Introduction to conditions and loops in python

This article mainly introduces conditions and loops in python to obtain any key-value pairs in the dictionary.

>>> x={'a':1,'b':2}>>> key,value=x.popitem()>>> key,value('a', 1)>>> del x[key]Traceback (most recent call last):  File "
 
  ", line 1, in 
  
       del x[key]KeyError: 'a'>>> x{'b': 2}>>> x[key]=value>>> x{'a': 1, 'b': 2}>>> del x[key]
  
 

Incremental assignment

>>> x=2>>> x+=1>>> x*=2>>> x>>> fnord='foo'>>> fnord+='bar'>>> fnord*=2>>> fnord'foobarfoobar'

Conditional execution if statement

>>> name=raw_input('?')?Yq Z>>> if name.endswith('Z'):  \   print 'Hello,Mr.Z'Hello,Mr.Z

Else clause

>>> name=raw_input('what is your name?')what is your name?Yq Z>>> if name.endswith('Z'):    print 'Hello,Mr.Z'else:    print 'Hello,stranger'    Hello,Mr.Z

Elif clause

>>> num=input('Enter a number: ')Enter a number: 5>>> if num>0:    print 'The number is position'elif num<0:    print 'The number is negative'else:    print 'The number is zero'    The number is position

Conditional nested statement

>>> name=raw_input('What is your name?')What is your name?Yq Z>>> if name.endswith('Yq'):    if name.startswith('Z'):        print 'Hello,Yq Z'    elif name.startswith('K'):        print 'Hello,Zyq'    else:        print 'Hello,Yq'else:    print 'Hello,stranger'    Hello,stranger

>>> number=input('Enter a number between 1 and 10:')Enter a number between 1 and 10:6>>> if number<=10 and number>=1:    print 'Great!'else:    print 'Wrong!'    Great!

>>> Age = 10 >>> assert 0> age =-1 >>> assert 0 ", line 1, in
 
  
Assert 0
  
While loop
  
>>> x=1>>> while x<=100:    print x    x+=1
  
>>> while not name:    name=raw_input('Please enter your name:')    print 'Hello,%s !' % name    Please enter your name:zyqHello,zyq !
  
For loop
  
>>> words=['this','is','an','ex','parrot']>>> for word in words:    print word    thisisanexparrot>>> range(0,10)[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]>>> for i in range(1,8):    print i246
  
Dictionary loop (iteration)
  
>>> d={'x':1,'y':2,'z':3}>>> for key in d:    print key,'corresponds to',d[key]    y corresponds to 2x corresponds to 1z corresponds to 3
  
Parallel iteration
  
>>> names=['Anne','Beth','George','Damon']  >>> ages=[12,19,18,20]>>> for i in range(len(names)):    print names[i],'is',ages[i],'years old'    Anne is 12 years oldBeth is 19 years oldGeorge is 18 years oldDamon is 20 years old
  
>>> zip(names,ages)[('Anne', 12), ('Beth', 19), ('George', 18), ('Damon', 20)]>>> for name,age in zip(names,ages):    print name,'is',age,'years old'    Anne is 12 years oldBeth is 19 years oldGeorge is 18 years oldDamon is 20 years old>>> zip(range(5),xrange(100))[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)]
  
Number iteration
  
>>> D [1, 2, 4, 4] >>> for x in d: if x = 4: d [d. index (x)] = 6 >>> d [1, 2, 6, 6] >>> S = ['skj', 'kiu', 'olm ', 'piy'] >>>> index = 0 >>> for s1 in S: if 'K' in s1: S [index] = 'hh 'index + = 1 >>> S ['hh', 'hh ', 'olm', 'piy'] >>> for index, s2 in enumerate (S): # The enumerate function provides the index-value pair if 'H' in s2: S [index] = 'df '>>> S ['df ', 'DF', 'olm', 'piy']
  
Flip, sort iteration
  
>>> sorted([4,3,6,8,3])[3, 3, 4, 6, 8]>>> sorted('Hello,world!')['!', ',', 'H', 'd', 'e', 'l', 'l', 'l', 'o', 'o', 'r', 'w']>>> list(reversed('Hello,world!'))['!', 'd', 'l', 'r', 'o', 'w', ',', 'o', 'l', 'l', 'e', 'H']>>> ''.join(reversed('Hello,world!'))'!dlrow,olleH'
  
Break bounce cycle
  
>>> for n in range(99,0,-1):    m=sqrt(n)    if m==int(m):        print n        break
  
While True/break
  
>>> while True:    word=raw_input('Please enter a word:')    if not word:break    print 'The word was '+word    Please enter a word:fThe word was fPlease enter a word:
  
Loop else statement
  
>>> for n in range(99,81,-1):    m=sqrt(n)    if m==int(m):        print m        breakelse:    print 'h'    h
  
List derivation-lightweight loop
  
>>> [x*x for x in range(10)][0, 1, 4, 9, 16, 25, 36, 49, 64, 81]>>> [x*x for x in range(10) if x%3==0][0, 9, 36, 81]>>> [(x,y) for x in range(3) for y in range (3)][(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]>>> result=[]>>> for x in range(3):    for y in range(3):        result.append((x,y))>>> result [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]>>> girls=['Alice','Bernice','Clarice']>>> boys=['Chris','Arnold','Bob']>>> [b+'+'+g for b in boys for g in girls if b[0]==g[0]]['Chris+Clarice', 'Arnold+Alice', 'Bob+Bernice']
  
Pass
  
>>> If name = 'nss': print 'Welcome! 'Elif name = 'U': # passelif name = 'Bill ': print 'Access Denied' else: print 'Nobody! '
  
Del x and y point to a list at the same time, but deleting x does not affect y. Only the name is deleted, not the list itself (value)
  
>>> x=['Hello','world']>>> y=x>>> y[1]='Python'>>> x['Hello', 'Python']>>> del x>>> y['Hello', 'Python']
  
Exec
  
>>> Exec "print 'Hello, world! '"Hello, world! >>> From math import sqrt> exec "sqrt = 1"> sqrt (4) Traceback (most recent call last): File"
   
    
", Line 1, in
    
     
Sqrt (4) TypeError: 'int' object is not callable # Add a dictionary, for namespace >>>> from math import sqrt >>>scope ={}>> exec 'sqrt = 1' in scope >>> sqrt (4) 2.0 >>> scope ['qrt ']
    
   
  
Note: a namespace is called a scope. You can think of it as a place to save variables, similar to an invisible Dictionary. When executing a value assignment statement such as x = 1, the key x and value 1 are placed in the current namespace, which is generally a global namespace.
  
>>> len(scope)2>>> scope.keys()['__builtins__', 'sqrt']
  
Eval evaluate
  
>>> scope={}>>> scope['x']=2>>> scope['y']=3>>> eval('x*y',scope)>>> scope={}>>> exec 'x=2' in scope>>> eval('x*x',scope)
  

  
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