Anton Kasyanov, Introduction to Python, Lecture4

Introduction to Python

Lecture 4
Kasyanov Anton

IASA 2011

Plan

 Lists
 Tuples
 Dictionaries

Lists

 Python has a way of grouping similar items
called a list.
 Denoted by:
list_name =
[list_elt0,list_elt1, ...,
list_eltn]

Lists

 To get to the i-th element of a list we use:
list_name[i-1]
 We use i-1 because lists are indexed from 0.
 This means to refer to the elements of a 4
element list named list_name we use
list_name[0], list_name[1],
list_name[2], list_name[3]
 Lists are mutable.

Lists

 You can also have an empty list: [].
 You can index into lists from the back.
 list_name[-i] returns the ith element from the
back.
 Lists are mixed:
 That is, the elements in a list need not be the same
type, can have ints and strings.
 Can even have lists themselves.
 + and * operators are overloaded for lists

Functions

 Lists come with lots of useful functions and
methods.
 len(list_name), as with strings, returns the
length of the list.
 min(list_name) and max(list_name)
return the min and max so long as the list is
well defined.
 sum(list_name) returns the sum of elements
so long as they're numbered.
 Not defined for lists of strings.

Methods

 append(value) – adds the value to the end
of the list.
 sort() - sorts the list so long as this is well
defined. (need consistent notions of > and ==)
 insert(index, value) – inserts the
element value at the index specified.
 remove(value) – removes the first instance
of value.
 count(value) – counts the number of
instances of value in the list.

Looping

 Often we want to do a similar operation to every
element of the list.
 Python allows us to do this using for loops.
for item in list:
block
 This is equivalent to:
item = list[0]
block
item = list [1]
block
...

Looping

 To do that, we use the range() function.
 range(i) returns an ordered list of ints ranging
from 0 to i-1.
 range(i,j) returns an ordered list of ints ranging
from i to j-1 inclusive.
 range(i,j,k) returns a list of ints ranging from i
to j-1 with a step of at least k between ints.
 So range(i,k)==range(i,k,1)
 To modify a list element by element we use:
for i in range(len(list)):
list[i] = ...

Slicing

 Sometimes we want to perform operations on a
sublist.
 To refer to a sublist we use list slicing.
 y=x[i:j] gives us a list y with the elements
from i to j-1 inclusive.
 x[:] makes a list that contains all the elements of the original.
 x[i:] makes a list that contains the elements from i to the end.
 x[:j] makes a list that contains the elements from the beginning
to j-1.

Tuples

 Sometimes we want our lists to be immutable.
 To do that we can make a tuple.
 tuple_name=(item0,item1,item2,...)
 Items are referenced by tuple_name[i] not
tuple_name(i)
 Single element tuples must be defined with a
comma to avoid ambiguity
 (8+3) vs. (8+3,)

Strings

 Strings can be considered tuples of individual
characters. (since they are immutable).
 In particular, this means that we can use the list
knowledge that we gained, an apply it to
strings.
 Can reference individual characters by string[+/-i].
 Strings are not mixed, they can only contain
characters.
 min() and max() defined on strings, but sum() is not.
 You can slice strings just as you can lists.

Strings

 Now that we know that we can index into
strings, we can look at some more string
methods.
 find(substring): give the index of the first character
in a matching the substring from the left or -1 if no
such character exists.
 rfind(substring): same as above, but from the right.
 find(substring,i,j): same as find(), but looks only in
string[i:j].

Nested lists

 Because lists are mixed, we can have lists of
lists.
 This is useful if we want matrices, or to
represent a grid or higher dimensional space.
 We then reference elements by list_name[i][j] if
we want the jth element of the ith list.
 So then naturally, if we wish to loop over all the
elements we need nested loops:
for item in list_name:
for item2 in item:
block

Dictionaries

 In one sentence, dictionaries are (key, value)
pairs. Sometimes they are called maps.
 Python syntax:
{key0 : value0, key1 : value1, ...,
keyn : valuen}
 Dictionaries are of type dict
 Since they have a type, they can be assigned to a
variable.
 To refer to a value associated with a key in a
dictionary we use dictionary_name[key]

Dictionaries

 Dictionaries are unsorted.
 Dictionary keys must be immutable, but the
values can be anything.
 Cannot be None.
 Once you've created a dictionary you can add
key-value pairs by assigning the value to the
key.
dictionary_name[key] = value
 Keys must be unique.

Dictionary methods
 len(dict_name) works in the same way as it
does for strings and lists.
 + and * are not defined for dictionaries.
 dict.keys() - returns the keys in some
order.
 dict.values() - returns the values in some
order.
 dict.items() - returns the (key, value) pairs
in some order.
 All of these methods have iter* variants that return
the keys|values|key-value pairs one by one.

Dictionary methods

 dict.has_key(key) - returns True iff the
dictionary has the key in it.
 dict.get(key) – returns the value that is
paired with the key, or None if no such key
exists.
 get(key, d) returns d rather than None if no
such key exists.
 dict.clear() - removes all the key-value
pairs from the dictionary.

Files

 How to read file line by line
 File = open(filename)
 For line in file.readlines():
 Print line

Home assignment

 You have to create a simple dictionary
program.
 Dictionary in file:
 Home=дом
 Table=стол
 Etc.
 Program have to translate words from one
language to another and vice versa.

Home assignment

 Any other improvements are welcome
 Use modules if you want
 Use str.split() and list.index() functions
 Create two versions:
 Using lists
 Using dicts
 Send files to mind_master@ukr.net due to
Monday

Anton Kasyanov, Introduction to Python, Lecture4

Anton Kasyanov, Introduction to Python, Lecture4

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Anton Kasyanov, Introduction to Python, Lecture4