Learning Python in anger - Part 2 - Types and operations

Python supports 5 data types, including numbers, strings, lists, tuples and dictionaries. As a dynamically typed language, there is no need to declare the type of an identifier. The type will be automatically determined when used.

For defined variables, if the variable is not used, you can use del deletion to release the occupied resources, such as

var = 10del var

Python numbers (numbers): supports int, long, float and complex, It is relatively simple and does not give examples.

Python’s string (string): As mentioned earlier, it can be defined by a variety of quotation marks, and supports [p] (get the character at p position), [l:r] (get the string starting from l and ending at r ), *t (get t strings linked together by this string) and +s (get the string linked with s) method (methods, why use method? It is related to ruby, .methods of all objects can Display all the operations that can be performed, so I also started to call the operations that an object can perform), as in the following example

s = 'This is a test case.'PRint s[0]             #Output the first character of s' T'print s[0:4]               #Output the string consisting of the first to the fourth characters of s 'This'print s*2                                       #Output the new string composed of the two s string strings print s + ' Good! ' #Output the string after s is linked with 'Good!'

Python list (list): defined with [], it looks like a C++ container vector, except that this container can hold different types of values, and at the same time Definitions can be nested, one list contains another list, the method is similar to string, the example is given below

l = ['wyp', 1, 3.14, [2, 'cc']]print l               #Output the entire lprint l[0]             #Output the first element of l 'wyp'print l[3][0]               #Output the first element of the fourth element of l 2print l[0:3]               #Output the first element of l Element to the third element print l*2                 #Output a list of two l linked together print l+['.']             #Output a list of l and list ['.'] linked together

Python tuple ( tuple): defined with (), the method is similar to the list, the only difference is that it cannot be assigned twice, so no example is given.

Python's dictionary: Use {} to define, the definition method is like json, and it is used like a C++ container map. The method is given below

d = {1:'wyp', 'two':'is ', 3.0:'Coder'}
d['four'] = 'like'd[5] = 'farmer'print d           #Output the entire dictionary dprint d[1]       #Output the value with key 1 'wyp'print d .keys() #Output all keys print d.values() #Output all values ​​

Python type conversion: Visual inspection is not used much, here is an example

int(x) #Convert x to int Type, directly round, regardless of the following mantissa long(x) #Convert x to long type, directly round, regardless of the following mantissa float(x) #Convert x to float type complex(x,y) #Create a x is the real part and y is the imaginary part of the complex number chr(x) #Convert to the character ord(c) corresponding to the ascii code x #Convert to the ascii code unichr(x) corresponding to c #Convert x to the unicode character hex(x )         #Convert x into its hexadecimal string oct(x)           #Convert x into its octal string str(x)           #Convert x into a string eval(str)       #Convert a string Calculate the value of the expression str tuple(s) #Convert the s of the sequence structure into a tuple list(s) #Convert the s of the sequence structure into a list dict(s) #Convert the sequence structure and each element It is a tuple of s that is converted into a dictionary

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Python operations The method is very similar to C++. It is divided into arithmetic operations, comparison operations, logical operations and bit operations. On this basis, membership operations and identity operations are added. Examples that are different from C++ are given below

Arithmetic operations: extra The operators include ** and //, so **= and //= are added to give examples

a ** b #That is, a^b, a raised to the b power a // b #First a/b and then Then directly round to the integer

Comparison operation: The extra operator is "", which has the same function as "!=", so no example is given

Logical operations: There is a little difference here. All logical operations are expressed in English, namely and (and), or (or) and not (not). The usage is the same and I will not give examples.

Bit operations: the same as C++

Member operation: The operators are in and not in. Taking in as an example, if an element is in the sequence, it is true, otherwise it is false, and the other one is negated. The example is given below

t = [1, 2, 3, 4] 1 in t ​ #Return true1 not in t ​ #Return false0 in t ​ #Return false0 not in t ​ #Return true

Identity operation: The operators are is and not is, that is, determine whether the two variables are the same storage unit, in Here I found a small detail of Python. All its integers from 0 to 256 are already stored in memory, while the others are directly allocated new memory when they are defined. An example is given below

a = -1b = -1a is b       #The value is false, here it is found that -1 is not a pre-stored in the memory by python += 1b += 1while (a is b): #It is true before 256
  a += 1
  b += 1 print str(a) + ', ' + str(b) + 'n'

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