Re: Python

Pourtant si je regarde ce que vaut customer + seller à l'entrée de la fonction pays et à la sortie, cette quantité est invariante.

Pour en avoir le cœur net j'ai écrit une alternative à pays qui met cela bien en évidence:

def pays(customer, seller, base):
 amount = 0
 # no advantage for the poorest as no poorest
 if seller == customer:
 amount = base * RATE
 # poorest receive the money so add DRATE in the transfer
 elif seller > customer:
 amount = base * (RATE + DRATE)
 # poorest pay the money so deduct DRATE in the transfer
 else:
 amount = base * (RATE - DRATE)
 return (seller + amount, customer - amount)

et ajouté des print dans le programme pour tracer:

for _ in range(0, NB_LOOP):
 # select two random people
 a = randint(0, NB_PEOPLE - 1)
 b = randint(0, NB_PEOPLE - 1)

base = min(people[a], people[b])
``` invariant1 = people[a] + people[b]
print(f"1st people[a], people[b] {people[a]}, {people[b]}")
u, v = pays2(people[a], people[b], base)
print(f"u,v ={u}, {v}")
people[a], people[b] = u, v
print(f"2nd people[a], people[b] {people[a]}, {people[b]}")
invariant2 = people[a] + people[b]
assert(invariant1 == invariant2)
~~~

et ça me fait un truc du genre:

1st people[a], people[b] 2000, 2000
u,v =2400.0, 1600.0
2nd people[a], people[b] 2400.0, 1600.0
1st people[a], people[b] 1600.0, 2000
u,v =2400.0, 1200.0
2nd people[a], people[b] 2400.0, 1200.0
1st people[a], people[b] 2000, 2000
u,v =2400.0, 1600.0
2nd people[a], people[b] 2400.0, 1600.0
1st people[a], people[b] 2400.0, 1200.0
u,v =1380.0, 2220.0
2nd people[a], people[b] 1380.0, 2220.0
1st people[a], people[b] 2000, 2000
u,v =2400.0, 1600.0
2nd people[a], people[b] 2400.0, 1600.0
1st people[a], people[b] 2400.0, 2400.0
u,v =2880.0, 1920.0
2nd people[a], people[b] 2880.0, 1920.0
1st people[a], people[b] 2000, 2880.0
u,v =3380.0, 1500.0
2nd people[a], people[b] 3380.0, 1500.0
1st people[a], people[b] 1920.0, 2400.0
u,v =2880.0, 1440.0
2nd people[a], people[b] 2880.0, 1440.0
1st people[a], people[b] 1500.0, 1600.0
u,v =1975.0, 1125.0
2nd people[a], people[b] 1975.0, 1125.0
1st people[a], people[b] 1600.0, 3380.0
u,v =3780.0, 1200.0
2nd people[a], people[b] 3780.0, 1200.0
1st people[a], people[b] 2000, 1440.0
u,v =1656.0, 1784.0
2nd people[a], people[b] 1656.0, 1784.0
1st people[a], people[b] 2000, 1200.0
u,v =1380.0, 1820.0
2nd people[a], people[b] 1380.0, 1820.0
1st people[a], people[b] 1380.0, 1380.0
u,v =1656.0, 1104.0
2nd people[a], people[b] 1104.0, 1104.0
Traceback (most recent call last):
 File "/Users/michael/Desktop/example.py", line 70, in <module>
 assert(invariant1 == invariant2)

On voit que l'affectation de deux variables en même temps genre marche de façon bizarre dans la dernière trace: même si u et v ont les bonnes valeurs (on a 1656 +たす 1104 =わ=わ 1380 +たす 1380) tout se casse la gueule quand on fait l'affectation à people[a], people[b]. Que se passe-t-il?

PS: Mon commentaire fait foirer markdown de façon aussi magistrale qu'inattendue!