Python Notes (4) - Lists and Dictionaries

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The note covers two python data types - lists and dictionaries.

Python notes of open courses @Codecademy.

Lists

Lists a datatype you can use to store a collection of different pieces of information as a sequence under a single variable name, e.g, list_name = [item_1, item_2].Access by Index list_name[index]List indices begin with 0, not 1.New Neighbors A list index behaves like any other variable name! It can be used to access as well as assign values.list_name[index] = valueLate Arrivals & List Length: A list doesn’t have to have a fixed length. You can add items to the end of a list using list_name.append('sth').List Slicing To access a portion of a list using list_name[1:2].We start at the index before the colon and continue up to but not including the index after the colon.

Slicing Lists and Strings

Strings can be treated as lists of characters: each character is a sequential item in the list, starting from index 0.

If your list slice includes the very first or last item in a list (or a string), the index for that item doesn’t have to be included. E.g.,

my_list[:2] # Grabs the first two items my_list[3:] # Grabs the fourth through last items

Maintaining Order

list.index(item): can find the index of item. The index can be assigned to a variable.list.insert(index, item): can insert the item at that index.list.sort(): can make strings in alphabetical order, or make lists in numerical order.

For One and All

for variable in list_name:

A variable name follows the for keyword; it will be assigned the value of each list item in turn.

# A sample loop would be structured as follows: a = ["List of some sort”] for x in a: # Do something for every x

in list_name: designates list_name as the list the loop will work on. The line ends with a colon : and the indented code that follows it will be executed once per item in the list.

Removing Elements from Lists list.pop(index) will remove the item at index from the list and return it. list.remove(item): removes the first item from list that matches the item. Note that .remove(item) does not return anything.del(list[index]) is like .pop in that it will remove the item at the given index, but it won’t return it.Python Range Function range(): is just a shortcut for generating a list. It three different versions: range(stop)range(start, stop)range(start, stop, step) In all cases, the range() function returns a list of numbers from start up to (but not including) stop. Each item increases by step.If omitted, start defaults to zero and step defaults to one.

Iterating Over a List in a Function

Method 1 - for item in list:

for item in list: print item Method 1 is useful to loop through the list, but it’s not possible to modify the list this way.

Method 2 - iterate through indexes:

for i in range(len(list)): print list[i] Method 2 uses indexes to loop through the list, making it possible to also modify the list if needed.

Dictionaries

A dictionary is similar to a list, but you access values by looking up a key instead of an index.

A key can be any string or number.

Dictionaries are enclosed in curly braces, like so:

# Example d = {'key1' : 1, 'key2' : 2, 'key3' : 3}

This is a dictionary called d with three key-value pairs. The key ‘key1’ points to the value 1, ‘key2’ to 2, and so on.

Dictionaries are great for things like phone books (pairing a name with a phone number), login pages (pairing an e-mail address with a username), and more!New Entries dict_name[new_key] = new_valueAn empty pair of curly braces {} is an empty dictionary, just like an empty pair of [] is an empty list.The length len() of a dictionary is the number of key-value pairs it has. Each pair counts only once, even if the value is a list.

del dict_name[key_name]: removes items from a dictionary. This will remove the key key_name and its associated value from the dictionary.

# The key "fish" has a list, the key "cash" has an int, and the key "luck" has a string. my_dict = { "fish": ["c", "a", "r", "p"], "cash": -4483, "luck": "good" } # print "c" print my_dict["fish"][0]

for loop on a dictionary: to loop through its keys with for key in dictionary:

Note that dictionaries are unordered, meaning that any time you loop through a dictionary, you will go through every key, but you are not guaranteed to get them in any particular order.

Tip: The \ character is a continuation character. The following line is considered a continuation of the current line.

External Resources

An Introduction to Python ListsDictionaries, Oh Lovely Dictionaries