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Python Learning Progress – Part 4

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Last time I talked about getting input and a little about conditional statement and looping that should’ve enabled you to start creating the basic application and implement some basic algorithm in Python. This time, I am going to do a review on some of the basic data structures in Python.

The basic data structures in Python are: list, tuples, and dictionaries.

Let me start with dictionaries. Dictionaries basically defines a one-to-one relationship between a key and a value, just like an ordinary dictionary, where a word is associated with its definition. To define a dictionary in Python, the code are:

# to define a dictionary, declare a
# variable with its value enclosed
# between two curly braces '{}', and declare the 'entries' with
# the syntax, {key:value}, and separate each entry with a comma,
# as you can see, the key and value can be mixed of any data type
dict = {'name':'John','age':20,'sex':'male',1:'number'}

# to access it, call the variable name with the key in a bracket '[]'
print dict['name'] #will print John
print dict['age'] #will print 20
print dict[1] #will print number

# to change or delete the value, use the same method
dict['name'] = 'jack'
del dict['age']

Some things to note about dictionary are, dictionary keys are case-sensitive, and it is unordered.

Next is the list. Lists are, well, lists, like dictionary, a list contains an array of items. It is like the array list of other programming languages. To define a list, declare the contents of the list within a square brackets and separate it with commas.

# declare a list of fruits
ls = ["apple", "orange", "banana", "pear"]

# to access it, use the index, starting from 0
print ls[0] #apple
print ls[2] #banana

# enter a negative index to access the list from the last element
print ls[-1] #pear
print ls[-2] #banana

# accessing a subset of the list (known as slicing)
print ls[0:3] #access the list from index 0-2
print ls[1:4] #access the list from index 1-3
print ls[:4] #access the list from the beginning until index 3
print ls[1:] #access the list from index 1 until the last element

# to change it, use the same syntax
ls[0] = "mango"

# or to delete it
del ls[3]

# to add an element to the end of the list, use append() method
ls.append("tomato")

# to extend the list with another list, use the extend() method
ls.extend(["strawberry","grapes"])

# to insert an element into the list, use the insert() method, which
# takes the index of the list to insert, and the item to insert
ls.insert(1,"lemon") #insert lemon into index #1

In addition to these basic operation, a list provides a lot of other functions as well, one of them is the function to search for an element inside a list.

li = ["apple","banana","pear","mango"]

# to search for an index of an element
li.index("banana") #will return 1
li.index("orange") #will throws an exception, because orange isnt in the list

# to check if an element is inside a list
"apple" in li #return true
"orange" in li #return false

There are several variations in removing an item from a list:

li = ['apple','banana','orange','pear','grape']

# the basic del operation, which will remove an element at a specified index
del li[0] #will remove apple

# the remove() method, which will remove the first occurance of an element
# this statement will remove the first occurance of banana
# if there are more than one occurance of banana, only the first one will be removed
# if there isnt any banana inside the list, it will raise an exception
li.remove('banana')

# the pop() method, which will remove the last value from the list,
# and at the same time return that value
li.pop() # will return grape, and remove it from the list

There are other operations on a list as well,

li = [1,2,3,4]
ls = [5,6,7,8]

# this will return the list[1,2,3,4,5,6,7,8]
# similar to the extend() method, but it returns a new list
# instead of directly performing the operation on the list
li = li+ls

# this will repeat the list 3 times, so the content of li
# will be [1,2,1,2,1,2]
li = [1,2]*3

Next is the tuple, a tuple is basically a list that cant be changed. It has almost the same function the same functions as a list, but the content can’t be changed, so a tuple doesn’t have the remove(), pop(), append() or any other method that make changes to a list. Tuples are usually used to create a constant list, whose values wont change. In general, tuples are faster than lists, and there are also other specific uses of a tuples that will be explored later. To create a tuple, the syntax is similar to a list, but instead of a square bracket, a tuple is declared with ordinary brackets.

tp = ('a','b','c')
# it can be accessed the same way
tp[0] #a
tp[-1] #c
tp.pop() #will throw an error

Those are the basic data structures in Python, each of those are kind of similar to each other, but they have specific purposes that are unique to each of them. For example, a tuple can contain a list of another list, and a list can also nest several level of lists.

This entry is posted in 2012.

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