A sample answer of Coding Bat

array_front9.py

"""
CodingBat: Warmup-2
array_front9
"""

# I think it's okay in Python.
def array_front9(nums):
    return 9 in nums[:4]

# Recursive case.
def array_front9(nums):
  # base case: length = 0 
  if len(nums) == 0:
      return False
  # base case: length = 1
  if len(nums) == 1:
      return nums[0] == 9
  # base case: 2 <= length <= 4
  if len(nums) <=4 and nums[-1] == 9:
      return True
  # recursive case: 5 <= length
  else:
    return array_front9(nums[:-1])

# Loop case. This is a sample answer.
def array_front9(nums):
  # First figure the end for the loop
  end = len(nums)
  if end > 4:
    end = 4
  
  for i in range(end):  # loop over index [0, 1, 2, 3]
    if nums[i] == 9:
      return True
  return False

diff21.py

"""
CodingBat: Warmup-1
diff21
"""


# Not need abs function. It's obvious n is less than 21
def diff21(n):
  if n < 21:
    return abs(n - 21)
  else:
    return abs(n - 21) *2

front3.py

"""
CodingBat: Warmup-1
front3
"""

def front3(str):
  # base case
  if len(str) <= 3:return 3*str
  # recursive case
  # strのlenが3より長い場合は後ろかあら1文字削ってfront3を呼ぶ
  return front3(str[:-1])

front_back.py

"""
CodingBat: Warmup-1
front_back
"""


def front_back(str):

  if str ==  "":
    return str

  front = str[0]
  back = str[-1]
  ans = ''
  
  for i in range(len(str)):
    if i == 0:
      ans += back
    elif i == len(str)-1:
      ans += front
    else:
      ans += str[i]
  return ans



"""
Error answer

def front_back(str):

  if str ==  "":
    return str

  front = str[0]
  back = str[-1]
  ans = ''
  
  for i in range(len(str)):
    if i == 0:
      ans += back
    if i == len(str)-1:
      ans += front
    # こうすると、一個上のifとこのelseでif-elseの構造になってしまう。なので２個目のifがfalseの場合に常にelseでansに文字が追加されてしまう。
    else:
      ans += str[i]
  return ans
"""

front_times.py

"""
CodingBat: Warmup-2
front_times
"""

# Slice
def front_times(str, n):
  return str[:3] * n

# Recursive
def front_times(str, n):
  # base case
  if len(str) <= 3:
    return str * n
  # recursive case
  else:
    return front_times(str[:-1], n)

# Loop case. This is a sample answer.
def front_times(str, n):
  front_len = 3
  if front_len > len(str):
    front_len = len(str)
  front = str[:front_len]

  result = ""
  for i in range(n):
    result = result + front
  return result

last2.py

"""
CodingBat: Warmup-2
last2
"""


# Get the last 2 charcters of a sring
# https://stackoverflow.com/questions/7983820/get-the-last-4-characters-of-a-string
def last2(str):
  
  sub = str[-2:]
  cnt = 0
  for i in range(len(str)-2):
    if (sub == str[i:i+2]):
      cnt += 1
  return cnt

makes10.py

"""
CodingBat: Warmup-1
makes10
"""


def makes10(a, b):
  if (a == 10 or b ==10) or (a + b == 10):
    return True
  else:
    return False

missing_char.py

"""
CodingBat: Warmup-1
missing_char
"""

# 指定した箇所の文字をremoveして詰める処理はslice + concatでOK
def missing_char(str, n):
  return str[:n] + str[n+1:]

monkey_trouble.py

"""
CodingBat: Warmup-1
monkey_trouble
"""


def monkey_trouble(a_smile, b_smile):
  return a_smile == b_smile  

near_hundred.py

"""
CodingBat: Warmup-1
near_hundred
"""

def near_hundred(n):
  return abs(n - 100) <= 10 or abs(n - 200) <= 10

not_string.py

"""
CodingBat: Warmup-1
not_string
"""

def not_string(str):
  # sliceはout of indexが起きないので、strの長さが3文字より短くても大丈夫
  if str[:3] == 'not':
    return str
  else:
    return "not " + str

parrot_trouble.py

"""
CodingBat: Warmup-1
parrot_trouble
"""


def parrot_trouble(talking, hour):
  return not (7 <= hour <= 20) and talking

pos_neg.py

"""
CodingBat: Warmup-1
pos_neg
"""

def pos_neg(a, b, negative):
  if negative:
    # AND
    return (a < 0 and b < 0)
  else:
    # XOR
    # https://en.wikipedia.org/wiki/Exclusive_or
    return (not (a < 0) and (b < 0)) or ((a < 0) and not (b < 0))

sleep_in.py

"""
CodingBat: Warmup-1
sleep_in
"""


def sleep_in(weekday, vacation):
  # Don't forget not oprator
  return not weekday or vacation

string_bits.py

"""
CodingBat: Warmup-2
string_bits
"""

def string_bits(str):
  # base case
  if(len(str)==1 or len(str)==0):
    return str
  # recursive case
  if(len(str)%2==0):
    return string_bits(str[:-1])
  else:
   return string_bits(str[:-1]) + str[-1]

string_splosion.py

"""
CodingBat: Warmup-2
string_splosion
"""

# これもrecursionの典型例
# 文字列生成などはrecursionが相性がいい気がする
def string_splosion(str):
  if len(str) == 1:
    return str
  else:
    return string_splosion(str[:-1]) + str
  

"""
# loopを使う場合
# この問題はrecursionの方が若干イメージしやすい気がする
def string_splosion(str):
  result = ""
  # On each iteration, add the substring of the chars 0..i
  for i in range(len(str)):
    result = result + str[:i+1]
  return result
"""

string_times.py

"""
CodingBat: Warmup-2
string_times
"""


# solution1
# recursionの典型的な型
def string_times(str, n):
  if n == 0:
    return ''
  elif n == 1:
    return str
  else:
    return str + string_times(str, n-1)

# solution2
# Pythonであれば基本はこれでいいと思う
def string_times(str, n):
  return str * n 

# solution1は以下のloopをrecursionに置き換えたもの
"""
def string_times(str, n):
  result = ""
  for i in range(n):  # range(n) is [0, 1, 2, .... n-1]
    result = result + str  # could use += here
  return result
"""

sum_double.py

"""
CodingBat: Warmup-1
sum_double
"""


def sum_double(a, b):
  # ternary conditional operator
  # {True value} if {condition} else {False value}
  #
  # https://stackoverflow.com/questions/394809/does-python-have-a-ternary-conditional-operator
  return (a+b)*2 if a == b else a+b