Can you use a for loop to implement a sentinel controlled loop in Python?

Question

I'm not too sure if there might have been another duplicate, but question as per topic.

The purpose of this question is not to find out whether you should or should not use for loop to implement sentinel control.

But rather to see if it can be done and hence have a better understanding of the distinction between the for and the while loop.

Community · Accepted Answer · 2017-05-23 12:28:34Z

1

Using itertools it is possible:

>>> import itertools
>>>
>>> SENTINEL = 0
>>> for i in itertools.count():
....:    if SENTINEL >= 10:
....:        print "Sentinel value encountered! Breaking..."
....:        break
....:    
....:    SENTINEL = SENTINEL + 1
....:    print "Incrementing the sentinel value..."
....: 
Incrementing the sentinel value...
Incrementing the sentinel value...
Incrementing the sentinel value...
Incrementing the sentinel value...
Incrementing the sentinel value...
Incrementing the sentinel value...
Incrementing the sentinel value...
Incrementing the sentinel value...
Incrementing the sentinel value...
Incrementing the sentinel value...
Sentinel value encountered! Breaking...

(Inspired by stack overflow question "Looping from 1 to infinity in Python".)

edited May 23, 2017 at 12:28

CommunityBot

11 silver badge

answered Mar 13, 2015 at 5:23

tbc

1,7793 gold badges21 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Calvintwr Over a year ago

what does itertools.count() do?

tbc Over a year ago

It returns an Iterator, which is "An object representing a stream of data. Repeated calls to the iterator’s next() method return successive items in the stream." from python docs . In this case the stream is infinite, and we break the loop with the sentinel value.

Calvintwr Over a year ago

sounds like a pretty big workaround to make a for loop sentinel-controlled.

tbc Over a year ago

I didn't say it was a good idea, I just showed you one way it is possible :-)

Calvintwr Over a year ago

yup exactly what i needed to see.

Calvintwr · Accepted Answer · 2015-05-07 06:49:34Z

0

Without importing any modules, you can also do a "sentinel" controlled looped with for by making loop to infinity and break with a condition:

infinity = [0]
sentinelValue = 1000
for i in infinity:
    if i == sentinelValue:
        break

    # like a dog chasing the tail, we move the tail...
    infinity.append(i+1)

    print('Looped', i, 'times')

print('Sentinel value reached')

Although this would create a very large infinity list that eats away at the memory.

answered May 7, 2015 at 6:49

Calvintwr

8,8485 gold badges33 silver badges48 bronze badges

Collectives™ on Stack Overflow

Can you use a for loop to implement a sentinel controlled loop in Python?

2 Answers 2

5 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related