HomePython Page 6 - Python 101 (part 3): A Twist In The Tail
Twist And Turn - Python
Find out more about adding flow control to your Python programswith the "for" and "while" loops, see how the range() function can be usedto generate number ranges, and learn all about list objects. And bring anold flame along for the ride.
In most programming languages, a "for" loop is used to execute a set ofstatements a certain number of times. Unlike a "while" loop, whichcontinues to run for so long as the specified conditional expressionevaluates as true, a "for" loop comes with a specific limit on the numberof times it can iterate.
Python's "for" loop conforms to this basic requirement; however, as withmost things in Python, there's a twist in the tail. A Python "for" loop isdesigned only to iterate over built-in "sequence objects" like strings andlists, and is structured like this:
for temp_var in sequence_obj:
Or, in English, "take each element of the sequence sequence_obj, place itin the variable temp_var, and execute the indented code block on temp_var".
An example might help to make this clearer:
>>> superheroes = ['Incredible Hulk', 'Green Lantern', 'Human Torch',
>>> for myhero in superheroes:
... print myhero, "rocks!"
Incredible Hulk rocks!
Green Lantern rocks!
Human Torch rocks!
Captain America rocks!
In this case, I've first initialized a list containing four elements. Next,I've used a "for" loop to iterate through the list; on each successiveiteration, one element of the list is assigned to the temporary variable"myhero" and then printed to the console via a print() call. Once all theelements of the list have been processed, the loop is automaticallyterminated.
You can use a "for" loop with any "sequence object" - this next exampledoes something similar with a string.
>>> str = "abracadabra"
>>> for char in str:
... print char, "_",
a _ b _ r _ a _ c _ a _ d _ a _ b _ r _ a _