Cannot yield the right values with Python

Question

I experience something strange from my point of view

I need to generate alternatively True and False values like this:

False, True, False, False, True, True, False, False, False, True, True, True, False, False, False, False, True, True, True, True, ...

I need to get one False, one True, then two False, two True, then three False, three True, etc

In my mind, using the yield keyword in a function was a good way to achive this:

def generateFalseTrueRangesAlternatively(limit):
    limitForChange = 0
    index = 0
    rv = True
    for i in range(0, limit):
        if index == limitForChange:
            rv = not rv 
            index = 0
            if not rv:
                limitForChange += 1
        index += 1
        yield rv

for i in range(0,4):
    g =  generateFalseTrueRangesAlternatively(20)
    print(next(g))

But I get only False values:

False
False
False
False

As a workaround, I wrote the following code, which generates a list :

def generateFalseTrueRangesAlternativelyList(limit):
    limitForChange = 1
    index = 0
    rv = False
    out = []
    for i in range(0, limit):
        if index == limitForChange:
            rv = not rv 
            index = 0
            if not rv:
                limitForChange += 1
        index += 1
        out.append(rv)
    return out
l = generateFalseTrueRangesAlternativelyInList(6)
for i in range (0,6):
    print(l[i])

Which outputs:

False
True
False
False
True
True

Questions

What is wrong with my generateFalseTrueRangesAlternatively function ?

How could it be modified to behave like the generateFalseTrueRangesAlternativelyInList but without creating a list ?

Is there something I did not understand with yield usage and what ?

Actually, my curiosity needs to be satisfied by answering this fundamental question: why ?

(My Python version: 3.5.4)

@cᴏʟᴅsᴘᴇᴇᴅ it is a pity that you deleted your answer as it seemed to be much more pythonic than my code. Thanks for your suggestion — lauhub
– lauhub, Commented Oct 19, 2017 at 23:25

Uriel · Accepted Answer · 2017-10-19 23:04:48Z

5

You code is fine; the usage of generator is wrong.

Just iterate it:

for i in generateFalseTrueRangesAlternatively(20):
    print(i)

answered Oct 19, 2017 at 23:04

Uriel

16.3k6 gold badges29 silver badges48 bronze badges

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

1 Comment

lauhub Over a year ago

Wow, I can't believe I was so dumb ! LOL Thanks a lot

Lawrence D'Oliveiro · Accepted Answer · 2017-10-19 23:04:01Z

5

In the first snippet, you should only call the generator once, outside the for-loop, then use the next calls to obtain successive yielded values:

g = generateFalseTrueRangesAlternatively(20)
for i in range(4):
    print(next(g))

Also note you can omit the lower bound of the range if it is zero.

answered Oct 19, 2017 at 23:04

Lawrence D'Oliveiro

2,8141 gold badge17 silver badges13 bronze badges

1 Comment

lauhub Over a year ago

That is the correct answer but just one minute slower than the accepted one, thanks !

score 0 · Accepted Answer · 2017-10-20 00:55:17Z

0

I'd suggest a simplified implementation like this because it's much easier to understand and your original generateFalseTrueRangesAlternatively doesn't yield the correct values:

def alternate_trues_and_falses(limit):
    for i in range(1, limit + 1):
        for j in range(i):
            yield True
        for j in range(i):
            yield False

>>> list(alternate_trues_and_falses(2))
[True, False, True, True, False, False]

>>> for value in alternate_trues_and_falses(2):
...     print(value)
...
True
False
True
True
False
False

edited Oct 20, 2017 at 0:55

answered Oct 19, 2017 at 23:36

user8651755

1 Comment

TemporalWolf Over a year ago

I'd explicitly skip i == 0 (which is essentially a no-op) with range(1, limit + 1).

Collectives™ on Stack Overflow

Cannot yield the right values with Python

3 Answers 3

1 Comment

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related