Python Strings

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Index

1. Introduction
2. raw-string
3. aritmetic operators
4. comparison operators
5. index
6. slicing
7. loops
8. Built-in functions

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Introduction

Strings main characteristics:

1. Contained by quotes: Strings are defined within single(’) or double quotes(”).

2. Sequence of characters: A string is a sequence of characters, meaning they can contain text characters, numbers, special symbols, and more.

3. Iterable Object: can be divided in samller parts

4. Immutable:* a string is an immutable object, which means that once you create a string object, you cannot modify its contents.

5. Ordinated: admits the repetition of elements and can be sliced and indexed

   string = "hello"
   string[0] = "H"    # Output: error (you can't change a string)

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raw-string

Python Raw String

read: Python Backslash (escaping character) and Python Escaping Sequences

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Definition

In Python, when you prefix a string with the letter r or R such as r'...' and R'...', that string becomes a raw string. Unlike a regular string, a raw string treats the backslashes (\) as literal characters.

Raw strings are useful when you deal with strings that have many backslashes, for example directory paths on Windows.

Example:

s = "lang\tver\nPython\t3 print(s)"

Output:

Output:

lang    ver 
Python  3

However, raw strings treat the backslash (\) as a literal character. For example:

s = r"lang\tver\nPython\t3"
 
print(s) #Output: lang\tver\nPython\t3

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Convert a regular string into a raw string

To convert a regular string into a raw string, you use the built-in repr() function. For example:

s = '\n'
raw_string = repr(s)
 
print(raw_string) #Output: \n

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Link to original

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Concatenation and Replication

Python Operators

Note:

A string is an immutable object, which means that when you concatenate two strings you are creating a new one

Operator	Description	Example
+	Concatenate two strings	”hello” + “world” → “hello world”
*	Repeat a string for a certain number of times	”spam” * 3 → “spamspamspam”

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Comparison Operators

In Python, you can compare two strings using comparison operators. The comparison operators return a Boolean value (True or False) depending on whether the comparison is true or false.

The comparison operators available for strings are:

1. == (equal to): returns True if the two strings are equal, and False otherwise.
2. != (not equal to): returns True if the two strings are not equal, and False otherwise.
3. < (less than): returns True if the first string comes before the second string in alphabetical order, and False otherwise.
4. > (greater than): returns True if the first string comes after the second string in alphabetical order, and False otherwise.
5. <= (less than or equal to): returns True if the first string comes before or is equal to the second string in alphabetical order, and False otherwise.
6. >= (greater than or equal to): returns True if the first string comes after or is equal to the second string in alphabetical order, and False otherwise.
7. in: returns True if a substring is found within a string, and False otherwise.
8. not in: returns True if a substring is not found within a string, and False otherwise.

Here are some examples:

str1 = "apple"
str2 = "banana"
 
print(str1 == str2)  # Output: False
print(str1 != str2)  # Output: True
print(str1 < str2)   # Output: True
print(str1 > str2)   # Output: False
print(str1 <= str2)  # Output: True
print(str1 >= str2)  # Output: False
 
str3 = "Hello, World!"
str4 = "World"
str5 = "Python"
 
print(str4 in str3)   # Output: True
print(str5 in str3)   # Output: False
print(str4 not in str3)   # Output: False
print(str5 not in str3)   # Output: True

Note:

Note that when comparing strings, Python uses lexicographical order, which means it compares the ASCII values of the characters in the strings.

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Built-in functions

Python offers numerous built-in functions for working with strings. Here are some of the most commonly used functions:

len()

The len() function returns the length of the string. For example:

my_string = "Hello, World!"
print(len(my_string)) # Output: 13

str()

The str() function converts a value to a string. For example:

my_number = 42
my_string = str(my_number)
print(my_string) # Output: "42"

count()

The count() function counts the number of occurrences of a substring within a string. For example:

my_string = "Hello, World!"
print(my_string.count("l")) # Output: 3

join()

The join() function joins a list of strings into a single string using a specific separator. For example:

my_list = ["apple", "banana", "cherry"]
print(", ".join(my_list)) # Output: "apple, banana, cherry"

split()

The split() function splits a string into a list of strings using a specific separator. For example:

my_string = "Hello, World!"
print(my_string.split(",")) # Output: ["Hello", " World!"]

upper() and lower()

The upper() and lower() functions convert a string to uppercase or lowercase letters. For example:

my_string = "Hello, World!"
print(my_string.upper()) # Output: "HELLO, WORLD!"
print(my_string.lower()) # Output: "hello, world!"

islower() and isupper()

The islower() and isupper() functions return True if all characters in the string are lowercase or uppercase respectively, otherwise False. For example:

my_string1 = "hello, world!"
my_string2 = "HELLO, WORLD!"
print(my_string1.islower()) # Output: True
print(my_string2.isupper()) # Output: True

capitalize()

The capitalize() function converts the first letter of a string to uppercase. For example:

my_string = "hello world"
print(my_string.capitalize()) # Output: "Hello world"

swapcase()

The swapcase() function converts uppercase letters to lowercase and vice versa within a string. For example:

my_string = "Hello, World!"
print(my_string.swapcase()) # Output: "hELLO, wORLD!"

replace()

The replace() function replaces a substring with another substring within a string. For example:

my_string = "Hello, World!"
print(my_string.replace("World", "Python")) # Output: "Hello, Python!"

find()

The find() function returns the index of the first occurrence of a substring within a string, or -1 if the substring is not found. For example:

my_string = "Hello, World!"
print(my_string.find("World")) # Output: 7
print(my_string.find("Python")) # Output: -1

title()

The title() function converts the first letter of each word to uppercase within a string. For example:

my_string = "hello world"
print(my_string.title()) # Output: "Hello World"

startswith() and endswith()

The startswith() and endswith() functions check if a string starts or ends with a certain substring and return True or False. For example:

my_string = "Hello, World!"
print(my_string.startswith("Hello")) # Output: True
print(my_string.endswith("World!")) # Output: True

format()

The format() function formats a string by inserting values into specified positional or named placeholders. For example:

name = "Alice"
age = 30
print("My name is {} and I'm {} years old.".format(name, age)) # Output: "My name is Alice and I'm 30 years old."

replace()

The replace() function replaces a substring with another one within a string. For example:

my_string = "Hello, World!"
print(my_string.replace("World", "Python")) # Output: "Hello, Python!"

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Slicingof a string

Note:

Slicing a string does not modify the original string - it returns a new string that contains only the specified portion of the original string, because strings are immutable

You can slice a string in Python using the syntax string[start:end:step], where:

• start is the index of the first character you want to include in the slice
• end is the index of the first character you want to exclude from the slice
• step is the number of characters to skip between each character included in the slice

Here are some examples:

string = "Hello, World!"
 
# slice the entire string
print(string[:])           # Output: Hello, World!
 
# slice from index 0 to index 5 (excluding 5)
print(string[0:5])         # Output: Hello
 
# slice from index 7 to the end
print(string[7:])          # Output: World!
 
# slice from index 2 to index 10, skipping every other character
print(string[2:10:2])      # Output: lo,W
 
# slice from the beginning to the end, skipping every other character
print(string[::2])         # Output: Hlo ol!
 
# slice from the end of the string to the beginning
print(string[::-1])        # Output: !dlroW ,olleH
 
# slice from the end of the string to index 2 (excluding 2), skipping every other character
print(string[-1:1:-2])     # Output: !lo le
 
# slice from index -5 (5th character from the end) to index -1 (excluding -1), skipping every other character
print(string[-5:-1:2])     # Output: Wo
 
# slice from index -10 (10th character from the end) to index -7 (excluding -7)
print(string[-10:-7])      # Output: orl
 
# slice from index -3 (3rd character from the end) to the end of the string
print(string[-3:])         # Output: ld!

Note:

That you can mix positive and negative indices in a single slice, and you can omit any of the three parameters (start, end, and step) as needed.

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Quartz Index Example of a string

In Python, a string is a sequence of characters, and you can access individual characters in the string by their position or index within the sequence.

• The index of a string starts at 0, which means that the first character in the string has an index of 0
• The second character has an index of 1, and so on.

  +---+---+---+---+---+
  | h | e | l | l | o |
  +---+---+---+---+---+
  | 0 | 1 | 2 | 3 | 4 |
  +---+---+---+---+---+
  |-5 |-4 |-3 |-2 |-1 |
  +---+---+---+---+---+

To access a specific character in a string, you can use its index within square brackets [], like this:

string = "hello"
print(string[0])   # Output: "h"
print(string[1])   # Output: "e"
print(string[2])   # Output: "l"
print(string[3])   # Output: "l"
print(string[4])   # Output: "o"
 
print(string[-5])   # Output: "h"
print(string[-4])   # Output: "e"
print(string[-3])   # Output: "l"
print(string[-2])   # Output: "l"
print(string[-1])   # Output: "o"

In this example, we access each character in the string string by its index within square brackets [].

Note:

The index must be an integer, and it must be within the range of valid indices for the string (from 0 to len(string) - 1). Otherwise, a IndexError will be raised.

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Loops in a string

In Python, a loop is a programming construct that allows you to iterate over a sequence of items, such as the characters in a string.

There are two types of loops in Python: the for loop and the while loop.

• we use a for loop to iterate over each character in the string.

  string = "hello"
  for char in string:
      print(char)

• while loop is designed to repeat a block of code while a specific condition is true

  string = "hello"
  index = 0
  while index < len(string):
      print(string[index])
      index += 1

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