One, foreword
Hello, everyone, I am Li Peng Li from fate.
This course will focus on strings in Python, so let’s get started.
This article is about 3500 words and takes about 15 minutes to read. Enjoy reading it.
2. String
Before we move on to today’s topic, let’s take a look at what a string is.
The language that we speak, the language that we communicate with other people are strings.
It’s just that in Python, strings become a data type, but now that we know that strings become a data type,
But how is this type used in computers? Is it ok for me to use Chinese on my computer?
Let’s start by exploring the encoding format in Python.
2.1 the ASCII coding
First of all, the bottom of the computer can not receive Chinese, even text can not process.
It has to be converted to numbers for representation, which we call bytes.
Note:
- 1 byte = 8 bits
- The largest integer represented by a word is 255 (binary 11111111).
- The largest integer that can be represented by two bytes is 65, 535
- The largest integer that can be represented by four bytes is 4 294 967 295
At the beginning of the computer, only 127 characters were stored in the computer, including numbers, letters, special symbols and so on.
This table is collectively referred to as THE ASCII code.
The thing to notice is that there are only three of them that we need to remember.
| Decimal representation | content | meaning |
|---|---|---|
| 48 | 0 | 0 |
| 65 | A | Letter A (uppercase) |
| 97 | a | Letter A (lowercase) |
By remembering these three things, we can deduce the ASCII numbers 0 to 9, A to Z, and A to Z, respectively.
2.2 the Unicode
Now that WE have ASCII coding, is that enough?
Of course not. If China had its own code table (GB2312 code), Japan had its own code table (Shift_JIS), and Korea had its own Korean code table (EUC-KR), wouldn’t that be chaos?
What you write in China becomes a pile of gibberish when you go to Japan or Korea. I believe that our work and life will be greatly affected.
So what do we do?
That’s when the International Organization for Standardization (ISO) and the Unicode Consortium, a consortium of multilingual software manufacturers, came along and joined forces with others to work on harmonization, and the result was what is known as Unicode.
With Unicode, it is possible to communicate characters and languages between different countries.
But some countries use Chinese characters, while some countries use English for daily communication, so there is a big difference.
Remember that Chinese characters have gone beyond ASCII encoding, and most require two bytes to represent, some even require four quads, while English requires only one byte.
The ASCII code for the letter A is 65 in decimal and 01000001 in binary;
The ASCII encoding for character 0 is 48 in decimal and 00110000 in binary. Note that the character ‘0’ is different from the integer 0;
Chinese characters are beyond the ASCII code and are encoded in Unicode as 20013 in decimal and 01001110 00101101 in binary.
Should we use Unicode encoding for chinese-related content?
ASCII is preferred when writing in English or numbers.
This brings us to a new standard, UTF-8.
2.3 UTF-8
Because to save space, content that should be encoded in ASCII should not be encoded in Unicode.
ASCII code of A: 01000001 Unicode code of A: 00000000 01000001Copy the codeSo how does a computer tell when to use Unicode and when to use ASCII?
To solve this problem, an intermediate Format character set, known as UTF (Unicode Transformation Format), emerged.
Common UTF formats are:
- UTF-7
- UTF – 7.5 –
- UTF-8
- UTF-16
- UTF-32
One of the things we need to focus on is utF-8 encoding, or “variable length encoding”.
Utf-8 encodes a Unicode character into 1-6 bytes, depending on the numeric size.
Common English letters are encoded into 1 byte, Chinese characters are usually 3 bytes, and more obscure characters are encoded into 4-6 bytes.
One thing to note here:
Python was born before the Unicode standard was published, so the original Python only supported ASCII encoding,
The ordinary string ‘ABC’ is ASCII encoded inside Python.
An added benefit of using UTF-8 encoding is that ASCII encoding can actually be considered part of UTF-8 encoding,
So, a lot of legacy software that only supports ASCII can continue to work in UTF-8.
This is why we need to declare UTF-8 at the beginning of the file every time we use Chinese.
2.4 “Slicing” of Strings
Once we’ve figured out the coding quirks of programming, we’re ready to use our strings.
Let’s say I declare a variable name.
name = "lipeng"
Copy the codeThat one day my student Fei elder sister wants to make fun of me, want to take out part of my name, this time actually need to use “slice”.
Note that strings, lists, and tuples all support slicing.
Let’s start with the syntax of slicing:
Syntax of slicing: [Start: End: step] Note: The selected interval belongs to left-closed and right-open type, that is, from"Start"Bit start, go"The end"The end of the preceding bit of the bit (excluding the end bit itself).Copy the codeSo let’s do a real example.
Last login: Tue Aug 7 09:27:48 on ttys000
➜ ~ python
>>> name = "lipeng"
>>> print(name[0:3])
lip
>>>
Copy the codeWe’ve got characters with subscripts 0 to 2, so let’s test characters 0 to 4.
Last login: Tue Aug 7 09:27:48 on ttys000
➜ ~ python
>>> name = "lipeng"
>>> print(name[0:5])
lipen
>>>
Copy the codeOne more three or four characters.
Last login: Tue Aug 7 09:27:48 on ttys000
➜ ~ python
>>> name = "lipeng"
>>> print(name[3:5])
en
>>>
Copy the codeWhat if we need all the data from a certain location?
Last login: Tue Aug 7 09:27:48 on ttys000
➜ ~ python
>>> name = "lipeng"
>>> print(name[2:])
peng
>>>
Copy the codeSuppose, too, that we don’t know how long the total is, but we know how many digits we need to cut to the end. What do we do?
Last login: Tue Aug 7 09:27:48 on ttys000
➜ ~ python
>>> name = "lipeng"
>>> print(name[1:-1])
ipen
>>>
Copy the codeIf you think you know how to slice, try these questions for yourself:
>>> print(name[:3])
lip
>>> print(name[::2])
lpn
>>> print(name[5:1:2])
"" # empty
>>> print(name[1:5:2])
ie
>>> print(name[::-2])
gei
>>> print(name[5:1:-2])
ge
>>>
Copy the code2.5 Common methods in Strings
2.5.1 ord() : Integer representation of a character
➜ ~ python
>>> ord('a')
97
>>> ord('A')
65
>>> ord('0'48) > > >Copy the code2.5.2 CHR () : The encoding is converted to the corresponding character
>>> chr(69)
'E'
>>> chr(103)
'g'
>>>
Copy the code2.5.3 find() : Queries whether the corresponding text exists
Checks whether STR is included in mystr, returning the starting index if it is, or -1 otherwise.
Mystr.find (STR, start=0, end=len(mystr))
>>> name = "my name is MR_LP"
>>> name.find("MR_LP"11) > > >Copy the code2.5.4 index() : Queries whether the corresponding text exists
Check whether STR is included in mystr, if it returns the starting index value, otherwise an error is reported
Mystr.index (STR, start=0, end=len(mystr))
>>> name = "my name is MR_LP"
>>> name.index("MR_LP")
11
>>> name.index("ZZZ")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: substring not found
>>>
Copy the code2.5.5 count() : Returns the number of occurrences of characters
Returns the number of occurrences in mystr between start and end
Mystr.count (STR, start=0, end=len(mystr))
>>> name = "my name is MR_LP"
>>> name.count('m'2) > > >Copy the code2.5.6 Replace () : Replace the text no more than count times
Replace str1 in mystr with str2, not more than count times if count is specified.
Replace (str1, str2, mystr.count(str1))
>>> name = "my name is MR_LP"
>>> name.replace("MR_LP"."233")
'my name is 233'
>>> name.replace("m"."233"1),'233y name is MR_LP'
>>>
Copy the code2.5.7 split() : Splits the string according to the delimiter
Slice mystr with STR as the delimiter, separating only maxsplit substrings if maxsplit has a specified value
Mystr.split (STR =” “, 2)
>>> name = "my name is MR_LP"
>>> name.split(' ')
['my'.'name'.'is'.'MR_LP']
>>> name.split(' ', 2) ['my'.'name'.'is MR_LP'] > > >Copy the code2.5.8 Capitalize () : Capitalize the first letter of a string
Capitalize the first character of the string
Standard syntax: mystr.capitalize()
>>> name = "my name is MR_LP"
>>> name.capitalize()
'My name is mr_lp'
>>>
Copy the code2.5.9 Title () : Capitalize the first letter of each word in the string
Capitalize each word of the string
Standard syntax: name.title()
>>> name = "my name is MR_LP"
>>> name.title()
'My Name Is Mr_Lp'
>>>
Copy the code2.5.10 startswith() : checks whether the string startswith a certain content
Checks if the string begins with obj, returning True if it does, and False otherwise
Standard syntax: mystr.startswith(obj)
>>> name = "my name is MR_LP"
>>> name.startswith('my')
True
>>> name.startswith('My')
False
>>>
Copy the code2.5.11 endswith() : checks whether the string endswith a certain content
Checks if the string ends in obj, True if so, False otherwise
Standard syntax: mystr.endswith(obj)
>>> name = "my name is MR_LP"
>>> name.endswith('LP')
True
>>> name.endswith('lp')
False
>>>
Copy the code2.5.12 lower() : Uppercase characters are lowercase characters
Convert all uppercase characters in mystr to lowercase
Mystr.lower ()
>>> name = "my name is MR_LP"
>>> name.lower()
'my name is mr_lp'
>>>
Copy the code2.5.13 All Uppercase Characters are uppercase
Converts all uppercase characters in mystr to uppercase
Standard syntax: mystr.upper()
>>> name = "my name is MR_LP"
>>> name.upper()
'MY NAME IS MR_LP'
>>>
Copy the code2.5.14 ljust() : Left-align and fill the string
Return a left-aligned string and fill it with Spaces to a new string of length width
Standard syntax: mystr.ljust(width)
>>> s = "lipeng"
>>> s.ljust(10)
'lipeng '
>>>
Copy the code2.5.15 Rjust () : Right-align and fill the string
Return a right-aligned string filled with Spaces to the new string of length width
Standard syntax: mystr.rjust(width)
>>> s = "lipeng"
>>> s.rjust(10)
' lipeng'
>>>
Copy the code2.5.16 Center () : Center and fill the string
Returns a centered string filled with Spaces to a new string of length width
Standard syntax: mystr.center(width)
>>> s = "lipeng"
>>> s.center(20)
' lipeng '
>>>
Copy the code2.5.17 lstrip() : Clears the left whitespace
Remove the whitespace character to the left of mystr
Standard syntax: mystr.lstrip()
>>> str = ' lipeng '
>>> str.lstrip()
'lipeng '
>>>
Copy the code2.5.18 rstrip() : Clears blank characters on the right
Remove the whitespace character to the right of mystr
Standard syntax: mystr.rstrip()
>>> str = ' lipeng '
>>> str.rstrip()
' lipeng'
>>>
Copy the code2.5.19 Strip () : Clears blank characters on the left and right sides
Remove whitespace characters on both sides of mystr
Standard syntax: mystr.strip()
>>> str = ' lipeng '
>>> str.strip()
'lipeng'
>>>
Copy the code2.5.20 rfind() : Checks whether the corresponding text exists on the right
Check whether the corresponding text exists on the right
Mystr.rfind (STR, start=0,end=len(mystr))
>>> name = "lipeng"
>>> name.rfind("p"2) > > >Copy the code2.5.21 rindex() : Queries the corresponding text on the right
It’s the same as index(), but it starts on the right
Mystr.rindex (STR, start=0,end=len(mystr))
>>> name = "lipeng"
>>> name.rindex("e")
3
>>> name.rindex("LI")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: substring not found
>>>
Copy the code2.5.22 Partition () : Divides a string into three parts based on the corresponding text
Split mystr with STR into three parts, before STR, STR, and after STR
Mystr.partition (STR)
>>> s = "i'm lipeng, you can call me MR_LP"
>>> s.partition("can")
("i'm lipeng, you ".'can'.' call me MR_LP') > > >Copy the code2.5.23 Rpartition () : Splits a string into three parts based on the corresponding text on the right
From the right, mystr is divided into three parts with STR, before STR, STR, and after STR
Syntax: mystr.rpartition(STR)
>>> s = "i'm lipeng, you can call me MR_LP"
>>> s.rpartition("can")
("i'm lipeng, you ".'can'.' call me MR_LP') > > >Copy the code2.5.24 splitlines() : splitlines
Returns a list of rows as elements, separated by row
Standard syntax: mystr.splitlines()
>>> s = "MR_LP.\nmr_lp"
>>> print(s)
MR_LP.
mr_lp
>>> s.splitlines()
['MR_LP.'.'mr_lp'] > > >Copy the code2.5.25 isalpha() : Checks whether it contains all letters
Return True if all characters of mystr are letters, False otherwise
Standard syntax: mystr.isalpha()
>>> s = "abc"
>>> s.isalpha()
True
>>> s = "123"
>>> s.isalpha()
False
>>> s = "abc 123"
>>> s.isalpha()
False
Copy the code2.5.26 isDigit () : Checks whether all digits are digits
Return True if mystr contains only numbers, False otherwise.
Standard syntax: mystr.isDigit ()
>>> s = "abc"
>>> s.isdigit()
False
>>> s = "123"
>>> s.isdigit()
True
>>> s = "abc123"
>>> s.isdigit()
False
Copy the code2.5.27 isalnum() : Checks whether all numbers or letters are contained
Mystr returns True if all characters are letters or numbers, False otherwise
Standard syntax: mystr.isalnum()
>>> s = "123"
>>> s.isalnum()
True
>>> s = "abc"
>>> s.isalnum()
True
>>> s = "abc123"
>>> s.isalnum()
True
>>> s = "abc 123"
>>> s.isalnum()
False
Copy the code2.5.28 isspace() : Checks whether only Spaces are included
Return True if mystr contains only Spaces, False otherwise.
Standard syntax: mystr.isspace()
>>> s = "abc123"
>>> s.isspace()
False
>>> s = ""
>>> s.isspace()
False
>>> s = "" # Spaces
>>> s.isspace()
True
>>> s = "" # the TAB key
>>> s.isspace()
True
Copy the code2.5.28 Join () : Checks whether only Spaces are included
Return True if mystr contains only Spaces, False otherwise.
Standard syntax: mystr.isspace()
>>> s = ""
>>> li = ["my"."name"."is"."LIPENG"]
>>> s.join(li)
'my name is LIPENG'
>>> s = "_"
>>> li = ["my"."name"."is"."LIPENG"]
>>> s.join(li)
'my_name_is_LIPENG'
Copy the code