python--about Dict

This article extracts from the MU class net "The introduction of Python"

1. Dict characteristics

Dict is denoted by curly braces {} and then Key:value, written out. The last comma of a key:value can be omitted.

①,dict search speed is fast , regardless of whether the dict has 10 elements or 100,000 elements, the search speed is the same . The search speed of the list decreases as the element increases.

The disadvantage of Dict is that it takes up a lot of memory, and it wastes much content , the list is the opposite, the memory is small, but the search speed is slow

②,dict to find value by key, so key cannot be duplicated, and value can be repeated

③,dict stored "key:value" is unordered , that is, not available index number slices.

Python's basic types, such as strings, integers, and floating-point numbers, are immutable and can be used as keys. But the list is mutable and cannot be a key.

The lookup table for "name"-"score" is shown in dict as follows:

D = {

' Adam ': Up, #key: value

' Lisa ': 85,

' Bart ': 59

}

We call the name key, the corresponding result is called value,dict is the key to find value.

2. Visit dict

Create a dict that represents the correspondence between the name and the score:

D = {

' Adam ': 95,

' Lisa ': 85,

' Bart ': 59

}

Use D[key] to find the corresponding value, which is similar to the list, where thelist must return the corresponding element using the index, and Dict uses key:

23:30:12

Note: The value of dict is accessed by key, and Dict returns the corresponding value as long as the key exists. If key does not exist, it will directly error: Keyerror.

There are two ways to avoid keyerror:

First, determine whether the key exists, with the in operator:

If ' Paul ' in D:

Print d[' Paul '

If ' Paul ' is not present, the IF statement evaluates to False, and the print d[' Paul ' is not executed naturally, thus avoiding the error.

The second is to use the dict itself to provide a get method, when the key does not exist, return to none:

>>> print d.get (' Bart ')

59

>>> print d.get (' Paul ')

None

The dict is mutable and you can add new Key-value to the dict at any time. For example, there are dict:

D = {

' Adam ': 95,

' Lisa ': 85,

' Bart ': 59

}

To add a new classmate ' Paul ' score 72, use the assignment statement:

d[' Paul '] = 72

Then look at the contents of Dict:

>>> Print D

{' Lisa ': $, ' Paul ': $, ' Adam ': Up, ' Bart ': 59}

If the key already exists, the assignment replaces the original value with the new value:

>>> d[' Bart '] = 60

>>> Print D

{' Lisa ': $, ' Paul ': $, ' Adam ': Up, ' Bart ': 60}

>>> d = {' Adam ': $, ' Lisa ': $, ' Bart ': 59}
>>> for key in D:
Print key, '-', D[key]

Lisa-85
Adam-95
Bart-59

Ⅱ,values ()/Itervalues () method: Returns the value of Dict

>>> d = {' Adam ': $, ' Lisa ': $, ' Bart ': 59}
>>> Print d.values ()
[85, 95, 59]
>>> for V in D.values ():
Print V

85
95
59

Replace the values () method with the itervalues () method, which is exactly the same as the iteration effect. and itervalues ()

Ⅲ,items ()/Iteritems () method: Returns the key and value of Dict

The value returned by the items () method of the Dict object:

>>> d = {' Adam ': $, ' Lisa ': $, ' Bart ': ' D.items ' >>> print ' [(' Lisa ', ' $ '), (' Adam ', '), (' Bart '), 59)]

As you can see, the items () method converts the Dict object to a list that contains a tuple and iterates over the list to obtain both key and value:

>>> for key, value in D.items (): ... print key, ': ', Value ... lisa:85 adam:95 bart:59

Like the values () has a itervalues (), items () also have a corresponding iteritems (), Iteritems () does not convert the dict to a list, but is constantly given in the iterative process Tuple, so Iteritems () does not occupy additional memory.

python--about Dict