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Python - String

Python - String

Strings

Strings in python are surrounded by either single quotation marks, or double quotation marks.
- 'hello' is the same as "hello".
Multiline Strings

a = """Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua."""

print(a)

a = '''Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua.'''
print(a)

Strings are Arrays

Strings in Python are arrays of bytes representing unicode characters.
However, Python does not have a character data type, a single character is simply a string with a length of 1.

a = "Hello, World!"
print(a[0]) #H
print(a[1]) #e
print(a[len(a)-1]) #!

for x in a:
    print(x)

# H
# e
# !
# H
# e
# l
# l
# o
# ,

# W
# o
# r
# l
# d
# !

String: More than one line

Three quotational marks.

xStr = ''' Hello,
    world'''

print(xStr)
#  Hello,
#     world

String Length: `len()`

To get the length of a string, use the len() function.

a = "Hello, World!"
print(len(a))   #13

String Check: `in`

To check if a certain phrase or character is present in a string, we can use the keyword in.

txt = "The best things in life are free!"

print("free" in txt)    # true
print("expensive" not in txt)   # true

if "free" in txt:
  print("Yes, 'free' is present.")

if "expensive" not in txt:
  print("No, 'expensive' is NOT present.")

String Slicing

The first character has index 0.
The last character does not include.

b = "Hello, World!"

print(b[0]) # H
print(b[0:1]) # H not inclusive
print(b[2:5]) # llo
print(b[:5]) # Hello
print(b[2:]) # llo, World!
print(b[-5:-2]) # orl

Slice a string with step
- str[begin_index: last_index: step]

arr = "ABCDEFG"

print(arr[::])  # ABCDEFG, grap all characters
print(arr[::1])  # ABCDEFG, grap all characters
print(arr[::2])  # ACEG, grap characters from beginning to the end with step of 2
print(arr[::-1])  # GFEDCBA, with step of -1 means reverse all order
print(arr[::-2])  # GECA, reverse with step of 2

String Function

# String function

print("\n--------Upper Case--------\n")
a = "Hello, World!"
print(a.upper())    # HELLO, WORLD!


print("\n--------Lower Case--------\n")
a = "Hello, World!"
print(a.lower())    # hello, world!


print("\n--------Remove Whitespace--------\n")
a = "    Hello, World!    "
print(a.strip())    # "Hello, World!"


print("\n--------Replace String--------\n")
a = "Hello, World!"
print(a.replace("H", "J"))  # Jello, World!


print("\n--------Split String--------\n")
a = "Hello, World!"
print(a.split(",")) # ['Hello', ' World!']

String Concatenation

# String Concatenation

print("\n--------+--------\n")
a = "Hello"
b = "World"
c = a + b
print(c)    # HelloWorld

age = 36
txt = "My name is John, I am"
print(txt,age)  # My name is John, I am 36

String Format

str.format() Method

combine strings and numbers by using the format() method!
- The format() method takes the passed arguments, formats them, and places them in the string where the placeholders {}.
- The format() method takes unlimited number of arguments, and are placed into the respective placeholders
Index Numbers
- use index numbers {0} to be sure the arguments are placed in the correct placeholders
Named Indexes
- use named indexes by entering a name inside the curly brackets {carname}, but then must use names when you pass the parameter values txt.format(carname = "Ford").

# String Format

age = 36
txt = "My name is John, and I am {}"
print(txt.format(age))  # My name is John, and I am 36


quantity = 3
itemno = 567
price = 49.95
myorder = "I want {} pieces of item {} for {} dollars."
print(myorder.format(quantity, itemno, price))  # I want 3 pieces of item 567 for 49.95 dollars.


quantity = 3
itemno = 567
price = 49.95
myorder = "I want to pay {2} dollars for {0} pieces of item {1}."
print(myorder.format(quantity, itemno, price))  # I want to pay 49.95 dollars for 3 pieces of item 567.

myorder = "I have a {carname}, it is a {model}."
print(myorder.format(carname = "Ford", model = "Mustang"))
# I have a Ford, it is a Mustang.

F-string Method

prefix the string with the letter F, the string becomes the f-string itself.
- The f-string can be formatted in much same as the str.format() method.
- The F-string offers a convenient way to embed Python expression inside string literals for formatting.
Speed
- The reason for adapting this formatting style is its speed. The f-string evaluates at runtime rather than constant values. It embeds expression inside string literals, using minimal syntax. It is fast because it evaluates at runtime, not a constant value.
Python expressions
- can put all valid Python expressions in them. 大括号中可以是变量, 表达式, 函数, 对象
Dictionary
- There is a different quotation to use dictionary keys and f-string. 注意: 在引用字典键时, 使用引号需要和字符串的引号不同.
Braces
- To make appear braces in the code, you should use the double quotes as follows. 需要显示大括号时,需要双括号.
- If we use the triple braces, it will display single braces in our string.
- can display the more braces if we use more than triple braces.

print("\n--------f-string--------\n")

print("\n--------variable--------")
val = 'Geeks'
# GeeksforGeeks is a portal for Geeks.
print(f"{val}for{val} is a portal for {val}.")
name = 'Tushar'
age = 23
# Hello, My name is Tushar and I'm 23 years old.
print(f"Hello, My name is {name} and I'm {age} years old.")


print("\n--------expressions--------")
print(f"{2 * 30}")  # 60


print("\n--------fuction--------")


def upercase(input):
    return input.upper()


name = "Sachin Tendulkar"
print(f"{upercase(name)} is great.")    # SACHIN TENDULKAR is great.


print("\n--------fuction--------")


class Actor:
    def __init__(self, first_name, last_name, movie):
        self.first_name = first_name
        self.last_name = last_name
        self.movie = movie

    def __str__(self):
        return f"{self.first_name} {self.last_name}'s superhit movie is {self.movie}."

    def __repr__(self):
        return f"{self.first_name} {self.last_name}  {self.movie}. Superhi!"


ac = Actor('Keenu', 'Reevs', 'Matrix')
print(f"{ac}")  # Keenu Reevs's superhit movie is Matrix.


print("\n--------Dictionary--------")

detail = {"name": "John", "age": 19}
# John is 19 years old.
print(f"{detail['name']} is {detail['age']} years old.")


print("\n--------Braces--------")

print(f"{70 + 40}")         # 110
print(f"70")       # {70 + 40}
print(f"}")     # {110}
print(f"}}")    # 70

Escape Character

An escape character is a backslash \ followed by the character you want to insert.

Code	Result
`\'`	Single Quote
`\\`	Backslash
`\n`	New Line
`\r`	Carriage Return
`\t`	Tab
`\b`	Backspace
`\f`	Form Feed
`\ooo`	Octal value
`\xhh`	Hex value

txt = "We are the so-called \"Vikings\" from the north."
print(txt)  # We are the so-called "Vikings" from the north.

Prefix: `b""`, `r""`

b prefix: the given string is in bytes’ format.

print(("abcdefg"))          # b'abcdefg'
print(type("abcdefg"))      # <class 'str'>

bprint((b"abcdefg"))         # b'abcdefg'
print(type(b"abcdefg"))     # <class 'bytes'>

r prefix: the string is to be treated as a raw string, which means all escape codes will be ignored.
```
print(r'\s') # \s
print(type(r'\s')) # <class 'str'>
```

String Methods

Return a new string.
- The original string remains.

Method	Description
`capitalize()`	Converts the first character to upper case
`casefold()`	Converts string into lower case
`center()`	Returns a centered string
`count()`	Returns the number of times a specified value occurs in a string
`encode()`	Returns an encoded version of the string
`format_map()`	Formats specified values in a string
`ljust()`	Returns a left justified version of the string
`lower()`	Converts a string into lower case
`lstrip()`	Returns a left trim version of the string
`maketrans()`	Returns a translation table to be used in translations
`partition()`	Returns a tuple where the string is parted into three parts
`replace()`	Returns a string where a specified value is replaced with a specified value
`rfind()`	Searches the string for a specified value and returns the last position of where it was found
`rindex()`	Searches the string for a specified value and returns the last position of where it was found
`rjust()`	Returns a right justified version of the string
`rpartition()`	Returns a tuple where the string is parted into three parts
`rsplit()`	Splits the string at the specified separator, and returns a list
`rstrip()`	Returns a right trim version of the string
`split()`	Splits the string at the specified separator, and returns a list
`splitlines()`	Splits the string at line breaks and returns a list
`startswith()`	Returns true if the string starts with the specified value
`strip()`	Returns a trimmed version of the string
`swapcase()`	Swaps cases, lower case becomes upper case and vice versa
`title()`	Converts the first character of each word to upper case
`translate()`	Returns a translated string
`upper()`	Converts a string into upper case
`zfill()`	Fills the string with a specified number of 0 values at the beginning

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