python判断とループ

じょうけんけってい
一般書式

if :           # if test    
         # Associated block
elif :         # Optional elifs    
    
else:                 # Optional else    
    

if/else三元式

    A = Y if X else Z

例

#    
>>> if choice == 'spam':
...     print(1.25)
... elif choice == 'ham':
...     print(1.99)
... elif choice == 'eggs':
...     print(0.99)
... elif choice == 'bacon':
...     print(1.10)
... else:
...     print('Bad choice')

#     
>>> A = 't' if '' else 'f'
>>> A
'f'

ループ
while一般フォーマット

while :           # Loop test    
           # Loop body
else:                   # Optional else   
           # Run if didn't exit loop with break

breakは、最近存在するループから飛び出します(ループ文全体をスキップします).

continueは、最近のループの先頭にジャンプします(ループの最初の行に来ます).

passは何もしないで、ただ空占位文です.バージョン違いのヒント:Python 3.0(Python 2.6ではなく)式を使用できる場所で使用できます...(3つの連続するポイント番号)でコードを省略します.

ループelseブロックは、ループが正常に離れたときにのみ実行される(すなわちbreak文に遭遇しない).

while一般フォーマット

while :   
       
    if : break     # Exit loop now, skip else   
    if : continue  # Go to top of loop now, to test1
else:   
             # Run if we didn't hit a 'break'

while例

while x:                   # Exit when x empty    
    if match(x[0]):        
        print('Ni')        
        break              # Exit, go around else    
    x = x[1:]
else:    
    print('Not found')     # Only here if exhausted x

for一般フォーマット

for  in 

for例

#  
>>> for x in ["spam", "eggs", "ham"]:
...     print(x, end=' ')
...spam eggs ham

#   
S = "lumberjack"
>>> for x in S: print(x, end=' ')   # Iterate over a string...l u m b e r j a c k

#  
>>> T = [(1, 2), (3, 4), (5, 6)]
>>> for (a, b) in T:                   # Tuple assignment at work
...     print(a, b)
...1 2
   3 4
   5 6

#  
>>> D = {'a': 1, 'b': 2, 'c': 3}
>>> for key in D:
...    print(key, '=>', D[key])          # Use dict keys iterator and index
...a => 1
   c => 3
   b => 2

#items      ( ,  )     ，           。
>>> for (key, value) in D.items():
...    print(key, '=>', value)            # Iterate over both keys and values
...a => 1
   c => 3
   b => 2
   
#       
>>> S = 'abcdefghijk'
>>> for c in S[::2]: 
    print(c, end=' ')
...a c e g i k

# python3   
>>> for (a, *b, c) in [(1, 2, 3, 4), (5, 6, 7, 8)]:
...    print(a, b, c)
...1 [2, 3] 4
   5 [6, 7] 8

range反復器
rangeは反復器で、必要に応じて要素が生成されます

>>> list(range(-5, 5))
[-5, -4, -3, -2, -1, 0, 1, 2, 3, 4]

>>> list(range(0, len(S), 2))
[0, 2, 4, 6, 8, 10]

>>> for i in range(3):
...     print(i, 'Pythons')
...0 Pythons
   1 Pythons
   2 Pythons
 
#     
>>> L = [1, 2, 3, 4, 5]
>>> for i in range(len(L)):     # Add one to each item in L
...     L[i] += 1               # Or L[i] = L[i] + 1
...
>>> L
[2, 3, 4, 5, 6]
   

zipパラレル反復器

>>> L1 = [1,2,3,4]
>>> L2 = [5,6,7,8]

>>> zip(L1, L2)


>>> list(zip(L1, L2))             # list() required in 3.0, not 2.6
[(1, 5), (2, 6), (3, 7), (4, 8)]

>>> for (x, y) in zip(L1, L2):
...     print(x, y, '--', x+y)
...1 5 -- 6
   2 6 -- 8
   3 7 -- 10
   4 8 -- 12

zipによる辞書の構築

# python2.2 before
>>> keys = ['spam', 'eggs', 'toast']
>>> vals = [1, 3, 5]
>>> list(zip(keys, vals))[('spam', 1), ('eggs', 3), ('toast', 5)]
>>> D2 = {}
>>> for (k, v) in zip(keys, vals): D2[k] = v
...

>>> D2
{'toast': 5, 'eggs': 3, 'spam': 1}

#python 2.2 later
>>> keys = ['spam', 'eggs', 'toast']
>>> vals = [1, 3, 5]
>>> D3 = dict(zip(keys, vals))

>>> D3
{'toast': 5, 'eggs': 3, 'spam': 1}

Enumerate関数
Enumerate関数はジェネレータを返します.オブジェクトは簡単に言えば、次の組み込み関数によって呼び出されるnextメソッドがあり、ループで反復するたびに1つの(index,value)メタグループが返されます.

>>> S = 'spam'
>>> for (offset, item) in enumerate(S):
...     print(item, 'appears at offset', offset)
...s appears at offset 0
   p appears at offset 1
   a appears at offset 2
   m appears at offset 3