Lists & Strings
Brandon Krakowsky
Lists
Property of Penn Engineering |
Lists - Review
• If you recall, lists are a type of data structure in Python
- Lists are the most common sequence
- Lists are mutable, which means, once defined, the individual elements can be changed
Property of Penn Engineering | 3
Lists - Review
• If you recall, lists are a type of data structure in Python
- Lists are the most common sequence
- Lists are mutable, which means, once defined, the individual elements can be changed
• To create a list, specify comma separated values, in between square brackets []
Property of Penn Engineering | 4
Lists - Review
• If you recall, lists are a type of data structure in Python
- Lists are the most common sequence
- Lists are mutable, which means, once defined, the individual elements can be changed
• To create a list, specify comma separated values, in between square brackets []
• Values included do not need to be all of the same type
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Lists - Review
• If you recall, lists are a type of data structure in Python
- Lists are the most common sequence
- Lists are mutable, which means, once defined, the individual elements can be changed
• To create a list, specify comma separated values, in between square brackets []
• Values included do not need to be all of the same type
• Each list item is assigned an index value, starting at 0
list1 = [‘1’, ‘dog’, ‘cat’, 789]
print(list1)
print(len(list1)) #get the length of a list
print(list1[1]) #get the 2nd item in the list
print(list1[4]) #get the 5th item in the list – doesn’t exist!
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Lists - Review
• If you recall, lists are a type of data structure in Python
- Lists are the most common sequence
- Lists are mutable, which means, once defined, the individual elements can be changed
• To create a list, specify comma separated values, in between square brackets []
• Values included do not need to be all of the same type
• Each list item is assigned an index value, starting at 0
list1 = [‘1’, ‘dog’, ‘cat’, 789]
print(list1)
print(len(list1)) #get the length of a list
print(list1[1]) #get the 2nd item in the list
print(list1[4]) #get the 5th item in the list – doesn’t exist!
• You can look up the index of a value using the built-in list index method
print(list1.index('dog’))
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Lists - Review
• You can add items to a list
list1.append(“hello”)
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Lists - Review
• You can add items to a list
list1.append(“hello”)
• Get the length of a list
print(len(list1))
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Lists - Review
• You can add items to a list
list1.append(“hello”)
• Get the length of a list
print(len(list1))
• Remove items from a list
list1.pop() #removes the last item in the list
print(list1)
list1.pop(1) #removes the 2nd item in the list
print(list1)
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Lists - Review
• You can add items to a list
list1.append(“hello”)
• Get the length of a list
print(len(list1))
• Remove items from a list
list1.pop() #removes the last item in the list
print(list1)
list1.pop(1) #removes the 2nd item in the list
print(list1)
• Insert an item at a specific location in a list
list1.insert(2, 'inserted item’) #insert at 3rd location
print(list1)
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Lists - Review
• You can add items to a list
list1.append(“hello”)
• Get the length of a list
print(len(list1))
• Remove items from a list
list1.pop() #removes the last item in the list
print(list1)
list1.pop(1) #removes the 2nd item in the list
print(list1)
• Insert an item at a specific location in a list
list1.insert(2, 'inserted item’) #insert at 3rd location
print(list1)
• Check if an item is in a list
print(‘dog’ in list1)
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Lists - More Operations
• You can add lists
ls1 = [2, 3, 4]
ls2 = [7, 8, 9]
ls3 = ls1 + ls2
print(ls3)
- This creates a new list ls3 with the values of ls2 appended to the end of ls1, i.e. [2, 3, 4, 7, 8,
9]
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Lists - More Operations
• You can add lists
ls1 = [2, 3, 4]
ls2 = [7, 8, 9]
ls3 = ls1 + ls2
print(ls3)
- This creates a new list ls3 with the values of ls2 appended to the end of ls1, i.e. [2, 3, 4, 7, 8,
9]
• And multiply lists
ls4 = ls3 * 3
print(ls4)
- This creates a new list ls4 with the values of ls3 repeated three times,
i.e. [2, 3, 4, 7, 8, 9, 2, 3, 4, 7, 8, 9, 2, 3, 4, 7, 8, 9]
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Lists - More Functions
• You can extend lists using the extend function
ls1.extend(ls2)
- This is similar to adding lists, except it will actually update ls1 and append the values of ls2 to
the end of ls1
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Lists - More Functions
• You can extend lists using the extend function
ls1.extend(ls2)
- This is similar to adding lists, except it will actually update ls1 and append the values of ls2 to
the end of ls1
• Iterate over the elements of updated ls1 to see it’s been updated
for i in ls1:
print(i) #prints each element of the list
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Assignment by Value
• How are variables assigned in Python?
• For simple values (e.g. integers, floats, booleans, etc.), variables are assigned by value
a=5
b = a #copies the value
a=a+1
print(a) #6
print(b) #5
- a stores the value 6
- b stores the value 5
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Assignment by Reference
• For more complex types (e.g. lists, strings, etc.), variables are assigned by reference
- This is equivalent to the concept of pointers in C
a = [1, 2, 3]
b = a #copies the reference
a[1] = 5
print(a) #[1, 5, 3]
print(b) #[1, 5, 3]
- a stores a reference to object [1, 5, 3]
- b stores a reference to the same object [1, 5, 3]
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Assignment by Reference
• How do you make a true copy of a list?
lst1 = [1, 2, -1]
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Assignment by Reference
• How do you make a true copy of a list?
lst1 = [1, 2, -1]
• This DOES NOT COPY lst1 to lst2
lst2 = lst1 #lst1 and lst2 store references to the same list [1, 2, -1]
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Assignment by Reference
• How do you make a true copy of a list?
lst1 = [1, 2, -1]
• This DOES NOT COPY lst1 to lst2
lst2 = lst1 #lst1 and lst2 store references to the same list [1, 2, -1]
• This DOES COPY lst1 to lst3 using the copy method
lst3 = lst1.copy() #lst1 and lst3 store references to different lists
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Assignment by Reference
• How do you make a true copy of a list?
lst1 = [1, 2, -1]
• This DOES NOT COPY lst1 to lst2
lst2 = lst1 #lst1 and lst2 store references to the same list [1, 2, -1]
• This DOES COPY lst1 to lst3 using the copy method
lst3 = lst1.copy() #lst1 and lst3 store references to different lists
• How do we know?
lst1[0] = 2 #update original list
print(lst1) #print original list
print(lst2) #print new reference to same list
print(lst3) #print new reference to different list
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Assignment by Reference
• You can also iterate over a list and copy the values to a new list this way
lst4 = []
for i in lst1:
lst4.append(i)
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Assignment by Reference
• You can also iterate over a list and copy the values to a new list this way
lst4 = []
for i in lst1:
lst4.append(i)
• Or more concisely using a list comprehension
lst5 = [i for i in lst1]
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Assignment by Reference
• You can also iterate over a list and copy the values to a new list this way
lst4 = []
for i in lst1:
lst4.append(i)
• Or more concisely using a list comprehension
lst5 = [i for i in lst1]
• How do we know?
print(lst4) #print new reference to different list
print(lst5) #print new reference to different list
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Assignment by Reference
• What if our list is more complex?
complex_lst = [1, [2, 3], -1]
• Perform a deep copy
import copy
complex_lst2 = copy.deepcopy(complex_lst)
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Assignment by Reference
• What if our list is more complex?
complex_lst = [1, [2, 3], -1]
• Perform a deep copy
import copy
complex_lst2 = copy.deepcopy(complex_lst)
• Update the original list
complex_lst[1][1] = 4
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Assignment by Reference
• What if our list is more complex?
complex_lst = [1, [2, 3], -1]
• Perform a deep copy
import copy
complex_lst2 = copy.deepcopy(complex_lst)
• Update the original list
complex_lst[1][1] = 4
• Print results
print(complex_lst) #print original list
print(complex_lst2) #print new reference to different list
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Call by Value
• When we call a function, simple values (e.g. integers, floats, booleans, etc.) are passed by value
• When you pass variables with simple values as arguments to a function, the values themselves
are put into the function parameters
• If the parameters are changed within the function (which isn’t good style), new local
variables are created
• The changes are not put back into the original arguments
def alter(x):
x = x + 1 #does not affect a
a=5
print(a) #5
alter(a)
print(a) #still 5
- a stores value 5 and x stores value 6
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Call by Reference
• When we call a function, complex types (e.g. lists, strings, etc.) are passed by reference
• When you pass variables with more complex types as arguments to a function, the arguments
become references to the objects
def alter(x):
x[1] = 99 #changes contents of b
x = [4, 5, 6] #BUT, you can’t change the original reference, so this
creates new local variable x and does not affect b
b = [1, 2, 3]
print(b) #[1, 2, 3]
alter(b)
print(b) #[1, 99, 3]
- b stores a reference to [1, 99, 3] and x stores the same reference to [1, 99, 3]
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Identity vs. Equality
• How can we compare two variables?
• Use the identity testing operator “is” if you want to ensure that two variables hold references to the same
object
a = [1, 2, 3]
b=a
print(a is b) #True
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Identity vs. Equality
• How can we compare two variables?
• Use the identity testing operator “is” if you want to ensure that two variables hold references to the same
object
a = [1, 2, 3]
b=a
print(a is b) #True
• Use the “==“ operator if you want to ensure that two variables have the same value (even if they are not
the same object)
print(a == b) #True
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Identity vs. Equality
• How can we compare two variables?
• Use the identity testing operator “is” if you want to ensure that two variables hold references to the same
object
a = [1, 2, 3]
b=a
print(a is b) #True
• Use the “==“ operator if you want to ensure that two variables have the same value (even if they are not
the same object)
print(a == b) #True
• a is b implies a == b, but not necessarily the other way around!
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Identity vs. Equality
• How can we compare two variables?
• Use the identity testing operator “is” if you want to ensure that two variables hold references to the same
object
a = [1, 2, 3]
b=a
print(a is b) #True
• Use the “==“ operator if you want to ensure that two variables have the same value (even if they are not
the same object)
print(a == b) #True
• a is b implies a == b, but not necessarily the other way around!
• What happens if we update a?
a[2] = 5
print(a is b)
print(a == b)
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Lists – Exercise
• Here’s a sneaky_list function that accepts 2 list parameters. The function does some very
“sneaky” things with those lists.
def sneaky_list(lst1, lst2):
lst1 = lst2 #creates new local variable lst1 and points to value of
lst2
lst1.append(7) #appends to new local lst1
lst2.append(5) #appends to lst2
print(lst1 is lst2) #do they hold references to same list?
print(lst1 == lst2) #do they have the same value?
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Lists – Exercise
• Calling the sneaky_list function shows interesting results. Let’s see what happens!
def main():
lst_one = [1, 2, -1]
lst_two = [7, 8, -4]
sneaky_list(lst_one, lst_two)
print(lst_one) #what does lst_one contain now?
print(lst_two) #and lst_two?
#program entry point
if __name__ == '__main__’:
main()
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Lists - Slice
• You can get a slice of a list by using a colon (:)
• Format: [start_index:end_index]
• start_index and end_index are both optional
• start_index is the index of the first value (included in slice)
• end_index is the index of the last value (not included in slice)
my_list = [‘b’, ‘a’, ’n’, ‘a’, ’n’, ‘a’, ’s’]
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Lists - Slice
• You can get a slice of a list by using a colon (:)
• Format: [start_index:end_index]
• start_index and end_index are both optional
• start_index is the index of the first value (included in slice)
• end_index is the index of the last value (not included in slice)
my_list = [‘b’, ‘a’, ’n’, ‘a’, ’n’, ‘a’, ’s’]
• Get elements from index 2 to 4
print(my_list[2:5]) #returns slice with elements 3 to 5
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Lists - Slice
• You can get a slice of a list by using a colon (:)
• Format: [start_index:end_index]
• start_index and end_index are both optional
• start_index is the index of the first value (included in slice)
• end_index is the index of the last value (not included in slice)
my_list = [‘b’, ‘a’, ’n’, ‘a’, ’n’, ‘a’, ’s’]
• Get elements from index 2 to 4
print(my_list[2:5]) #returns slice with elements 3 to 5
• Get elements from index 4 to end
print(my_list[4:]) #returns slice with elements 5 to end
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Lists - Slice
• Get elements from index 0 to end (entire list!)
print(my_list[:]) #returns slice with elements 1 to end
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Lists - Slice
• Get elements from index 0 to end (entire list!)
print(my_list[:]) #returns slice with elements 1 to end
• Get elements from index 0 to -4 (counts from right to left)
print(my_list[:-4]) #returns slice with elements from 1 to 3
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Lists - Slice
• Get elements from index 0 to end (entire list!)
print(my_list[:]) #returns slice with elements 1 to end
• Get elements from index 0 to -4 (counts from right to left)
print(my_list[:-4]) #returns slice with elements from 1 to 3
• Another way to copy a list
copy_my_list = my_list[:] #creates new list from slice with elements 1
to end
print(copy_my_list)
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Lists - Slice
• Get elements from index 0 to end (entire list!)
print(my_list[:]) #returns slice with elements 1 to end
• Get elements from index 0 to -4 (counts from right to left)
print(my_list[:-4]) #returns slice with elements from 1 to 3
• Another way to copy a list
copy_my_list = my_list[:] #creates new list from slice with elements 1
to end
print(copy_my_list)
• Let’s test it
print(copy_my_list is my_list) #same references?
print(copy_my_list == my_list) #same values?
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Lists – Slice
• You can also update list elements by specifying an index or slice
• Here we have a list of odd numbers
odd_numbers = [2, 4, 6, 8]
• wait … what? Let’s make some changes!
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Lists – Slice
• You can also update list elements by specifying an index or slice
• Here we have a list of odd numbers
odd_numbers = [2, 4, 6, 8]
• wait … what? Let’s make some changes!
• Of course, we can update (a single) element at index 0
odd_numbers[0] = 1
print(odd_numbers) #should output [1, 4, 6, 8]
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Lists – Slice
• You can also update list elements by specifying an index or slice
• Here we have a list of odd numbers
odd_numbers = [2, 4, 6, 8]
• wait … what? Let’s make some changes!
• Of course, we can update (a single) element at index 0
odd_numbers[0] = 1
print(odd_numbers) #should output [1, 4, 6, 8]
• We can also update (multiple) elements from index 1 to 3
odd_numbers[1:4] = [3, 5, 7]
print(odd_numbers) #should output [1, 3, 5, 7]
• Note: index 4 doesn’t exist in the list. Python doesn’t care!
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Strings
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Strings
• A string is a sequence of characters
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Strings
• A string is a sequence of characters
• A string is kind of like a list – just imagine a string as a list of characters!
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Strings
• A string is a sequence of characters
• A string is kind of like a list – just imagine a string as a list of characters!
• Unlike lists, strings are immutable, which means, once defined, you cannot change the individual
elements (characters) of a string
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Strings
• A string is a sequence of characters
• A string is kind of like a list – just imagine a string as a list of characters!
• Unlike lists, strings are immutable, which means, once defined, you cannot change the individual
elements (characters) of a string
• For example, if we have a list:
my_menu_choices = [‘burger’, ‘fries’, ‘coke’]
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Strings
• A string is a sequence of characters
• A string is kind of like a list – just imagine a string as a list of characters!
• Unlike lists, strings are immutable, which means, once defined, you cannot change the individual
elements (characters) of a string
• For example, if we have a list:
my_menu_choices = [‘burger’, ‘fries’, ‘coke’]
• We can get a single value:
main_course = my_menu_choices[0]
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Strings
• A string is a sequence of characters
• A string is kind of like a list – just imagine a string as a list of characters!
• Unlike lists, strings are immutable, which means, once defined, you cannot change the individual
elements (characters) of a string
• For example, if we have a list:
my_menu_choices = [‘burger’, ‘fries’, ‘coke’]
• We can get a single value:
main_course = my_menu_choices[0]
• We can also update a single value:
my_menu_choices[0] = ‘cheese burger’
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Strings
• However, if we have a string:
my_restaurant_choice = ‘Mcdonalds’
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Strings
• However, if we have a string:
my_restaurant_choice = ‘Mcdonalds’
• We CAN get a single value (character):
my_restaurant_choice_third_letter = my_restaurant_choice[2]
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Strings
• However, if we have a string:
my_restaurant_choice = ‘Mcdonalds’
• We CAN get a single value (character):
my_restaurant_choice_third_letter = my_restaurant_choice[2]
• But we CAN’T directly update a single value (character) – this won’t work:
my_restaurant_choice[2] = ‘D’
- You will get an error because strings are immutable
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Slicing Strings
• Like a list, we can get a slice from a string!
• This is called a substring
• Use the same colon (:) syntax
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Slicing Strings
• Like a list, we can get a slice from a string!
• This is called a substring
• Use the same colon (:) syntax
• Format: [start_index:end_index]
• start_index is the index of the first value (included in slice)
• end_index is the index of the last value (not included in slice)
s = ‘Hello world!’
• Get characters from index 0 to 5
print(s[:5]) #returns substring with characters 1 to 5
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Slicing Strings - Exercise
• Set a variable name to the value of your first and last name
• Print the substring containing just your first name, without counting the letters in your first name
• Hint: Use the built-in string index method to locate the space
name = 'Brandon Krakowsky’
first_space = name.index(' ') #get the index of the first space in the
string
print(name[0:first_space]) #use the first_space index when getting the
substring
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Slicing Strings - Exercise
• Write code to print the 3rd to the 16th letters of the alphabet
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Slicing Strings - Exercise
• Write code to print the 3rd to the 16th letters of the alphabet
• First, import the string module, to gain access to additional useful string methods
import string
• Then you can use the following variable to get all lowercase letters
alphabet = string.ascii_lowercase
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Slicing Strings - Exercise
• Write code to print the 3rd to the 16th letters of the alphabet
• First, import the string module, to gain access to additional useful string methods
import string
• Then you can use the following variable to get all lowercase letters
alphabet = string.ascii_lowercase
• alphabet is a string
print(type(alphabet))
• Print the new string by slicing the 3rd to the 16th letters
print(alphabet[2:16])
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Some String Functions
• Here are some useful built-in string methods:
• string.capitalize() – capitalizes first letter of string
• string.startswith(prefix) – determines if string starts with prefix
• string.endswith(suffix) – determines if string ends with suffix
• string.isupper() – determines if all characters in the string are uppercase
• string.islower() – determines if all characters in the string are lowercase
• string.find(str) – determines if str occurs in string
• string.index(str) – determines index of str in string
• string.replace(old, new) – replaces all occurrences of old in string with new
• string.strip() – trims whitespace from beginning and end of string
• string.upper() - returns uppercased string from given string
• string.lower() - returns lowercased string from given string
• All strings have these built-in methods!
For reference: https://docs.python.org/3/library/stdtypes.html#string-methods
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Some String Functions
• split is a useful string method used to split a single string into a list of multiple strings
colors = 'blue,red,green’
colors_list = colors.split(‘,’) #splits string into list of strings
using comma separator
print(colors_list)
print(colors_list[2])
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Some String Functions
• split is a useful string method used to split a single string into a list of multiple strings
colors = 'blue,red,green’
colors_list = colors.split(‘,’) #splits string into list of strings
using comma separator
print(colors_list)
print(colors_list[2])
• Conversely, join creates a single string from a list of multiple strings
separator = ‘,’
new_colors = separator.join(colors_list) #joins list of strings using
separator value
print(new_colors)
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Some String Functions
• In a previous example, we tried to update a character in a string – this wouldn’t work:
my_restaurant_choice = ‘Mcdonalds’
my_restaurant_choice[2] = ‘D’
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Some String Functions
• In a previous example, we tried to update a character in a string – this wouldn’t work:
my_restaurant_choice = ‘Mcdonalds’
my_restaurant_choice[2] = ‘D’
• We CAN first convert the string to an actual list
- Note: Calling the split function with an empty string (‘’) will throw an error – so this won’t work
my_restaurant_choice_list = my_restaurant_choice.split(‘’)
- Instead, use Python’s built-in list function to convert the string to a list
my_restaurant_choice_list = list(my_restaurant_choice)
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Some String Functions
• In a previous example, we tried to update a character in a string – this wouldn’t work:
my_restaurant_choice = ‘Mcdonalds’
my_restaurant_choice[2] = ‘D’
• We CAN first convert the string to an actual list
- Note: Calling the split function with an empty string (‘’) will throw an error – so this won’t work:
my_restaurant_choice_list = my_restaurant_choice.split(‘’)
- Instead, use Python’s built-in list function to convert the string to a list
my_restaurant_choice_list = list(my_restaurant_choice)
• Now we can update the third letter
my_restaurant_choice_list[2] = ‘D’
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Some String Functions
• In a previous example, we tried to update a character in a string – this wouldn’t work:
my_restaurant_choice = ‘Mcdonalds’
my_restaurant_choice[2] = ‘D’
• We CAN first convert the string to an actual list
- Note: Calling the split function with an empty string (‘’) will throw an error – so this won’t work:
my_restaurant_choice_list = my_restaurant_choice.split(‘’)
- Instead, use Python’s built-in list function to convert the string to a list
my_restaurant_choice_list = list(my_restaurant_choice)
• Now we can update the third letter
my_restaurant_choice_list[2] = ‘D’
• Then convert back to a string using join
my_restaurant_choice = ‘’.join(my_restaurant_choice_list)
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