####################################################################
####################################################################
###  1. Iterators
####################################################################
####################################################################


####################################
## Several built-in iterable types:
## list, str, dict, set, tuple
####################################
"""
l = [1,3,2]

li = iter(l)
li          #list_iterator object
next(li)
next(li)
next(li)
next(li) #raises StopIteration

for e in l:   #new iterator created here
    print(e)  #for knows to "swallow" StopIteration
"""



####################################################################
####################################################################
###  2. Generators
####################################################################
####################################################################

def countdown_gen():
    ''' calling it creates an iterator for the values 5,4,3,2,1,'launch' '''
    yield 5
    yield 4
    yield 3
    yield 2
    yield 1
    yield 'launch'






# a function which produces a list of all positive even numbers up to n
def Evens_list(n):
    ''' a list of evens up to n '''
    return [num for num in range(n) if num%2==0]



# a generator function (includes a "yield" statement) which produces a generator that generates
#     all positive even numbers up to n
def Evens_gen(n):
    ''' a generator of evens up to n '''
    for i in range(n):
        if i%2 == 0:
            yield i




# A function that produces a generator using a generator expression.
#  The generator is exactly the same as the one produces by Evens_gen
def Evens_gen2(n):
    ''' a generator of evens up to n '''
    return (num for num in range(n) if num%2==0)







# a generator which produces the **infinite** sequence of all positive even numbers
def All_evens_gen():
    i=0
    while True:
        yield i
        i+=2



















###You may open task manager and watch memory/CPU usage
"""
l = Evens_list(10**8) #memory up
for x in l:
    pass
del l            #memory down

l = Evens_gen(10**8) #memory stays down all the time
for x in l:
    pass
del l
"""