Create range without certain numbers

Question

I want to create a range x from 0 ... n, without any of the numbers in the list y. How can I do this?

For example:

n = 10
y = [3, 7, 8]
x = # Do Something

Should give the output:

x = [0, 1, 2, 4, 5, 6, 9]

One naive way would be to concatenate several ranges, each spanning a set of numbers which have been intersected by the numbers in y. However, I'm not sure of what the simplest syntax to do this is in Python.

Solution 1

You can use a list comprehension to filter the range from 0 to n: range(n) generates a list (or, in Python 3, a generator object) from 0 to n - 1 (including both ends):

x = [i for i in range(n) if i not in y]

This filters out all numbers in y from the range.

You can also turn it into a generator (which you could only iterate over once but which would be faster for (very) large n) by replacing [ with ( and ] with ). Further, in Python 2, you can use xrange instead of range to avoid loading the entire range into memory at once. Also, especially if y is a large list, you can turn it into a set first to use O(1) membership checks instead of O(n) on list or tuple objects. Such a version might look like

s = set(y)
x = (i for i in range(n) if i not in s)

Solution 2

hlt's answer is ideal, but I'll quickly suggest another way using set operations.

n = 10
y = [3, 7, 8]
x = set(range(n)) - set(y)

x will be a set object. If you definitely need x to be a list, you can just write x = list(x).

Note that the ordering of a set in Python is not guaranteed to be anything in particular. If order is needed, remember to sort.