Strings. General concepts. String declaration. Operations with strings. Examples
- 1. String declaration. The purpose of strings
- 2. What string types are supported in Python?
- 3. How to declare a variable of type “string“? General form
- 4. Examples of declaring string variables
- 5. Is there a type in Python that describes a single character (e.g. char)?
- 6. Basic operators for working with strings. Table
- 7. An example of using the operator + concatenation of strings
- 8. Example of using the operator * of string repeating
- 9. An example of using the  operator to pull a string item by its index
- 10. An example of using the [:] operator to extract a substring from a string
- 11. Example of string traversal using the for loop statement
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1. String declaration. The purpose of strings
In the Python programming language, a string is a built-in type that is designed to store and present character or text information in an ordered manner. From a syntactic point of view, a string is a sequence of characters taken in single or double quotes.
Strings provide the use of everything that can be represented in text form, for example:
- information in files;
- data on names, descriptions, comments in databases;
- Internet domain names;
- information texts in documents that support Unicode encoding;
Strings belong to a class of objects called sequences. String literals taken in single or double quotation marks are interchangeable, that is, this is an object of the same type.
Examples of literal strings.
"Hello world!" 'bestprog.net' '1234567890' "1234567890" "I'm" # I'm a string - single quotes in double quotes 'I"m' # I'm a string - double quotes in single quotes
2. What string types are supported in Python?
Python supports three types of strings:
- strings of type str – intended to represent text in Unicode format and other coding systems. This format contains ASCII characters and characters in other encodings;
- bytes strings – intended to represent binary data;
- strings of type bytearray – intended to represent binary data, taking into account changes in the type of bytes.
Python 2.6 uses the unicode type to represent Unicode text.
3. How to declare a variable of type “string“? General form
To declare a variable of type string, just use the assignment operator =. The general form of variable declaration is as follows
variable_name = string
- variable_name – a name for the variable. In the future, this name is used in the program and is associated with the string;
- string – string (literal), taken in single or double quotes.
4. Examples of declaring string variables
a = 'abcde' # single quote declaration b = "abcde" # double quute declaration line = '' # empty string block_s = """abcde""" # triple quote block s1 = b'hello' # byte string in versions 3.0 and later s2 = u'hello' # string with Unicode characters s3 = 's\np\ta\x00m' # escape sequences s4 = r'c:\myfolder\myfile1.txt' # unformatted string
5. Is there a type in Python that describes a single character (e.g. char)?
No, there is not. To describe a single character, the same string is used, which contains only one character (single-character string), for example:
firstLetter = 'A' lastLetter = 'Z' zero = '0'
6. Basic operators for working with strings. Table
It is possible perform typical operations on strings. To do this, the corresponding operators are overloaded in Python.
The table below lists the operators for working with strings.
|Use in programs||Explanation|
|[ ]||s[i]||Access to the character in string s at index i|
|[:]||s[i:j]||Pulling a substring from position i to position j|
7. An example of using the operator + concatenation of strings
The concatenation or addition operator is indicated by the + symbol. The operator can be used in expressions of varying complexity.
# Concatenation s1 = 'Hello' s2 = 'world!' s3 = s1 + ' ' + s2 # s3 = 'Hello world!'
8. Example of using the operator * of string repeating
The operator of string repetition is denoted by *. The operator forms a new string object, which is repeated a specified number of times.
Example. In the example, string s2 is equal to three lines s1
# Operator * of string repetition s1 = 'abc' s2 = s1*3 # 'abcabcabc'
9. An example of using the  operator to pull a string item by its index
To get a single character in a string, the index operation  is used. The following are examples of getting a string character by its index. The numbering starts with index 0.
Example. In the example, the variable c is assigned a character with index  of string s.
# Operator  - pulling a character in a string by its index s1 = 'abc' c = s1 # c = 'b'
10. An example of using the [:] operator to extract a substring from a string
The [:] operator is used to process substrings in strings. This operator has many varieties. For more information about the operator [:] is described here. This topic provides some limited examples of using the operator to extract a string from a substring.
# Operator [:] - take a character in a string by index s1 = '01234567890' s2 = s1[2:4] # s2 = '23' s3 = s1[:5] # s3 = '01234' s4 = s1[:-1] # s4 = '0123456789'
11. Example of string traversal using the for loop statement
The example shows how to view all characters in a string using the for loop statement. The problem of viewing the number of characters ‘z’ in a string is solved. The string is entered from the keyboard.
# Strings # Example of traversing string items in a loop # 1. Input string s = str(input("Enter string: ")) # 2. Display string for checking for c in s: print(c, end=' ') # characters output print() # 3. Determining the number of characters 'z' in a string count = 0; for c in s: if c=='z': count = count+1 # Display the result print("count = ", count)
The result of the program
Enter string: zero z1sdlkj 12+laksd z e r o z 1 s d l k j 1 2 + l a k s d count = 2