Python range() Function

range(n)
reversed()
range(start, stop)
range(start, stop, step

The python range(n) function creates a collection of numbers on the fly, like 0, 1, 2, 3 .. n-1. The numbers extend up to, but not including the n, UBNI. The numbers produced by range() are perfect for indexing into collections likes strings and lists. The range() function can be called in a few different ways shown below (the range feature is implemented as a class, but it's simplest to think of it as a function).

range(n) - 1 Parameter Form

The most common form is range(n), given integer n returns a numeric series starting with 0 and extending up to but not including n, e.g. range(6) returns 0, 1, 2, 3, 4, 5. With Python's zero-based indexing, the contents of a string length 6, are at index numbers 0..5, so range(6) will produce exactly the index numbers for a string length 6, like this:

>>> s = 'Python'
>>> len(s)
6
>>> for i in range(len(s)):
...   print(i, s[i])
... 
0 P
1 y
2 t
3 h
4 o
5 n

The input to the range() must be int. The output of range() is an "iterable" producing the series of int values. The iterable is similar to a list, but more lightweight. The iterable will work in a for-loop, or in common functions like sum() or min(). You can pass the iterable to list() to make a heavyweight list out of it if necessary. Below are more examples calling range(). The examples below use list() cosmetically, so the numbers will print out. This use of list() is only for printing, not needed to use range() in a loop.

>>> list(range(5))
[0, 1, 2, 3, 4]
>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> list(range(2))
[0, 1]

What is range(0)? Well range(n) returns n numbers, so this case returns no numbers at all - like the empty list.

>>> list(range(2))
[0, 1]
>>> list(range(1))
[0]
>>> list(range(0))   # n <= 0, no numbers
[]
>>> list(range(-42))
[]

reversed() Variation

Probably the second most common problem is to go through the standard index numbers, but in reverse order. The reversed() function takes in a linear collection and returns a reversed form of it. This works nicely with range() to go over the regular numbers in reverse order:

>>> s = 'Python'
>>> len(s)
6
>>> for i in reversed(range(len(s))):
...   print(i, s[i])
... 
5 n
4 o
3 h
2 t
1 y
0 P

range(start, stop) - 2 Parameter Form

Range with 2 parameters specifies a start number other than 0, but is otherwise like the 1 parameter form above, going up to but not including the stop number.

>>> list(range(5, 10))
[5, 6, 7, 8, 9]
>>> list(range(5, 7))
[5, 6]
>>> list(range(5, 6))
[5]
>>> list(range(5, 5))  # start >= stop, no numbers
[]
>>> list(range(0, 5))
[0, 1, 2, 3, 4]
>>> list(range(0, 0))
[]
>>> list(range(0, -1))
[]

Mnemonic: the "stop" number is strong, so as soon as the numbers hit or exceed the stop the range is done (even if the start number initiates things in that position).

range(start, stop, step) - 3 Parameter Form

The 3 parameter form begins with start number, up to but no including the stop number as usual. The difference is the "step" amount between numbers is now custom. Once the number is equal or goes beyond the stop, the range ends. As before, the stop number itself is always omitted.

>>> list(range(0, 10, 2))
[0, 2, 4, 6, 8]
>>> list(range(0, 10, 6))
[0, 6]
>>> list(range(200, 800, 100))
[200, 300, 400, 500, 600, 700]

If the step is negative, the range decreases from start down to stop. As always, numbers reaching or beyond the stop are omitted, but now step is decreasing.

>>> list(range(10, 5, -1))
[10, 9, 8, 7, 6]
>>> list(range(10, 5, -2))
[10, 8, 6]
>>> list(range(6, 5, -2))
[6]
>>> list(range(5, 5, -2))  # equal to stop is omitted
[]
>>> list(range(4, 5, -2))  # beyond the stop is omitted
[]

If you want to loop over the index numbers of a string or list backwards, it's easier to use reversed() above, vs. trying to do it manually with a negative increment.