Skip over a value in the range function in python

Question

What is the pythonic way of looping through a range of numbers and skipping over one value? For example, the range is from 0 to 100 and I would like to skip 50.

Edit: Here's the code that I'm using

for i in range(0, len(list)):
    x= listRow(list, i)
    for j in range (#0 to len(list) not including x#)
        ...

Solution 1

You can use any of these:

# Create a range that does not contain 50
for i in [x for x in xrange(100) if x != 50]:
    print i

# Create 2 ranges [0,49] and [51, 100] (Python 2)
for i in range(50) + range(51, 100):
    print i

# Create a iterator and skip 50
xr = iter(xrange(100))
for i in xr:
    print i
    if i == 49:
        next(xr)

# Simply continue in the loop if the number is 50
for i in range(100):
    if i == 50:
        continue
    print i

Solution 2

In addition to the Python 2 approach here are the equivalents for Python 3:

# Create a range that does not contain 50
for i in [x for x in range(100) if x != 50]:
    print(i)

# Create 2 ranges [0,49] and [51, 100]
from itertools import chain
concatenated = chain(range(50), range(51, 100))
for i in concatenated:
    print(i)

# Create a iterator and skip 50
xr = iter(range(100))
for i in xr:
    print(i)
    if i == 49:
        next(xr)

# Simply continue in the loop if the number is 50
for i in range(100):
    if i == 50:
        continue
    print(i)

Ranges are lists in Python 2 and iterators in Python 3.

Solution 3

It is time inefficient to compare each number, needlessly leading to a linear complexity. Having said that, this approach avoids any inequality checks:

import itertools

m, n = 5, 10
for i in itertools.chain(range(m), range(m + 1, n)):
    print(i)  # skips m = 5

As an aside, you woudn't want to use (*range(m), *range(m + 1, n)) even though it works because it will expand the iterables into a tuple and this is memory inefficient.

---

Credit: comment by njzk2, answer by Locke

Solution 4

for i in range(0, 101):
if i != 50:
    do sth
else:
    pass

View more solutions

Comments

[aestrivex]

The third suggestion throws TypeError because you did not explicitly create an iterator

[Daniel]

(1) creates two lists, (2) concatenates two lists, (3) does not work, xrange is not a iterator. (4) should use xrange to avoid creating a list, and is the so far best solution.

[aestrivex]

How does (1) create two lists? xrange(100) is not a list. And you can avoid creating the second list by returning a generator instead: for i in (x for x in xrange(100) if x is not 50)

[David]

#2 is perfect. Thanks

[njzk2]

@Daniel: 1/ creates one list only because of xrange. 2/ I know, not the most efficient, but clear. 3/ typo, thanks for pointing out 4/ probably, yes. (but for a 100 elements it is fine)

[loxosceles]

#2 with Python 3.x: ...list(range(50)) + list(range(51, 100)):

[njzk2]

@loxosceles or using itertools.chain(range(50), range(51, 100)) instead, to avoid creating lists (although my initial solution is indeed creating 2 lists)

[Asclepius]

#2 doesn't work in Python 3. The rest are all quite inefficient.

[njzk2]

@Acumenus this question is tagged python, not python3. If you have more efficient solutions, please share them so everyone can benefit :)