Section 07: Strings

Question 1

Overview of strings:

Answer 1

• Strings ordered collection of characters
• Each has an index, starting at 0
• Characters of a string can be retrieved using their index (or negative indices)
• Strings are immutable
• To create a new string, use slicing and string addition
• Strings have methods, including lower, upper, replace, split, count, find
• Comparing strings use == operator

Immutable sequence of Characters

Operators: + and *

• + for concatenation between two strings $s$ and $t$
• * between str(x) and int(x).

Question 2

What operators work on strings?

Answer 2

Operators: + and *

• + for concatenation between two strings $s$ and $t$
• * between str(x) and `int(x)

Question 3

String: Length

Answer 3

• Let s be a variable referencing a value of type string (str).

Useful function: len(s)

Takes a string s as input, and returns num of characters in s.

Question 4

Strings in a sequence

Answer 4

• String ordered collection of characters
• Each has an index (starting at zero)First characters: index 0 and last has index n-1

Question 5

Accessing Characters in a String:

Answer 5

Characters of a string can be retrieved using their index

• Can use negative indices to access characters → count backwards[-1] refers to the last character in a string

IndexError program stops if index DNE ⇒ error

Question 6

Errors

IndexError

Answer 6

IndexError program stops if index DNE ⇒ error

Question 7

Valid range for characters in a string:

Answer 7

[-len(s), len(s)-1]

Question 8

Strings

How are strings immutable?

Answer 8

Strings are immutable:

String objects are immutable. Immutable simply meansunmodifiable or unchangeable. Once String object is created its data or state can’t be changed but a new String object is created.

Question 9

Slicing

What is slicing?

Answer 9

from start until index 3

• Pieces off strings can be retrieved using a special form of indexing called “slicing”
• A start and end index is needed
  
  • If start omitted: slice begin start sentence
  • If end omitted: slipe continue till end

`»> fruit = ‘banana’

> > > print(fruit[:3])
ban

> > > print(fruit[3:])
ana`

Question 10

Slicing Strings

Step Value:

Answer 10

Step Value: thrid num when slicing → indicates how many indices to skip between characters

Same concept as range(start, end, step)

> > > s = ‘Hello world’
print(s[0:5:2])
Hlo

Question 11

Slicing Strings

Negative Skip Value

Answer 11

Negative Skip Value: decrease by the given amount

> > > s = ‘Hello world’
print(s[::-1])
dlrow olleH

Question 12

What are immutable objects?

Answer 12

• Strings immutable objects

• Cannot use square brackets to modify a character in string
  »> s = ‘cats’
  »> s[0] = ‘r’

TypeError

Working Solution:

python
s = 'cats' 
s = 'r' + s[1:] 

*Re-creating a new variable s and not changing the old one *

Question 13

How to create a new string?

Answer 13

To create a new string: use slicing and string addition

Question 14

Methods

s.lower()

Answer 14

(1) s.lower()

Returns a copy of s, all lower-case

> > > s = “HELLO”
s.lower()
‘hello’

Question 15

String Methods

(2) s.upper()

Answer 15

(2) s.upper()
Returns a copy of s, but all upper case

> > > s = ‘small’
s.upper()
‘SMALL’

Question 16

String Methods

(3) s.replace(old, new)

Answer 16

(3) s.replace(old, new)
string with all occurrences of substring old replaced by new

> > > s = ‘hello’
s.replace(‘l’, ‘y’)
‘heYYo’

Question 17

String Methods

(4) s.split(separator)

Answer 17

(4) s.split(separator)
Output: list of substrings in s separated by separator

> > > s = “Hi, I am a string”
s.split(‘,’)
[‘Hi’, ‘ I am a string’]

Question 18

String Methods

(5) s.count(c)

Answer 18

(5) s.count(c)

Returns num of non-overlapping occurences of c in s

> > > s = ‘Hello world’
s.count(‘l’)
3

Question 19

String Methods

(6) s.find(c)

Answer 19

(6) s.find(c)

Takes a substring as input and returns index where the first occurence of the substring s begins

If not a substring, then -1 returned

index of first occurence of substring c in s

> > > s = ‘Hello world’
s.find(‘ld’)
9

Question 20

String Methods

(7) s.find(c, start, end)

Answer 20

(7) s.find(c, start, end)

index of first occurrence of substring c in s between indices start and end (inclusive)

> > > s = ‘Hello world’
s.find(‘d’, 0, 5)
-1

Question 21

Compare strings

Answer 21

Comparing strings use the == operator

true if strings equal, false if strings not equal

• For characters, ASCII codes assigns each character a number
• D < c < d

> > > s = ‘Hello world’
s == ‘Hello world’
True

Question 22

The in opertor

Answer 22

• In Python, in is a keyword
• The in and not in operators test for membership

Use to test if one string is a substring of another

True or False

> > > ‘d’ in ‘Hello world’
True

Question 23

Random

uniform() method

Answer 23

uniform() method in Python isused to return a random floating-point number that is greater than or equal to the specified low boundary, and less than or equal to the specified high boundary

Generate a (pseudo)-random number
Using random module:

import random #Top of file -> Provides different functions

Question 24

Random

What is the random import

Answer 24

Using random module:

```python
import random #Top of file -> Provides different functions
~~~

1. random(): Returns random float value between 0.0 (inclusive) and 1.0 (exclusive)
2. randint(x, y): Retruns random int value between x and y, both included

Question 25

Random

Random Seed

Answer 25

Random Seed:

• Number generation not truley random → determined by initial value = **seed**
• If no seed specified: use generate the number is the current system time → then always differentFix seed function: random.seed()

Question 26

Traversing a String

How to traverse a string:

Answer 26

Process of going through a string one character at a time
Error type: IndexError

1. While loop, 2. for loop w index, 3. for loop w characters

Question 27

Traversing a string

(1) Using a while loop

Answer 27

```python
fruit = ‘banana’
index = 0
while index < len(fruit):
letter = fruit[index]
print(letter)
index = index + 1
~~~

Question 28

Traversing a String

(2) Using a for loop through the indices of a string

Answer 28

```python
s = ‘banana’
for i in range(len(s)):
print(s[i])
~~~

```python
s = ‘apple’
for i in range(len(s) -1):
print(s[i], end=” “)
#Output: a p p l
~~~

Question 29

Traversing a String

(3) Using a for loop through the characters of a string

Answer 29

```python
fruit = ‘banana’
for letter in fruit:
print(letter) #Output: print letters on separate lines
or …
print(letter, end = “ “) #Output: prints lets on the same line
~~~

Question 30

Traversal with a for loop comparison

Answer 30

For for loops:

1. Loop by index: for i in range (len(s))
   
   • Gives access to index
2. Loop by value for char in s
   
   • Saves from possible off-by-one (IndexError) Error