Lists and Tuples

DATA STRUCTURES:
• DATA STRUCTURES ARE PARTICULAR WAYS OF STORING DATA TO
MAKE SOME OPERATION EASIER OR MORE EFFICIENT. THAT IS,
THEY ARE TUNED FOR CERTAIN TASKS
• DATA STRUCTURES ARE SUITED TO SOLVING CERTAIN PROBLEMS,
AND THEY ARE OFTEN ASSOCIATED WITH ALGORITHMS.
ROUGHLY TWO KINDS OF DATA STRUCTURES:
• BUILT-IN DATA STRUCTURES, DATA STRUCTURES THAT ARE SO
COMMON AS TO BE PROVIDED BY DEFAULT
• USER-DEFINED DATA STRUCTURES (CLASSES IN OBJECT ORIENTED
PROGRAMMING) THAT ARE DESIGNED FOR A PARTICULAR TASK

THE PYTHON LIST DATA STRUCTURE
• PYTHON COMES WITH A GENERAL SET OF BUILT IN DATA
STRUCTURES:
• LISTS
• TUPLES
• STRING
• DICTIONARIES
• SETS
• OTHERS...
• A LIST IS AN ORDERED SEQUENCE OF ITEMS.
• YOU HAVE SEEN SUCH A SEQUENCE BEFORE IN A STRING. A
STRING IS JUST A PARTICULAR KIND OF LIST

SIMILARITIES WITH STRINGS
• CONCATENATE/+ (BUT ONLY OF LISTS)
• REPEAT/*
• INDEXING (THE [ ] OPERATOR)
• SLICING ([:])
• MEMBERSHIP (THE IN OPERATOR)
• LEN (THE LENGTH OPERATOR)

OPERATORS
[1, 2, 3] + [4]  [1, 2, 3, 4]
[1, 2, 3] * 2  [1, 2, 3, 1, 2, 3]
1 IN [1, 2, 3]  TRUE
[1, 2, 3] < [1, 2, 4]  TRUE
COMPARE INDEX TO INDEX, FIRST DIFFERENCE DETERMINES THE
RESULT

DIFFERENCES BETWEEN LISTS AND
STRINGS
• LISTS CAN CONTAIN A MIXTURE OF
ANY PYTHON OBJECT, STRINGS CAN
ONLY HOLD CHARACTERS
• 1,"BILL",1.2345, TRUE
• LISTS ARE MUTABLE, THEIR VALUES
CAN BE CHANGED, WHILE STRINGS
ARE IMMUTABLE
• LISTS ARE DESIGNATED WITH [ ],
WITH ELEMENTS SEPARATED BY
COMMAS, STRINGS USE " " OR ' '

INDEXING
• CAN BE A LITTLE CONFUSING, WHAT DOES THE [ ] MEAN, A LIST OR
AN INDEX?
[1, 2, 3][1]  2
• CONTEXT SOLVES THE PROBLEM. INDEX ALWAYS COMES AT THE
END OF AN EXPRESSION, AND IS PRECEDED BY SOMETHING (A
VARIABLE, A SEQUENCE)

LIST OF LISTS
MY_LIST = ['A', [1, 2, 3], 'Z']
• WHAT IS THE SECOND ELEMENT (INDEX 1) OF THAT LIST? ANOTHER LIST.
MY_LIST[1][0] # APPLY LEFT TO RIGHT
MY_LIST[1]  [1, 2, 3]
[1, 2, 3][0]  1

LIST FUNCTIONS
• LEN(LST): NUMBER OF ELEMENTS IN LIST (TOP LEVEL).
LEN([1, [1, 2], 3])  3
• MIN(LST): SMALLEST ELEMENT. MUST ALL BE THE SAME
TYPE!
• MAX(LST): LARGEST ELEMENT, AGAIN ALL MUST BE THE
SAME TYPE
• SUM(LST): SUM OF THE ELEMENTS, NUMERIC ONLY

ITERATION
YOU CAN ITERATE THROUGH THE ELEMENTS OF A LIST
LIKE YOU DID WITH A STRING:

CHANGE AN OBJECT'S CONTENTS
• STRINGS ARE IMMUTABLE. ONCE CREATED, THE OBJECT'S CONTENTS CANNOT
BE CHANGED. NEW OBJECTS CAN BE CREATED TO REFLECT A CHANGE, BUT THE
OBJECT ITSELF CANNOT BE CHANGED
MY_STR = 'ABC'
MY_STR[0] = 'Z' # CANNOT DO!
# INSTEAD, MAKE NEW STR
NEW_STR = MY_STR.REPLACE('A','Z')

LISTS ARE MUTABLE
UNLIKE STRINGS, LISTS ARE MUTABLE. YOU CAN CHANGE THE
OBJECT'S CONTENTS!
MY_LIST = [1, 2, 3]
MY_LIST[0] = 127
PRINT(MY_LIST)  [127, 2, 3]

LIST METHODS
• REMEMBER, A FUNCTION IS A SMALL PROGRAM (SUCH AS
LEN) THAT TAKES SOME ARGUMENTS, THE STUFF IN THE
PARENTHESIS, AND RETURNS SOME VALUE
• A METHOD IS A FUNCTION CALLED IN A SPECIAL WAY,
THE DOT CALL. IT IS CALLED IN THE CONTEXT OF AN
OBJECT (OR A VARIABLE ASSOCIATED WITH AN OBJECT)

AGAIN, LISTS HAVE METHODS
MY_LIST = ['A',1,TRUE]
MY_LIST.APPEND('Z')
the object that
we are calling the
method with
the name of
the method
arguments to
the method

SOME NEW METHODS
• A LIST IS MUTABLE AND CAN CHANGE:
• MY_LIST[0]='A' #INDEX ASSIGNMENT
• MY_LIST.APPEND()
• MY_LIST.EXTEND()
• MY_LIST.POP()
• MY_LIST.INSERT()
• MY_LIST.REMOVE()
• MY_LIST.SORT()
• MY_LIST.REVERSE()

MORE ABOUT LIST METHODS
• MOST OF THESE METHODS DO NOT RETURN A VALUE
• THIS IS BECAUSE LISTS ARE MUTABLE, SO THE METHODS
MODIFY THE LIST DIRECTLY. NO NEED TO RETURN
ANYTHING.
• CAN BE CONFUSING

UNUSUAL RESULTS
MY_LIST = [4, 7, 1, 2]
MY_LIST = MY_LIST.SORT()
MY_LIST  NONE # WHAT HAPPENED?
WHAT HAPPENED DID THE SORT OPERATION CHANGED THE
ORDER OF THE LIST IN PLACE (RIGHT SIDE OF ASSIGNMENT).
THEN THE SORT METHOD RETURNED NONE, WHICH WAS
ASSIGNED TO THE VARIABLE. THE LIST WAS LOST AND NONE IS
NOW THE VALUE OF THE VARIABLE.

RANGE
• WE HAVE SEEN THE RANGE FUNCTION BEFORE. IT
GENERATES A SEQUENCE OF INTEGERS.
• IN FACT WHAT IT GENERATES IS A LIST WITH THAT
SEQUENCE:
MYLIST = RANGE(1,5)
MYLIST IS [1,2,3,4]

SPLIT
• THE STRING METHOD SPLIT GENERATES A SEQUENCE OF
CHARACTERS BY SPLITTING THE STRING AT CERTAIN SPLIT-
CHARACTERS.
• IT RETURNS A LIST (WE DIDN'T MENTION THAT BEFORE)
SPLIT_LIST = 'THIS IS A TEST'.SPLIT()
SPLIT_LIST
 ['THIS', 'IS', 'A', 'TEST']

SORTING
ONLY LISTS HAVE A BUILT IN SORTING METHOD. THUS YOU OFTEN CONVERT YOUR
DATA TO A LIST IF IT NEEDS SORTING
MY_LIST = LIST('XYZABC')
MY_LIST ['X','Y','Z','A','B','C']
MY_LIST.SORT() # NO RETURN
MY_LIST 
['A', 'B', 'C', 'X', 'Y', 'Z']

REVERSE WORDS IN A STRING
JOIN METHOD OF STRING PLACES THE CALLING STRING
BETWEEN EVERY ELEMENT OF A LIST

SORTED FUNCTION
THE SORTED FUNCTION WILL BREAK A SEQUENCE INTO
ELEMENTS AND SORT THE SEQUENCE, PLACING THE RESULTS
IN A LIST
SORT_LIST = SORTED('HI MOM')
SORT_LIST 
[‘ ’,'H','I','M','M','O']

TUPLES
• TUPLES ARE SIMPLY IMMUTABLE LISTS
• THEY ARE PRINTED WITH (,)

LISTS AND TUPLE
• EVERYTHING THAT WORKS WITH A LIST WORKS WITH A
TUPLE EXCEPT METHODS THAT MODIFY THE TUPLE
• THUS INDEXING, SLICING, LEN, PRINT ALL WORK AS
EXPECTED
• HOWEVER, NONE OF THE MUTABLE METHODS WORK:
APPEND, EXTEND, DEL

COMMAS MAKE A TUPLE
FOR TUPLES, YOU CAN THINK OF A COMMA AS THE OPERATOR
THAT MAKES A TUPLE, WHERE THE ( ) SIMPLY ACTS AS A
GROUPING:
MYTUPLE = 1,2 # CREATES (1,2)
MYTUPLE = (1,) # CREATES (1)
MYTUPLE = (1) # CREATES 1 NOT (1)
MYTUPLE = 1, # CREATES (1)

MULTIPLE COLLECTS
[X+Y FOR X IN RANGE(1,4) FOR Y IN RANGE (1,4)]
IT IS AS IF WE HAD DONE THE FOLLOWING:
MY_LIST = [ ]
FOR X IN RANGE (1,4):
FOR Y IN RANGE (1,4):
MY_LIST.APPEND(X+Y)
 [2,3,4,3,4,5,4,5,6]

Lists and Tuples

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