[Tutor] Beginner - explaining 'Flip a coin' bug

[Alan Gauld]

On 12/02/14 15:25, Marc Eymard wrote:
> However, I still don't understand the bug since, in my understanding,
> both files are incrementing variable *count_flips* each time until the
> loop becomes false.

 > count_heads = 0
 > count_tails = 0
 > count_flips = 0
 >
 > while count_flips != 100:

This is usually a bad idea since if count_flips gets
incremented by 2 it could go over 100 and this test
will still be true and it will loop forever...
(see below).

Its better to have a test like:

while count_flips <100:

 >    coin_side = random.randint(1,2)
 >
 >    if coin_side == 1:
 >        count_heads += 1
 >        count_flips += 1
 >    else: count_tails += 1
 >
 >    count_flips += 1

Because this is outside the else block it gets incremented
twice every time a 1 gets 'thrown'. (see above)

That's why your solution of incrementing only once before
the if/else tests is better. Its a more accurate implementation
of the logic and avoids the double increment problem with
the while loop.

HTH
-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.flickr.com/photos/alangauldphotos