# looping through possible combinations of McNuggets packs of 6, 9 and 20

News123 news1234 at free.fr
Thu Aug 12 22:28:09 CEST 2010

```Hi Baba,

for testing your function you could do:

Below your uncorrected code and a test for it

for a in range (1,n_nuggets):
for b in range (1,n_nuggets):
for c in range (1,n_nuggets):
print "trying for %2d: %2d %2d %2d" % (n,a,b,c)
if 6*a+9*b+20*c==n_nuggets:
return True
else:
return False

# show the results for the first 50 numbers.
for n in xrange(50):
result = can_buy(n) # get the result (see Brian's comment)
print("result of can_buy(%2d) is %s" % (n,result))

with the print statement in can_buy() you should immediately see some
issues.
as soon as can_buy seems to work, you could comment the print statement.

I would return [a,b,c] if you got a result
and [] if you got no result.

So you don't only seem whether you can buy n nuggets, but you can also
see how (AND even verify the solution)

the print statement should show you what Peter said.
You just make one test and return immediately in the else statement

Instead you should continue until you found a solution or until you
tried all possibilities.

After having fixed the fact, that you don't try all options, you'll see
that your first success would be with 6+9+12 = 27 nuggets es you never
try with 0 boxes of a kind. (as Peter mentioned)

Finally as Roald says.

if you can reduce the upper limit of your search ranges.
if you have for example 13 nuggets you do not have to try 3 boxes of 6
as this is already 18, so trying only 0,1 or two boxes is enough

You will now also see, what
On 08/12/2010 09:42 PM, Roald de Vries wrote:
>
> On Aug 12, 2010, at 9:02 PM, Peter Otten wrote:
>
>> Baba wrote:
>>
>>
>>> Thank You for helping me out. Indeed i am not looking for the code but
>>> rather for hints that direct my reasoning as well as hints as to how
>>> to write basic programs like this.
>>>
>>> You have broken down the approach into 2 parts. I have tried to solve
>>> part 1 but i'm not quite there yet. Here's my code:
>>>
>>>    for a in range (1,n_nuggets):
>>>        for b in range (1,n_nuggets):
>>>            for c in range (1,n_nuggets):
>>>                if 6*a+9*b+20*c==n_nuggets:
>>>                    #print a,b,c,'n_nuggets=',n_nuggets
>>>                    return True
>>>                else:
>>>                    return False
>>>
>>>
>>>
>>> as you can see i am trying to loop through all combinations of values
>>> bewtween 1 and n_nuggets and when the equation resolves it should
>>> return True, else it should return False.
>>>
>>> I was hoping that when i then call my function and ask it to test a
>>> value nothing happens. What is wrong? My syntax? My semantic? Both?
>>
>> First, the function gives up too early; it should only return False
>> when all
>> combinations of a, b, c (technically: the product of the ranges) have
>> been
>> tried.
>>
>> Second, can_buy(0) should return True, but the solution 0*6 + 0*9 +
>> 0*20 is
>> never tried; fix your ranges accordingly.
>
> Moreover: a, b and c can range over n_nuggets/6, n_nuggets/9 and
> n_nuggets/20 respectively. This will work, but does too much work.
>
> Cheers, Roald
>

```