right()

def move_to_next_beeper_on_row():
"""
Pre: Karel is facing east on first column
Post: Karel is either on the next beeper, or
at the end of the row
"""
safe_move()
while no_beepers_present() and front_is_clear():
move()

def farm_beeper():
"""
Pre: Karel is standing on a beeper to be harvested
Post: Karel and the beeper are at the start of the row.
Karel is facing East
"""
pick_beeper()
turn_around()
move_to_wall()
turn_around()
put_beeper()

# Some common utility functions

def safe_move():
if front_is_clear():
move()

def move_to_wall():
while front_is_clear():
move()

def turn_around():
turn_left()
turn_left()


## 4. Python Problem (14 points)

def main():
curr_num = get_num_from_user()
while curr_num != 0:
if is_perfect(curr_num):
print(str(curr_num) + " is perfect!")
else:
print(str(curr_num) + " is not perfect!")
curr_num = get_num_from_user()

def get_num_from_user():
return int(input("Your number: "))

def is_perfect(n):
# this range gives the values 1, 2, ... , n-1
sum_factors = 0
for i in range(1, n):
# i is a factor if it it leaves no remainder
if n % i == 0:
sum_factors += i
return sum_factors == n


## 5. Lists Problem (12 points)

def make_ascending(old_list):
new_list = []
for next_value in old_list:
if is_ascending(new_list, next_value):
new_list.append(next_value)
return new_list

def is_ascending(new_list, next_value):
""" Returns true if next_value should be added to the new_list """
if len(new_list) == 0:
return True
return next_value >= new_list[-1]