What are Strings in Python

It’s a Sequence, a Sequence of Characters

Can access the characters using the braket operator
```
>>> fruit = 'banana'
>>> letter = fruit[1]
>>> print(letter)
a
```
You get that fruit[1] is a, the second letter in the string. Remeber counting starts from 0 in Python.
- the number inside the bracket is called the index.
- Indeces can be negative. But index $\in [-length+1, length -1]$.
len is the built-in function which gives the length of a string.
```
>>> fruit = 'banana'
>>> len(fruit)
6
```

How to travese through a string:

while loop:

index = 0
while index < len(fruit):
    letter = fruit[index]
    print(letter)
    index = index + 1

for loop:
```
for char in fruit:
  print(char)
```

How to slice a string?

>>> s = 'Monty Python'
>>> print(s[0:5])
Monty
>>> print(s[6:12])
Python
>>> print(s[:3])
Mon
>>> print(s[3:])
ty Python
>>> print(s[3:3])
''
>>> print(s[:])
Monty Python

What’s imutable about ’em'?

try this:

>>> greeting = 'Hello, world!'
>>> greeting[0] = 'J'
TypeError: 'str' object does not support item assignment

you get an error because strings are immutable, ie they can’t be changed.

instead you need to create a new string:

>>> new_greeting = 'J' + greeting[1:]
>>> print(new_greeting)
Jello, world!

Fighting for equality:

if word == 'banana':
  print('All right, bananas.')

this checks if the string word is exactly the string banana.

if word < 'banana':
  print('Your word,' + word + ', comes before banana.')

this checks the alphabetical order among strings.

All the uppercase letters come before all the lowercase letters

Are strings Python objects? What’s an object?

Strings are objects
Object contains two things
- data
- methods : functions built into objects and available to any instance
The dir shows avilable methods associated with the object
Methods have a different syntax:

Some string methods :

str.upper() : as shown above, return copy by turning all characters into uppercase.
str.capitalize() : returns copy with first character capitalized and the rest lowercased
str.casefold : removes all case distictions; umlauts, etc removed
str.count(sub[, start[, end]]) : Returns the number of non-overlapping occurrences of substring sub in the range $[start, end]$
str.find(sub[, start[, end]]) : Returns the lowest index in the string where substring sub is found within the slice $s[start:end]$. Returns -1 if sub is not found.
in operator (not a method):
```
>>> 'Py' in 'Python'
True
```
str.endswith(suffix[, start[, end]]) : Returns True if the string ends with the specified suffix, otherwise return False. suffix can also be a tuple of suffixes to look for.
str.format(*args, **kwargs) : replaces elements in {} with a string value. Eg
```
>>> "The sum of 1 + 2 is {0}".format(1+2)
'The sum of 1 + 2 is 3'
```
This is a relatively new inclusion to Python. More details later.
str.index(sub[, start[, end]]) : like find(), but returns ValueError when substring is not found.
str.isdigit() : returns True if all characters are digits and at least one character, False otherwise. Digits include decimals and special ones (like non-base 10 and superscript ones).
str.join(iterable) : Returns a string which is a concatenation of strings in iterable. Error TypeError if there is a non-string value in iterable.
str.partition(sep) : Splits the string at the first occurence of sep and returns a 3-tuple containing the part before separator, the seperator, and the part after separator.
str.replace(old,new[,count]) : Returns a copy of str with all occurances of old replaced by new. If count is provided then only the first count-number of replacements are held.
str.split(sep=None, maxsplit=-1) : Returns a list of words in the string, all of which are separated by sep. The defaults separates whitespace characters.
str.startswith : Like endswith(), but checks for prefix.
str.strip([chars]) : Returns a copy of the string with the leading and trailing characters removed. By default it removes the whitespaces at the begining and end of the string.

Parsing Strings

Exercises

My solutions:

fruit = 'banana'
index = len(fruit)
while index > 0:
    print(fruit[index -1])
    index = index - 1

Here in the first statement of the while loop I combined two statement and it works.

 def countletter(word, char):
     count = 0
     for letter in word:
         if letter == char:
             count = count + 1
     print(count)

 word = input('Word: ')

 char = input('Letter to count: ')

 countletter(word, char)

It’s a Sequence, a Sequence of Characters#

How to travese through a string:#

How to slice a string?#

What’s imutable about ’em'?#

Fighting for equality:#

Are strings Python objects? What’s an object?#

Some string methods :#

Parsing Strings#

Exercises#