Counter Loops: while and range

The range function is really a general tool that can be used in a variety of contexts. Although it’s used most often to generate indexes in a for, you can use it anywhere you need a list of integers. In Python 3.0, range is an iterator that generates items on demand, so we need to wrap it in a list call to display its results all at once (more on iterators in Chapter 14):

>>> list(range(5)), list(range(2, 5)), list(range(0, 10, 2))
([0, 1, 2, 3, 4], [2, 3, 4], [0, 2, 4, 6, 8])

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With one argument, range generates a list of integers from zero up to but not including the argument’s value. If you pass in two arguments, the first is taken as the lower bound. An optional third argument can give a step; if it is used, Python adds the step to each successive integer in the result (the step defaults to 1). Ranges can also be nonpositive and nonascending, if you want them to be:

>>> list(range(−5, 5))
[−5, −4, −3, −2, −1, 0, 1, 2, 3, 4]

>>> list(range(5, −5, −1))
[5, 4, 3, 2, 1, 0, −1, −2, −3, −4]

Although such range results may be useful all by themselves, they tend to come in most handy within for loops. For one thing, they provide a simple way to repeat an action a specific number of times. To print three lines, for example, use a range to generate the appropriate number of integers; for loops force results from range automatically in 3.0, so we don’t need list here:

>>> for i in range(3):
...     print(i, 'Pythons')
...
0 Pythons
1 Pythons
2 Pythons

range is also commonly used to iterate over a sequence indirectly. The easiest and fastest way to step through a sequence exhaustively is always with a simple for, as Python handles most of the details for you:

>>> X = 'spam'
>>> for item in X: print(item, end=' ')           # Simple iteration
...
s p a m

Internally, the for loop handles the details of the iteration automatically when used this way. If you really need to take over the indexing logic explicitly, you can do it with a while loop:

>>> i = 0
>>> while i < len(X):                             # while loop iteration
...     print(X[i], end=' ')
...     i += 1
...
s p a m

You can also do manual indexing with a for, though, if you use range to generate a list of indexes to iterate through. It’s a multistep process, but it’s sufficient to generate offsets, rather than the items at those offsets:

>>> X
'spam'
>>> len(X)                                        # Length of string
4
>>> list(range(len(X)))                           # All legal offsets into X
[0, 1, 2, 3]
>>>
>>> for i in range(len(X)): print(X[i], end=' ')  # Manual for indexing
...
s p a m

Note that because this example is stepping over a list of offsets into X, not the actual items of X, we need to index back into X within the loop to fetch each item.