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Thursday, August 15, 2019

String Variable In Python

1 - Define string in Python 

 A string variable in Python is sequence of characters that follow each other and delimited by double or single quotas "..." or '...'. A string which does not contain any characters is called an empty string. 


s1 = "Hello !" 
s2 = 'welcome !' 
print(s1) # this will display : 
Hello print(s2) # this will display : welcome

2 - Access the elements of a string variable 

The brackets can be used to access the string elements.

Example. get the character at position 1 

 (remember that the first character has the 0 position):

s = "Hello World!" 
print ("The second character of s is:", s [1]) 
# this will displays: "The second character of s  is: e 


A string is immutable: once it is created, its content can not be changed.


s = "welcome" s [1] = v # causes a run-time error, indicating that it is impossible to change the contents of a string

3 - Python functions for string object

The Python language is endowed with a large number of functions allowing the manipulation of chains of characters: calculation of the length of the chain, transformation in upper and lower case, to extract a sosus chain ... 

Here is a non-exhaustive list

1. capitalize ():   capitalize the first letter of the string
2. center (width, fill):   returns a string filled with spaces with the original string centered on the total width columns.
3. counts (str, beg = 0, end = len (string)):   counts the number of times str occurs in a string or a substring if the start of the start index and the end of the end index are indicated.
4. decode (encoding = 'UTF-8', errors = 'strict'):   decodes the string using the codec recorded for encoding. The default encoding is the default string encoding.
5. encode (encoding = 'UTF-8', errors = 'strict'):   returns the encoded version of the string; in case of error, the default value is to generate a ValueError value unless errors are indicated with "ignore" or "replace".
6. endswith (suffix, start = 0, end = len (string)):   determines whether a string or a substring string (if end indexes start and end indexes are indicated) ends with a suffix; returns true if yes and false otherwise.
7. expandtabs (tabsize = 8):   expands the tabs of a string into multiple spaces; The default value is 8 spaces per tab if tabsize is not provided.
8. find (str, beg = 0 end = len (string)):   determine if str appears in a string or a substring of strings if the start index and end index are specified, end returns if it is found and -1 otherwise.
9. format (string s):   replace the braces by the variable string s (see example below: [String.format])
10. index (str, beg = 0, end = len (string)):   same as find (), but raises an exception if str is not found.
11. isalnum ():   returns true if the string is at least 1 character and all characters are alphanumeric and false otherwise.
12. isalpha ():   returns true if the string has at least 1 character and all characters are alphabetic and false otherwise.
13. isdigit ():   returns true if the string contains only numbers and false otherwise.
14. islower ():   returns true if the string has at least 1 character in case and all case characters are lowercase and false otherwise.
15. isnumeric ():   returns true if a unicode string contains only numeric characters and false otherwise.
16. isspace ():   returns true if the string contains only space characters and false otherwise.
17. istitle ():   returns true if the string is correctly titlecased and false otherwise.
18. isupper (): Returns true if string contains at least one character and all characters are uppercase and false otherwise.
19. join (seq):   merges (concatenates) the string representations of elements in sequence seq into a string, with a separator string.
20. len (string):   returns the length of the string
21. ljust (width [, padding]):   returns a string filled with spaces with the original justified string to the left for a total of width columns.
22. lower ():   converts all uppercase letters in a string to lowercase.
23. lstrip ():   removes all spaces at the beginning of the string.
24. maketrans():   returns a translation table to use in the translation function.
25. max (str):    returns the maximum alphabetic character of string str.
26. min (str):   returns the minimum alphabetic character of string str.
27. replace (old, new [, max]):   replaces all occurrences of old in string with new or maximum max if given max.
28. rfind (str, beg = 0, end = len (string)):   same as find (), but look back in string.
29. rindex (str, beg = 0, end = len (string)):   same as index (), but look back in string.
30. rjust (width, [, padding]):   returns a string filled with spaces with the justified origin string on the right, with a total of width columns.
31. rstrip ():   removes all trailing spaces.
32. split (str = "", num = string.count (str)):   divides the string according to the str (if not supplied) delimiter and returns the list of substrings; divided into maximum substrings, if any.
33. splitlines (num = string.count ('\ n')):   breaks the string of all NEWLINE (or num) and returns a list of each line without the NEWLINE.
34. startswith (str, beg = 0, end = len (string)):   determines whether string or a substring string (if end indexes and end indexes are specified) begins with the sub-string string str; returns true if yes and false otherwise.
35. strip ([floats]):   performs lstrip () and rstrip () on string.
36. swapcase ():   reverses the case of all letters

4 - Example of using string functions

Example. transformation of a string into lower case

s = s.lower ()
print (s) # displays crmef oujda

Example. replacing one occurrence with another

s = s.replace ("CRMEF", "ENS")
print (s) # display: ENS OUJDA

Example. Number of characters in a string

n = len(s)
print ("the number of characters in the string s is:", n)
# displays the number of characters in the string s is: 11 

Example. String.format

name = "David"
age = 37
s = 'Hello, {}, you have {} ans'.format (name, age)
print (s) # display:  'Hello, David, you are 37'

Example. extract a sub string

s1 = s [6: 9]
print (s1) # displays OUJ
s2 = s [6:]
print (s2) # displays OUJDA
s3 = s [: 4]
print (s3) # displays CRME

Younes Derfoufi

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