Python Notes (4) - Lists and Dic

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] = value

• Late 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_value
  • An 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 Lists
• Dictionaries, Oh Lovely Dictionaries