.md test

1.basics/0.must-know.md

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+## Comments
+```py
+# This is a comment.
+
+# python editor
+# https://repl.it/languages/python3
+```
+
+```py
+'''
+This is not a
+multi line comment (also tripple double-quotes).
+This is still compiled by the interpreter
+but does nothing.
+https://www.python.org/dev/peps/pep-0257/#multi-line-docstrings
+'''
+```
+
+## `print` function
+
+```py
+print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)
+```
+
+Print objects to the text stream file, separated by sep and followed by end. `sep`, `end`, `file` and `flush`, if present, must be given as keyword arguments.
+
+https://docs.python.org/3/library/functions.html#print
+
+
+```py
+print(123)
+# => 123
+
+print("Hello World!")
+# Hello World!
+
+print("Hello", "World!")
+# => Hello World!
+
+print("Hello", "World!", sep=" - ")
+# => Hello - World!
+
+print( 4 * 2 )
+# => 8
+
+a, b = 4, 2
+print( "a * b =", a * b )
+# => a * b = 8
+
+# https://docs.python.org/3/tutorial/inputoutput.html
+print( "%d * %d = %d" %( a, b, (a * b) ) )
+# => 4 * 2 = 8
+
+print("{} * {} = {}".format( a, b, a * b ))
+# => 4 * 2 = 8
+
+print("{2}, {1} and {0}".format("mango", "bananas", "apple"))
+# => apple, bananas and mango
+
+print(1 == 1)
+# => True
+
+print( 1 != 1 )
+# => False
+```
+
+## Multi-line code
+The `\` character is used to put sequential code on the next line.
+
+```py
+a, b = 1, 2
+c = a + \
+    b
+print( "c =", c )
+
+# => c = 3
+```
+

5.advanced/iterator.py

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+# an iterator is an object which produces a value on succesive iteration 
+
+# sample iterator class
+# https://www.geeksforgeeks.org/iterators-in-python/
+class MyItr:
+    def __init__(self, limit):
+        self.limit = limit
+
+    # when iteration is initialized
+    def __iter__(self):
+        self.current = 0
+        return self
+
+    # get values sequentially
+    def __next__(self):
+        if self.current > self.limit:
+            raise StopIteration
+        else:
+            returnVal = self.current
+            self.current = self.current + 1
+            return returnVal
+
+# create iterator
+myItr5 = MyItr(5)
+
+# for loop
+print('for loop on interable', end=" :=> ")
+for elem in myItr5:
+    print( elem, end=" " )
+print('')
+
+# convert iterable to a list
+print( 'iterator: list(myItr5)', list(myItr5) )
+
+# check element exist
+print('iterator: 4 in myItr5', 4 in myItr5)
+
+# convert list to an iterable
+itr_list = iter([1,2,3,4,5])
+print('itr_list', end=" :=> ")
+for elem in itr_list:
+    print( elem, end=" " )
+print('')
+
+# https://www.geeksforgeeks.org/python-range-method/
+# https://stackoverflow.com/questions/31227536/what-is-the-difference-between-range0-2-and-listrange0-2
+# https://pynative.com/python-range-function/
+
+# in python 2.x, range() returns a list, which stores entire list in the memory.
+# in python 3, range() returns a class of immutable iterable object that lets you iterate over it. This does not store the list in memory. This produces the element on the fly as we iterate over them. (borrowd from: https://stackoverflow.com/questions/31227536/what-is-the-difference-between-range0-2-and-listrange0-2)
+# doc: https://docs.python.org/3/library/stdtypes.html#ranges
+
+# range(stop)
+a = range(5) # start from 0 to 5 (excluded)
+print( 'range(5)', list(a) )
+
+# range(start, stop)
+a = range(2, 5) # exclude 5
+print( 'range(2, 5)', list(a) )
+
+# range(start,stop,step)
+print( 'range(2,10)', list(range(2,10)) ) # default step: 1
+print( 'range(2,10,2)', list(range(2,10,2)) ) # ignores 9
+print( 'range(2,-10,-2)', list(range(2,-10,-2)) ) # ignores -9
+print( 'range(2,10,-2)', list(range(2,10,-2)) ) # empty
+#print( 'range(2,10,0)', list(range(2,10,0)) ) # ValueError for step 0
+