Anton Kasyanov, Introduction to Python, Lecture4

Anton Kasyanov, Introduction to Python, Lecture4

• 1.
  Introduction to Python Lecture 4 Kasyanov Anton IASA 2011
• 2.
  Plan  Lists  Tuples  Dictionaries
• 3.
  Lists  Python has a way of grouping similar items called a list.  Denoted by: list_name = [list_elt0,list_elt1, ..., list_eltn]
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  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.
• 5.
  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
• 6.
  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.
• 7.
  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.
• 8.
  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 ...
• 9.
  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] = ...
• 10.
  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.
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  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,)
• 12.
  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.
• 13.
  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].
• 14.
  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
• 15.
  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]
• 16.
  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.
• 17.
  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.
• 18.
  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.
• 19.
  Files  How to read file line by line  File = open(filename)  For line in file.readlines():  Print line
• 20.
  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.
• 21.
  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