[Python tutorial] 13. Control flow, Loop, Iterator

While Loops

i = 1
while i < 6:
  print(i)
  i += 1

break

i = 1
while i < 6:
  print(i)
  if i == 3:
    break
  i += 1

continue

i = 0
while i < 6:
  i += 1
  if i == 3:
    continue
  print(i)

else

i = 1
while i < 6:
  print(i)
  i += 1
else:
  print("i is no longer less than 6")

For Loop

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x)

Looping Through a String

for x in "banana":
  print(x)

break

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x)
  if x == "banana":
    break

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  if x == "banana":
    break
  print(x)

continue

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  if x == "banana":
    continue
  print(x)

range()

for x in range(6):
  print(x)

for x in range(2, 6):
  print(x)

for x in range(2, 30, 3): # increment 3
  print(x)

2
5
8
11
14
17
20
23
26
29

Iteration with range() & len()

a = ['a', 'b', 'c']

for i in range(len(a)):
  print(i, a[i])

enumerate()

for i, v in enumerate(['tic', 'tac', 'toe']):
    print(i, v)

Else in For Loop

for x in range(6):
  print(x)
else:
  print("Finally finished!")

0
1
2
3
4
5
Finally finished!

for x in range(6):
  if x == 3: break
  print(x)
else:
  print("Finally finished!")

0
1
2

Nested Loops

adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for x in adj:
  for y in fruits:
    print(x, y)

red apple
red banana
red cherry
big apple
big banana
big cherry
tasty apple
tasty banana
tasty cherry

pass

for x in [0, 1, 2]:
  pass

match - case

def http_error(status):
    match status:
        case 400:
            return "Bad request"
        case 404:
            return "Not found"
        case 418:
            return "I'm a teapot"
        case _: # wildcard
            return "Something's wrong with the internet"

case 401 | 403 | 404: # or
    return "Not allowed"

# point is an (x, y) tuple
match point:
    case (0, 0):
        print("Origin")
    case (0, y):
        print(f"Y={y}")
    case (x, 0):
        print(f"X={x}")
    case (x, y):
        print(f"X={x}, Y={y}")
    case _:
        raise ValueError("Not a point")

Iterators

• 셀 수 있는 값을 포함하는 개체
• 반복할 수 있는 개체
• __iter__() 및 __next__() 메서드로 구성된 반복자 프로토콜을 구현하는 객체

interator & iterable

• iterable: list, tuple, dictionary, set & string
• __inter__() 메서드를 가진 객체

iter() & next()

# tuple
mytuple = ("apple", "banana", "cherry")
myit = iter(mytuple)

print(next(myit))
print(next(myit))
print(next(myit))

# string
mystr = "banana"
myit = iter(mystr)

print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))

Looping Through an Iterator: for...

mytuple = ("apple", "banana", "cherry")

for x in mytuple:
  print(x)

mystr = "banana"

for x in mystr:
  print(x)

Create an Iterator

• 객체/클래스를 이터레이터로 생성하려면 __iter__() 및 __next__() 메서드를 객체에 구현
• __iter__() 항상 반복자 객체 자체를 반환해야 함
• __next__() 메서드로 작업을 수행하고 시퀀스의 다음 항목을 반환

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    x = self.a
    self.a += 1
    return x

myclass = MyNumbers()
myiter = iter(myclass)

print(next(myiter)) # 1
print(next(myiter)) # 2
print(next(myiter)) # 3
print(next(myiter)) # 4
print(next(myiter)) # 5

StopIteration

• 반복이 영원히 계속되는 것 방지 위해 StopIteration 문을 사용
• __next__() 메서드에서 반복이 지정된 횟수만큼 수행되면 오류를 발생시키는 종료 조건을 추가

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    if self.a <= 20:
      x = self.a
      self.a += 1
      return x
    else:
      raise StopIteration

myclass = MyNumbers()
myiter = iter(myclass)

for x in myiter:
  print(x)