Dictionary in Python

Dictionary in Python

The field is the only key-value type in the dictionary in Python. It is a very important data structure in Python. because it uses hash to store data, its complexity is O (1 ), fast. The following lists the common usage of dictionaries.

【List of common dictionary Methods]

 

# Method # description D. clear () # Remove all items in D. copy () # returns the copy of D. fromkeys (seq [, val]) # returns the dictionary of keys obtained from seq and values set to val. Class method call D. get (key [, default]) # If D [key] exists, return it; otherwise, return the given default value NoneD. has_key (key) # Check whether D has a given key keyD. items () # Return the list of (Key, value) pairs that represent item D. iteritems () # from D. the (Key, value) pair returned by items () returns an iteratable Object D. iterkeys () # Return an iteratable Object D from the D key. itervalues () # Return an iteratable Object D from the value of D. keys () # returns the list of keys D. pop (key [, d]) # Remove and return the value D corresponding to the given key or given default value D. popitem () # Remove any item from D and use it as a (Key, value) pair to return D. setdefault (key [, default]) # If D [key] exists, return it; otherwise, return the default value NoneD. update (other) # add each item in other to D. D. values () # returns the list of values in D.

【 Five ways to create a dictionary]

 

Method 1: General Method# This method is more convenient if you can spell out the entire dictionary in advance

>>> D1 = {'name':'Bob','age':40}

Method 2: Dynamic Creation# This method is convenient if you need to dynamically create a dictionary Field

>>> D2 = {}>>> D2['name'] = 'Bob'>>> D2['age']  =  40>>> D2{'age': 40, 'name': 'Bob'}

Method 3: Dict-Keyword format# The code is relatively small, but the key must be of the string type. Used for Function Assignment

>>> D3 = dict(name='Bob',age=45)>>> D3{'age': 45, 'name': 'Bob'}

Method 4: Dict-key-value sequence# This method is useful if you need to gradually build a sequence of key values, which is often used together with the zip Function

>>> D4 = dict([('name','Bob'),('age',40)])>>> D4{'age': 40, 'name': 'Bob'}

 

Or

 

>>> D = dict(zip(('name','bob'),('age',40)))>>> D{'bob': 40, 'name': 'age'}

Method 5: Dict -- fromkeys Method# If the key values are the same, it is better to use this method and you can use fromkeys for initialization.

 

>>> D5 = dict.fromkeys(['A','B'],0)>>> D5{'A': 0, 'B': 0}

If the key value is not provided, the default value is None.

 

>>> D3 = dict.fromkeys(['A','B'])>>> D3{'A': None, 'B': None}

【 Dictionary-type-value Traversal method]

>>> D = {'X': 1, 'y': 2, 'z': 3 }# method 1 >>> for key in D: print key, '=>', D [key] y => 2x => 1z => 3 >>> for key, value in D. items (): # method 2 print key, '=>', valuey => 2x => 1z => 3 >>> for key in D. iterkeys (): # method 3 print key, '=>', D [key] y => 2x => 1z => 3 >>> for value in D. values (): # Method 4 print value213 >>> for key, value in D. iteritems (): # Method 5 print key, '=>', valuey => 2x => 1z => 3

Note: using D. iteritems (), D. iterkeys () is much faster than without iter.
【 One of the common usage of the dictionary replaces switch]

 

In C/C ++/Java, there is a convenient function switch, for example:

 

public class test {public static void main(String[] args) {String s = "C";switch (s){case "A": System.out.println("A");break;case "B":System.out.println("B");break;case "C":System.out.println("C");break;default:System.out.println("D");}}}

To implement the same function in Python,

 

Method 1 is implemented using the if and else statements, for example:

 

from __future__ import divisiondef add(x, y):    return x + ydef sub(x, y):    return x - ydef mul(x, y):    return x * ydef div(x, y):    return x / ydef operator(x, y, sep='+'):    if   sep == '+': print add(x, y)    elif sep == '-': print sub(x, y)    elif sep == '*': print mul(x, y)    elif sep == '/': print div(x, y)    else: print 'Something Wrong'print __name__ if __name__ == '__main__':    x = int(raw_input("Enter the 1st number: "))    y = int(raw_input("Enter the 2nd number: "))    s = raw_input("Enter operation here(+ - * /): ")    operator(x, y, s)

Method 2: Use a dictionary to skillfully implement the same switch function, for example:

 

 

#coding=gbkfrom __future__ import divisionx = int(raw_input("Enter the 1st number: "))y = int(raw_input("Enter the 2nd number: "))def operator(o):    dict_oper = {        '+': lambda x, y: x + y,        '-': lambda x, y: x - y,        '*': lambda x, y: x * y,        '/': lambda x, y: x / y}    return dict_oper.get(o)(x, y) if __name__ == '__main__':      o = raw_input("Enter operation here(+ - * /): ")    print operator(o)