[python 기초] 조건문과 Loop문 + List Comprehension

Block 지정

---

- Indentation - level 당 4 spaces

조건문

---

1. 조건지정(Boolean 값)

1) False

- False, None, 0, "", {}, (), []

2) True

- True, 

- False가 아닌 기타의 모든 것

2. 조건실행

- if, elif, else

var = 100
if var<200:
    print("Expression value is less than 200")
    if var == 150:
        print("Which is 150")
    elif var == 100:
        print("Which is 100")
elif var < 50:
    print("Expression value is less than 50")
else:
    print("Could not find true expression")
 
# Expression value is less than 200
# Which is 100

3. 비교연산자

==

<.<

<=,>=

!=

is와 is not

in과 not in

4. Equality 연산자 : is

x = y = [1, 2, 3]
z = [1, 2, 3]
 
print(x == y) # True
print(x == z) # True
print(x is y) # True
print(x is z) # False

5. Membership: in

print('a' in 'apple') # True
print('b' in 'apple') # False

6. String 비교

print('apple' < 'banana') # True
print([1, 2] < [2, 1]) # True

Loop

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1. while loop

x = 1
while x <= 10:
    print(x)
    x += 1
    
# 1
# 2
# 3
# 4
# 5
# 6
# 7
# 8
# 9
# 10

2. for loop

code block 수행을 for each element of a set(or sequence, or other iterable object)

words = ('Think like a man of action and act like man of thought').split()
for word in words:
    print(word)
    
# Think
# like
# a
# man
# of
# action
# and
# act
# like
# man
# of
# thought

3. Loop에서 나오는 방법

1) break

2) continue

- while 에서는 True 이기 때문에, break를 걸어줘야함

word = input('Enter a word: ')
while word:
    print('the word was '+ word)
    word = input('Enter a word: ')

while True:
    word = input('Enter a word: ')
    if not word: break
    print('The word was '+ word)

1. pass()

2. exec()

직접 system 명령을 수행할 수 있음

보안 issue가 있기 때문에, 가급적 사용안함

3. eval()

print(eval(input('Enter an arithmetic expression: ')))
 
# Enter an arithmetic expression: 5+13
# 18

4.

Range

---

range(start, end, step)

print(range(1, 10, 3)) # range(1, 10, 3)
print(list(range(1, 10, 3))) # [1, 4, 7]

Dictionary에 대한 iteration

---

numbers = {1:'one', 2:'two', 3:'three', 4:'four', 5:'five'}
 
for key in numbers:
    print(key, 'corresonds to', numbers[key])
    
# 1 corresonds to one
# 2 corresonds to two
# 3 corresonds to three
# 4 corresonds to four
# 5 corresonds to five

iteration 관련 utilities

---

- Parallel iteration

animals = ['cat', 'dog', 'rabbit']
eat = ['fish', 'meat', 'carrot']
 
for i in range(len(animals)):
    print(animals[i], 'eats', eat[i])
    
# cat eats fish
# dog eats meat
# rabbit eats carrot

- zip

animals = ['cat', 'dog', 'rabbit']
eat = ['fish', 'meat', 'carrot']
 
print(list(zip(animals, eat))) 
# [('cat', 'fish'), ('dog', 'meat'), ('rabbit', 'carrot')]

- enumerate

animals = ['cat', 'dog', 'rabbit']
eat = ['fish', 'meat', 'carrot']
 
for i, j in enumerate(animals):
    print(i, j)
 
# 0 cat
# 1 dog
# 2 rabbit

- reverse, sort 

- sorted(), reversed()

# reverse, sort; in-place change
# sorted(), reversed()
 
print(sorted([4, 3, 2, 1, 5])) 
# [1, 2, 3, 4, 5]
print(sorted('hello'))
# ['e', 'h', 'l', 'l', 'o']
print(''.join(reversed('hello')))
# olleh

List Comprehension(짱짱 좋은 기능이랩)

---

print([x*x for x in range(10)])
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
 
# 3으로 나누어지는 x로 
print([x*x for x in range(10) if x % 3==0])
# [0, 9, 36, 81]
 
print([(x, y) for x in range(3) for y in range(2)])
# [(0, 0), (0, 1), (1, 0), (1, 1), (2, 0), (2, 1)]
 
result = []
for x in range(3):
    for y in range(2):
                   result.append((x,y))
print(result)
# [(0, 0), (0, 1), (1, 0), (1, 1), (2, 0), (2, 1)]

boys = ['chris', 'bob', 'arnold']
girls = ['alice', 'bernice', 'clarice']
 
print([b+':'+g for b in boys for g in girls if b[0]==g[0]])
# ['chris:clarice', 'bob:bernice', 'arnold:alice']

기타

---

1. pass()

2. exec()

직접 system 명령을 수행할 수 있음

보안 issue가 있기 때문에, 가급적 사용안함

3. eval()

print(eval(input('Enter an arithmetic expression: ')))
 
# Enter an arithmetic expression: 5+13
# 18

4. assert

위험요소를 미리!!! checkpoint로 활용

맞으면 별일 없이 넘어가고, 아니면 error

age = 10
assert 0 <age <100
 
age = -5
assert 0 < age <100
#AssertionError

5. 

x, y, z = 1, 2, 3
print(x, y, z) # 1 2 3
 
x, y = y, x  
print(x, y, z) # 2 1 3
 
x = 2
x += 3
print(x) # 5
 
somename ='son'
somename += 'jiyoung'
print(somename) # sonjiyoung
print(somename*5) #s onjiyoungsonjiyoungsonjiyoungsonjiyoungsonjiyoung