Python Note - Day 2

Dictionary

Python uses {} to represent dictionary:

>>> scores = {'Rick':98,'Bob':78,'Tom':92}
>>> scores['Rick']
98
>>> source.get('Rick')
98

Use in to check if the key is included in the dictionary:

>>> 'Angel' in scores
False

Use pop() to remove items from dictionary:

>>> scores.pop('Rick')
98
>>> scores
{'Bob':78,'Tom':92}

set

A dictionary contains both keys and values, but a set contains keys only, no values, also, you need to provide a list to get a set:

>>> s = set([1,2,3])
>>> s
{1,2,3}

Duplicate values in a list will be removed automatically:

>>> s = set([1,2,2,3,3,4])
>>> s
{1,2,3,4}

Add / Remove keys:

>>> s.add(5)
>>> s
{1,2,3,4,5}
>>> s.remove(2)
>>> s
{1,3,4,5}

Union or intersection:

>>> s1 = set([1,2,3,4])
>>> s2 = set([2,3,4,5])
>>> s1 & s2
{2,3,4}
>>> s1 | s2
{1,2,3,4,5}

Functions

In Python, a function name can be assigned to a variable:

>>> a = abs
>>> a(-1)
1

Declare a function

def myFunc(x): # myFunc is the function name, x is the parameter for the function.
    if x >= 0:
        return x
    else:
        return -x

print(myFunc(-1))

Arguments

Position Arguments

def power(a):
    return a * a

def powern(a,n):
    s = 1
    while n > 0:
        n = n -1
        s = s * a
    return s

All basic arguments are required when the function is invoked.

Default Arguments

def powern(a,n=2): # n is a default argument and the default value is 2.
    s = 1
    while n > 0:
        n = n -1
        s = s * a
    return s

When calling the function, the default argument can be ignored, if so, the default value will be used.

If a function has more than 1 default arguments, we might need to specify the argument name when providing values:

def newHires(name, age, gender='F', hometown='China')
    print('New Hire:', name, gender, age, 'Years old.', 'From', hometown)

If the gender is different, but hometown is same to the default value, we may invoke this function like:

newHires('Rick', 20, 'M')

If the hometown is different, but gender is same to the default value, we may invoke this function like:

newHires('Rick', 20, hometown='U.K.')

Multi Value Arguments

def calc(*numbers): # The * character indicates that the number of value in the argument is non-staionary.
    sum = 0
    for n in numbers:
        sum = sum + n * n
    return sum

For the above function, we can call it like:

>>> calc(1,2,3)
14
>>> calc(1,2,3,4,5)
55

Pass a list into the function:

>>> nums = [1,2,3]
>>> calc(nums[0],nums[1],nums[2])
14

Or:

>>> nums = [1,2,3]
>>> calc(*nums)
14