---
title: Introduction
description:
duration: 200
card_type: cue_card
---

# **Strings 2 (2 hours)**

[Class Recording](https://www.scaler.com/meetings/i/beginner-strings-2-13/archive)
[Lecture Notes](https://scaler-production-new.s3.ap-southeast-1.amazonaws.com/attachments/attachments/000/015/558/original/Strings_2.pdf?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIDNNIRGHAQUQRWYA%2F20221205%2Fap-southeast-1%2Fs3%2Faws4_request&X-Amz-Date=20221205T044912Z&X-Amz-Expires=86400&X-Amz-SignedHeaders=host&X-Amz-Signature=60169527e2359ff6276313309b10c3fa89ddd93838cb637c599959cddf38c7ea)
[Old Script](https://colab.research.google.com/drive/12GS8NQuNbxDrMQoDd4IdLxWZ-GQWOXYz?usp=sharing)

## **Content**
1. Various String methods.
2. Concept of Mutability and Immutability

### Meanwhile when people are joining (5-7 mins)
* Instructor's welcome.
* Breaking ice with students.
* Casual conversation to bond.

---
title: Quiz-1
description:
duration: 60
card_type: quiz_card
---

# Question
What will be the output of the following code?

message = "Hello World!"
i = 0
while i < len(message):
  if i % 2 == 0:
    print(message[i], end = "")
  i += 1

# Choices
- [x] HloWrd
- [ ] Hlo ol
- [ ] eolWrd
- [ ] el or

---
title: String Methods
description:
duration: 2400
card_type: cue_card
---

## String Methods

### .split() method
* The str.split() function converts a string into a list of characters based on a splitting criteria.

Code:

"4 5 6 7 8 9".split()

>Output:

['4', '5', '6', '7', '8', '9']

Code:

"This_is_a_underscore_separated_string".split("_")

>Output:

['This', 'is', 'a', 'underscore', 'separated', 'string']

### .join() method
* Opposite of splitting is joining.
* We can use str.join() to do so.

Code:

random = "This_is_a_underscore_separated_string".split("_")
" ".join(random)

>Output:

'This is a underscore separated string'

Code:

"-".join(random)

>Output:

'This-is-a-underscore-separated-string'

* While providing an iterable to .join() you need to make sure that the contents of that iterable only contain strings. Else it will throw an error.

Code:

# Error
a = ["random", 102, 282, "string"]
" ".join(a)

>Output:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-6c67814fded6> in <module>
      1 a = ["random", 102, 282, "string"]
----> 2 " ".join(a)

TypeError: sequence item 1: expected str instance, int found

### str() function

* We can use str() function to convert numbers to string.

Code:

str(45)

>Output:

'45'

### .find() method

* The .find() method can be used to find a substring inside a string that we can call it on.
* This function returns the starting index of the first occourance of the substring that we are trying to find inside the string.

Code:

"this is a random string".find("is")

>Output:

2

* Returns -1 if the substring is not present.

Code:

"this is a random string".find("adadsaf")

>Output:

-1

### .replace() method

* The method .replace() can be used to replace a substring from the original string with the input we provide.
* This method takes in two arguments, first the substring from the original string and second the string we want to replace it with.
* It replaces the string we provide and returns a new string.

Code:

random = "This is a random string that I have created"
# The orginal string is not updated.
random.replace("random", "SUPER RANDOM")

>Output:

'This is a SUPER RANDOM string that I have created'

### .count() method

* The method .count() can be used to count the number of occourances of a substring we provide inside a particular string.

Code:

random = "This is a random string that I have created"
random.count("a")

>Output:

5

Code:

random = "This is a random string that I have created random random random"
random.count("random")

>Output:

4

---
title: Question-1
description:
duration: 480
card_type: cue_card
---

* Take a string as input.
* Convert the string to lowercase without using any inbuilt function.

Code:

def custom_lower(s):
    result = ""
    for i in s:
        if ord(i) >= 65 and ord(i) <= 90:
            order = ord(i)
            order = order + 32
            result = result + chr(order)
        else:
            result = result + i
    return result
custom_lower("This is A stRIng")

>Output:

'this is a string'

---
title: Quiz-2
description:
duration: 60
card_type: quiz_card
---

# Question
What will be the output of the following code?

message = "Hello World!"
print(message.lower())

# Choices
- [ ] HELLO WORLD!
- [x] hello world!
- [ ] hELLO wORLD!
- [ ] Hello World!

---
title: Quiz-3
description:
duration: 60
card_type: quiz_card
---

# Question
What will be the output of the following code?

message = "Hello World!"
print(message.replace("o", "e"))

# Choices
- [ ] Helle Welle
- [ ] Helle Welle!
- [x] Helle Werld!
- [ ] Hllo Wrld!

---
title: Some more methods
description:
duration: 1800
card_type: cue_card
---

## Some more methods -

### .isdigit()

* .isdigit() returns True a character inside a string is a digit.
* This method works only for integers.

Code:

"6".isdigit()

>Output:

True

Code:

"a".isdigit()

>Output:

False

Code:

"aasda231321".isdigit()

>Output:

False

Code:

"124143222".isdigit()

>Output:

True

### .isalpha()

* isalpha() returns True if a certain string only contains alphabets

Code:

"aasda231321".isalpha()

>Output:

False

Code:

"a".isalpha()

>Output:

True

Code:

"abc".isalpha()

>Output:

True

### .isupper() and .islower()

Code:

"A".isupper()

Output:

True

Code:

"a".isupper()

Output:

False

Code:

"a".islower()

Output:

True

Code:

"A".islower()

Output:

False

### .isspace()

* Returns True if there is a space in the string.

Code:

"abc".isspace()

>Output:

False

Code:

" ".isspace()

>Output:

True

---
title: Question-2
description:
duration: 400
card_type: cue_card
---

* Take a string as input.
* Replace all the space with underscore.

Code:

string = "this is a random string"
# 1
def change_to_underscore(s):
    result = ""
    for i in s:
        if i.isspace():
            result = result + "_"
        else:
            result = result + i
    return result
change_to_underscore(string)

>Output:

'this_is_a_random_string'

Code:

# 2
string.replace(" ", "_")

>Output:

this_is_a_random_string

---
title: Quiz-4
description:
duration: 60
card_type: quiz_card
---

# Question
What will be the output of the following code?

message = "Not Hello World!"
print(message.startswith("Hello"))

# Choices
- [ ] True
- [x] False
- [ ] None
- [ ] Error

---
title: Quiz-5
description:
duration: 60
card_type: quiz_card
---

# Question
What will be the output of the following code?

message = "Hello World!"
print(message.find("o"))

# Choices
- [ ] 8
- [ ] 7
- [x] 4
- [ ] -1

---
title: Glossary of string methods
description:
duration: 90
card_type: cue_card
---

### Glossary of string methods -

<img src="https://d2beiqkhq929f0.cloudfront.net/public_assets/assets/000/039/775/original/glossary_of_str_methods.png?1689241237" width=600 height=350>

---
title: Mutability and Immutability
description:
duration: 900
card_type: cue_card
---

### Mutability and Immutability

Mutable: whose value can change.
Immutable: whose value can't change.

<img src="https://d2beiqkhq929f0.cloudfront.net/public_assets/assets/000/039/776/original/mutable_and_immutable_explaination.png?1689241538" width=600 height=150>
<img src="https://d2beiqkhq929f0.cloudfront.net/public_assets/assets/000/039/777/original/mutable_and_immutable_explaination_2.png?1689241570" width=600 height=350>

Code:

a = 3
a

>Output:

3

Code:

a = 4
a

>Output:

4

It seems that integers are mutable but NO. originally in python we are just changing the memory reference of a to point to 4 instead of changing its value to 4.

Only 3 data types in python are mutable

* list
* sets
* dictionaries

Other data types such as strings and tuples(you are going to learn in upcoming lectures) are immutable.

> id() function in python gives the memory address of the variable where it is stored

Code:

id(a)

>Output:

140424322138448

Code:

a = 6

id(a) # it can be seen that memory address is change hence now a is pointing to somewhere else in memory

>Output:

140424322138512

Code:

my_list =["Python", "C++", "Java"]
id(my_list)

>Output:

140423632449408

Code:

my_list.append("HTML")
my_list[0] = "Python 3.5"
print(my_list)
print(id(my_list))

>Output:

['Python 3.5', 'C++', 'Java', 'HTML', 'HTML']
140423632449408

As lists are mutable, we can see that the same list created in the first cell is modified and the values are changed in the same memory location.

Code:

my_string  = "Scaled"
my_string[5] = "r"  # Trying to change the character in the last index
print(my_string)

>Output:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-9-5b191cb176fc> in <cell line: 2>()
      1 my_string  = "Scaled"
----> 2 my_string[5] = "r"  # Trying to change the character in the last index
      3 print(my_string)

TypeError: 'str' object does not support item assignment

* We receive an error as strings are immutable and we cannot change the value in place.
* We can instead assign a new value to the same variable, which leads to the creation of a different memory space

Code:

print(id(my_string))

>Output:

140423625872112

Code:

my_string = "Scaler"
print(id(my_string))

>Output:

140423066082608