input letter 0 should not be considered as None in method WordMorphism.is_growing

[seblabbe]

Wolfgang Steiner wrote to me:

J'ai un problème très étrange avec SageMath :
L'évaluation de WordMorphism({0:[0,1],1:[1]}).fixed_point(0) ne termine
pas alors que WordMorphism({1:[1,0],0:[0]}).fixed_point(1) ou
WordMorphism({0:[0,1],1:[1,1]}).fixed_point(0) marche sans problème.
Le problème apparaît donc uniquement quand la lettre du point fixe
s'appelle 0 et l'autre lettre ne croît pas.

Amitiés,
Wolfgang

I confirm this is okay :

sage: m = WordMorphism({1:[1,0],0:[0]})
sage: m.fixed_point(1)
word: 1000000000000000000000000000000000000000...

Exchanging 0 and 1 does this:

sage: m2 = WordMorphism({0:[0,1],1:[1]})
sage: m2.fixed_point(0)           # this hangs !!!

The problem is in the method is_growing :

sage: m.is_growing(0)
False
sage: m.is_growing(1)
True
sage: m2.is_growing(0)         # this should return True
False
sage: m2.is_growing(1)
False

The issue is in this line:

sage/src/sage/combinat/words/morphism.py

Line 3053 in 0fc8ed5

if not letter:

which is supposed to test whether letter is None. But this evaluates to True when letter is 0...

It should be replaced by if letter is None.

[AyushVerma54]

i have did this change to the code i m looking forward if u could assign me this task

[seblabbe]

Thank you for proposing to fix the issue.

Please also add new doctests to illustrate the bug is fixed.

[AyushVerma54]

ok

[seblabbe]

Thank you for your commits.

Unfortunately, the new doctests are wrong. They do not check that the current bug is fixed.

Please remove them. Please instead use the morphism m2 = WordMorphism({0:[0,1],1:[1]}) defined above.

[AyushVerma54]

ok