strings and functions

wek 5 & 6

strings

sequence of characters

example x = “zenora is a girl”

Since strings are objects, they come with built-in methods:

text = "hello world"

• text.upper() # '“HELLO WORLD”

• text.lower() # “hello world”

• text.capitalize() #" “Hello World”

• text.replace(old word, the replacement)

  text.replace(“world”, “you”) # “hello you”

• text.split() #[“hello”, “world”

• text.strip() …. removes spaces before and after the text never in the middle # “hello”

datatype of string

string

Other related methods

.lstrip() removes spaces on the left

.rstrip() removes spaces on the right

strip vs slicing

.strip() removes whitespaces

slicing extract part of string using indexex

Characters Are Numbers Internally

A = 65

a = 97

ord(‘a’) = 97

char(65) = A

unicode

• text = "Hello World"

  print([ord(a) for a in text])]

• #[72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100]

processing string

• + to join string “hello” + “world”

  # “hello world”

• to repeat string “hello”*3

  hellohellohello

• to find the length len() # 5

Indexing a string

• strings are immutable

• “hello Zee”[6] = Z

• what you cannot do

• s = "Hello"

  s[0] = "Y" # ❌ This will raise an error!

• you can do this

• s = "Hello"

  s = "Y" + s[1:] # Now s = "Yello"

Iterating over string characters

• word = "Hello"

  for a_char in word:

  print(a_char * 2, end="")

• word = "Hello"

  for index in range(len(word)):

  print(word[index] * 2, end="")

string slicing vs string indexing

• slicing Syntax: string[start⏹step]

• s[0, 10, 2] this gets a substring

• indexing Syntax: string[index]

• s[4] this gets one character

Notes on String Slicing

s[start, stop, step]

• if the step is negative

  reverse the string

To change a sentence to a list,

you use split

• sentence = "I love learning new things"

  word_list = sentence.split()

  print(word_list)

sentence = "Apples, oranges, bananas, grapes"

item_list = sentence.split(", ")

print(item_list)

• x.split(you write whatever is separating the sentence)

to turn a word into a list

you use list(sentence)

• sentence = "Hello"

  char_list = list(sentence)

  print(char_list)

  [“h”, “e”, “l”, “l”, “o”]

string methods THEY ARE ALL CASE SENSETIVE

• s.count()

  count how many times an item appear in the string

• s = "banana"

  print(s.count("a")) # Output: 3

• s.find()

  find the first occurrence

• s = "hello world"

  print(s.find("world")) # Output: 6

• len()

  return the length of the string

• s.replace(old, new)

  replace old string with new string

  .REPLACE IS CASE SENSENTIVE

  s = "I like apples"

  print(s.replace("apples", "oranges")) # Output: "I like oranges"

string formatting

• General Syntax:

  "<template string>".format(<var1>, <var2>, ...)

• name = "Alice"

  age = 30

  formatted = "My name is {} and I am {} years old.".format(name, age)

  print(formatted)

• "{} is {} years old.".format("Bob", 25)

• "{1} is older than {0}.".format("Tom", "Jerry")

  # Output: Jerry is older than Tom.

• "Hello, {first} {last}!".format(first="John", last="Doe")

f-strings

name = "Alice"

age = 30

print(f"My name is {name} and I am {age} years old.")

Percent (%) formatting – Older style

name = "Alice"

age = 30

print("My name is %s and I am %d years old." % (name, age))

Percent (%)

Code	Meaning	Example
%s	String	"Hello, %s" % "Alice"
%d	Integer (whole number)	"Age: %d" % 2

Left-Aligned in Fixed Width Using .format()

"{0:<20}".format("one")

one———————

Right-aligned:)

“{0:>20}”.format(“one”)

—————————one

Center-aligned

“{0:20}”.format(“one”)

————one—————

Floating point number rounding

• {position:width.decimal place}

• {0:5.3f}

• print("|{0:5.3f}|".format(1.23456789))

  |1.235|

• print("|{0:8.3f}|".format(1.23456789))

  | 1.235|

built in functions

• print() – displays output

• round() – rounds a number

• len() – returns the length of a sequence

• type() – returns the data type

map

def square(x):

return x * x

numbers = [1, 2, 3, 4]

squared = map(square, numbers)

print(list(squared)) # Output: [1, 4, 9, 16]

Defining a Function

def some_function():

statement1

statement2

# More statements as needed

calling or evoking a functions

some_function()

parameters

• actual parameters/arguments

these are the real values you pass in when calling a function

greet("Alice") # "Alice" is the actual parameter (argument)

greet("Bob") # "Bob" is also an actual parameter

• formal parameters

these act like placeholders

def greet(name): # 'name' is a formal parameter

print("Hello", name)

Pass-By-Value

def change_value(x):

print("Before changing inside function:", x)

x = 99

print("After changing inside function:", x)

num = 10

print("Before function call:", num)

change_value(num)

print("After function call:", num)

pass by value

def some_function(a):

a = a + 1

print(a)

b = 12

some_function(b)

print(b)

13

12

local variable

def some_function():

a = 1 # 'a' is a local variable inside the function

print(a) # prints the value of 'a'

some_function() # calls the function, so it will print 1

global

?

Return Values

def add(a, b):

return a + b # returns the sum of a and b

result = add(10, 11)

print(result) # Output: 21

docstring

• used to document what a function does

• """triple quotes"""

• to access the docstring use function_name._doc_

• def greet(name):

  """

  Greets the user with the provided name.

  Parameters:

  name (str): The name of the user.

  Returns:

  None

  """

  print(f"Hello, {name}!")

• you can for the doc string

  print(greet.__doc__)

• output

  Greets the user with the provided name.

  Parameters:

  name (str): The name of the user.

  Returns:

  None

Nested Functions / Function Composition

• function inside function

• def cube(x):

  return x x x

  def square(x):

  return x * x

  def power(a, b):

  return a ** b

  print(power(cube(2), square(2)))

International Space Station

• first component was launched in 1998

• largest man made object at orbit

• can be seen with naked eye.

arrays in functions

• def tester(myArr):

  print("In function, before append", myArr)

  myArr.append('a') # modifies the original list (mutates it)

  print("In function, after append", myArr)

  myArr = ['a', 'b', 'c'] # reassigns local variable 'myArr' to a new list

  print("In function, after reassignment", myArr)

  arrMain = ["one", "two", "three"]

  print("Before function call", arrMain)

  tester(arrMain)

  print("After function call", arrMain)

• Before function call ['one', 'two', 'three']

  In function, before append ['one', 'two', 'three']

  In function, after append ['one', 'two', 'three', 'a']

  In function, after reassignment ['a', 'b', 'c']

  After function call ['one', 'two', 'three', 'a']

Enigma function

• def Enigma(lst, item):

  tmp = []

  for i in lst:

  if i != item:

  tmp.append(i)

  return tmp

• def main():

  list1 = [2, 4, 6, 6, 8, 10]

  list2 = ["Skipper", "Kowalski", "Rico", "Private", "King Julian"]

  list3 = [[1, 2], [3, 4], [5, 6]]

  print(Enigma(list2, "King Julian"))

• ['Skipper', 'Kowalski', 'Rico', 'Private']

  [2, 4, 6, 6, 8, 10]