iterating over two sequences at once

[Pierre-Frédéric Caillaud]

> for c, c2 in a, b:

	This is a valid syntax but it does not do I think you think it does :

	It will iterate over the list (in fact tuple) (a, b), so the loop will  
run two times, just as if you had typed "for c, c2 in (a, b):".

	At each iteration it will try to put the item in (c,c2), so if a and b  
are 2-item tuples, say :
	a= ("g",5)  and b=("f",6)

	you'll have :
	first iteration :
	c = "g" and c2=5
	second iteration :
	c = "f" and c2=6

	got it ?

	For can only manage one iterable (i'ts a one-level loop) so you'll either  
need to use indices :

	for pos, letter in enumerate(str1):
		letter2 = str2[letter*2]
		...

	Or you can use zip to join your two iterables in one :
	for letter1, letter2 in zip( str1, str2[::2]):
		...

	I'm not sure about the slice syntax [::2] but it means "every other  
char", chekc the docs.