Learning Log---Python (list parsing, generators, dictionaries, and collections)

The resulting list can be used with range ();

List parsing

A new list is generated for parsing a sequence, and the list parsing is much better than for performance;

A=[x for x in range (0,10) if x > 5]print aa=[x+1 for x in range (0,10) if x > 5]print A

Use the expressions in the list to get the list you want, without having to use a for loop;

Double nesting: Use simpler statements to get a list:

a=[(x, y) for x in range (3) for Y in range (3)]print a

There are a number of built-in functions available in the parser:

Filter (Function,iter)

Fuction is a function that accepts only one parameter, that is, the sequence of ITER that is passed in, and returns a filtered sequence;

The returned value is the value in ITER, which satisfies the return, fuction is true;

Map (Function,iter)

Functions are a function that takes all the elements of the ITER list in turn, generating a list of the corresponding return values;

def f (x): Return x%5==0def U (x): Return x+1a = Filter (F,range (0,100)) Print AA = Map (U,range (0,10)) print a

The disadvantage of list parsing is that memory efficiency is too low;

Builder expression

Each time only one element is loaded and returned, the only syntax that is different from the list parsing is the parentheses , which returns a list header pointer

, when the pointer is obtained, it needs to be obtained by the For loop;

A = (x for x in range (0,10) if x%3==0) print afor i in A:print i

Condition is not 0 o'clock, that is, the element to be returned is 0 o'clock, or false

Xrange

The generator version of range, with a wide range of requirements, with this; As with the use of range

For I in Xrange (0,9999999): If I%999==0:print I

Dictionaries and collections

Dictionary

Dictionary is an unordered key-value pair, based on the implementation of a hash table, key can be hash, hash function to calculate the value, and then the value;

Types that can be built (content is immutable):

The general recommendation is to use a string; class instance to do key, you need to implement hash method return hash value, list and dictionary can not do key, because list and dictionary variable; tuples can do key, cannot contain list inside. ；

The built-in hash () function can be used to determine whether it can be used as a key value;

Create a dictionary: {key:value}, or you can use the Dict () factory function to create

A={' A ': 1, ' B ': 2}print aa=dict (a= ' C ', b=2) print Aprint a.get (' C ', ' not exist ')

Dict () function is created so that the key is the default string, value is also the default, the number can be directly written;

In the dictionary does not confirm whether there is a key, you can use the dictionary get method to fetch, no error, return the default value, this is a very safe operation;

Del Dict[key] can be deleted, dict[' None ']=3, if the key is not, then create;

Find key:

The Keys function returns a list of keys that exist in the dictionary.

A={' A ': 1, ' B ': 2, ' C ': 3}b = A.keys () Print B

Collection (SET)

A = set ([+]) b = List ([1,2,3,4]) B.append (5) A.add (5)

The value inside the set set is unique, adding the element with the Add method, update updates a set of sequences at the same time;

A collection can be federated, intersect, set, and XOR.

Topic:

Design a function zip (LISTA,LISTB,REPL), enter two lists and placeholders, return a new list, each element of the list is a tuple, the elements of the tuple are the same order in the Lista and LISTB elements, if the length is not the same, use placeholders instead.

Def zip (LISTA,LISTB,REPL):     lena = len (lista)     lenb  = len (LISTB)     length = 0    list = []     if lena < lenb:         Length = lena    else:        length  = lenb    for i in range (0,length):         list.append ((lista[i],listb[i))     if lena >  Length:        for i in range (Length,lena):             list.append ((LISTA[I],REPL))      if lenb > length:        for i in  range (LENGTH,LENB): &NBSP;&NBsp;          list.append ((repl,listb[i))      print listzip ([1,2,3],[4,5,6], "a") zip ([1,2,3],[4], "B")

Learning Log---Python (list parsing, generators, dictionaries, and collections)