important chapter

2. DATA STRUCTURES IN PYTHON
Data Structures in Python :
Lists
Dictionary
Tuples
Sets

3. LISTS
 List is most versatile datatype available in Python, written as a list of comma-
separated values (items) between square brackets.
 Items in a list can be Heterogenous types(need not be of the same type).
 Lists are mutable
 Concatenation produces a new lists.
 Append function extends a list with a new value without changing it.

4. LISTS ARE MUTABLE
 A mutable object can be changed after it's created, and an immutable object
cannot be changed after its created.
 Examples of mutable objects are dictionary,lists etc
 Example of lists in mutable form is:

5. LIST FUNCTIONS
 list. append ( x) :Add an item to the end of the list.
 list. extend ( L): Extend the list by appending all the items in the given list.
 list. insert ( i, x): Insert an item at a given position.
 list. remove ( x) : Remove the first item
from the list whose value is x.
It gives an error if there is no such item.

6. LIST FUNCTIONS
 list. pop ( [i]): Remove the item at the given position in the list, and return it.
 list. clear ( ): Remove all items from the list. Equivalent to del a[:] .
Not supported in python 2.X.
 list.reverse(): Reverse a given list.
 list. index ( x): Return the index in the list of the first item whose value is x. It is an
error if there is no such item.
 list. count ( x): Return the number of times x appears in the list.
 list. sort ( ): Sort the items of the list in place.

7. DICTIONARY
 Dictionary is defined as an unordered set of key: value pairs, with the
requirement that the keys are unique .
 Dictionary is a Mutable datatype.
 Dictionaries are sometimes found in other languages as “associative memories”
or “associative arrays”.
 Dictionaries are indexed by keys, which can be any immutable type or could be
a string.
 A pair of braces creates an empty dictionary: {} , not [].
 INITIALISATION: example be
 Keys could be a string in dictionary. Example

8. DICTIONARY
 We can nest dictionaries. Example:
 Directly assign values to a dictionary.
 Dictionary comprehensions can be used to create dictionaries from arbitrary key
and value expressions:
 The dict() constructor builds dictionaries directly from sequences of key-value
pairs:

9. TUPLES
 Tuple is a sequence data type.
 Tuples are immutable, and usually contain a heterogeneous sequence of
elements.
 Simultaneous assignment is possible in tuples.
 In tuple, we can assign a tuple of value to a name.

10. TUPLES
 Tuples may be nested.
 A tuple consists of a number of values separated by commas.
 Extract positions in tuples using slices.

11. SETS
 A set is an unordered collection with no duplicate elements.
 Set objects also support mathematical operations like union, intersection,
difference, and symmetric difference.
 Curly braces or the set() function can be used to create sets.
 Example:
 Creates an empty set using
 Set membership in sets
 We can convert a list into sets.

12. SETS OPERATIONS
 Union : union of two sets is done using “set1|set2”
 Intersection : intersection of two sets can be done using “set1&set2”
 Set difference: In this, elements present in set2 is not included in the resultant set.
It can be don using “set1-set2
 Exclusive or: The syntax for exclusive or is “set1^set2”

13. STRINGS
 String is defined as a sequence or list of characters.
 String is immutable in nature means once defined , they cannot be changed.
 str is the type for strings in python.
 Define strings using quotes (“ or ‘ or “““)
>>> st = “Hello World”
>>> st = ‘Hello World’
>>> st = “““This is a multi-line
string that uses triple quotes.”””
‘ ‘ can be used to escape quotes:

14. STRINGS
 The string is enclosed in double quotes, if the string contains a single quote
and no double quotes, otherwise it is enclosed in single quotes.
 The print() function produces a more readable output, by omitting the enclosing
quotes and by printing escaped and special characters:
 ‘n’ is used to place a string in newline.

15. STRINGS
 If you don’t want characters prefaced by to be interpreted as special characters,
you can use raw strings by adding an r before the first quote:
 Two or more string literals (i.e. the ones enclosed between quotes) next to each
other are automatically concatenated.
 Attempting to use an index that is too large
result in an error.

16. ACCESSING A STRINGS
 To access substrings, use the square brackets for slicing along with the index or
indices to obtain your substring.
 Example −

17. POSITIVE AND NEGATIVE INDICES

18. OPERATIONS ON STRINGS
 CONCATENATION (+) :
 adds value on the either sde of operator.
 REPETITION (*) :
 creates new strings,concatenating multiple copies of the same strings.
 SLICE ([]) :
 gives the character from the given index
 RANGE SLICE ([ :]) :
 gives the character from the given range.

19. OPERATIONS ON STRINGS
 MEMBERSHIP (in) or (not in) : membership returns true or false if the character
exist in the given string.
 length (len): It is used to find the length of the string.
 upper() : upper() is used to uppercase the string .
 lower() : lower() operation is used to lowercase the string.
 THERE ARE MANY MORE OPERATIONS ON STRINGS

20. MODIFYING STRINGS
 Cannot update a string “in place” as strings are immutable in nature.
 Example:
 Instead ,use slices and concatenation for modification of strings:-
 Example :

21. SLICING IN STRINGS
 A slice is a segment of string.
 Slicing allows you to obtain substring
 Syntax:


Str[start : end]
Start: substring start from this element
End: end of substring excluding the element at this index
 S[i:j] starts at s[i] and ends at s[j-1]
 S[:j] means start at s[0], so s[0:j]
 S[i:] means ends at s[len(s)-1]

22. STRING FORMATTING
 str.format () is used to format the string.
 Examples: Replace argument by position in message string.
 Replace argument by names in message string.