Confused: appending to a list

[DataSmash]

I'm confused. Why is it that when I say "while len(list) < 5:", I get
5 items in my list.
If I say "while len(list) < 6:", I get 6 items in the list and so on.
I would think if I said "less than 5", I would get 4 items.
Can anyone explain this?
Thanks.
R.D.


# Start an empty list
list = []
while len(list) < 5:
# Get a random number between 1 & 100
num = random.randint(1,100)
# Make sure there are no duplicates
if num not in list:
# Append each number to the list
list.append(num)
print list

 

[Diez B. Roggisch]

I'm confused. Why is it that when I say "while len(list) < 5:", I get
5 items in my list.
If I say "while len(list) < 6:", I get 6 items in the list and so on.
I would think if I said "less than 5", I would get 4 items.
Can anyone explain this?

Yes - you loop until the condition is _not_ fullfilled anymore. Which means
that if the list has a length of five, the len(l) < 6 is true, and
appending makes it len(l) == 6 - which then will fail your condition.

So - you need to loop one time less, by doing

while len(l) < 6 - 1:
...

diez

 

[Felipe Almeida Lessa]

Em Qui, 2006-03-23 às 08:31 -0800, DataSmash escreveu:

I'm confused. Why is it that when I say "while len(list) < 5:", I get
5 items in my list.

"while len(list) < 5:" implies that the loop will stop only when
len(list) >= 5.

HTH,

 

[Rene Pijlman]

DataSmash:

I'm confused. Why is it that when I say "while len(list) < 5:", I get
5 items in my list.

Because the last time when len(list) was < 5, the block of code following
the while executed and did something to the list to give it a length >= 5
(otherwise the block of code would be executed again and it wouldn't have
been the last time).

list.append(num)

There you have it.

 

[skip]

R.D.> I'm confused. Why is it that when I say "while len(list) < 5:", I
R.D.> get 5 items in my list. If I say "while len(list) < 6:", I get 6
R.D.> items in the list and so on. I would think if I said "less than
R.D.> 5", I would get 4 items. Can anyone explain this? Thanks. R.D.

The loop exits when the condition becomes false. That happens when
len(list) >= 5 (or 6).

Skip

 

[adam.bachman]

You're wanting it to stop when the len(list) == 4, right? The easiest
way to change the logic would be to say

while len(list) != 4:

but that could get you into trouble later on. The problem with the
len(list) < 5 expression is that the loop will run "one more time" as
long as len(list) == 4 adding another item to the list giving you one
more than you wanted. If you wanted, you could put your len() check
inside the loop:

# Start an empty list
list = []
while 1:
# Get a random number between 1 & 100
num = random.randint(1,100)
# Make sure there are no duplicates
if num not in list:
# Append each number to the list
list.append(num)
if len(list) == 4:
# Break if we've reached desired length.
break
print list

I hope this gives you some ideas.

 

[Fredrik Lundh]

I'm confused. Why is it that when I say "while len(list) < 5:", I get
5 items in my list.
If I say "while len(list) < 6:", I get 6 items in the list and so on.
I would think if I said "less than 5", I would get 4 items.

except that you're saying "as long as there are less than 5 items
in the list, add another one"

</F>

 

[Dennis Lee Bieber]

# Start an empty list
list = []
while len(list) < 5:

The empty list has a length of 0...

0
1
2
3
4 4 < 5, but is the fifth element of the list

# Get a random number between 1 & 100
num = random.randint(1,100)
# Make sure there are no duplicates

You want no duplicates? Try this (untested):

lst = [] #don't use "list", that's a data type
for x in xrange(4): #use however many elements you really want
while True:
num = random.randint(1, 100)
if num not in lst:
lst.append(num)
break #added element, go back to outer loop
--

 

[DataSmash]

Thanks for explaining and all the additional ideas!
R.D.