Pascal's triangle in python?

Question

I seem to be having a problem with creating a pascal's triangle in python, and I'm really realyl frustrated not finding the problem. Please help. Thanks.

Heres the code:

inpt = input("Enter levels: ")   #number of levels in a triangle
list1 = []
list2 = [1]

for a in range(inpt):
    list1.append(1)
    for x in range(a+1):
        if (x == 0 or x == a):
            list1[x]
        elif (x > 0 or x < a):
            list1[x] = list2[x] + list2[x-1]
    print list1
    list2 = list1

and it prints something like this:

[1]
[1, 1]
[1, 2, 1]
[1, 3, 4, 1]
[1, 4, 8, 9, 1]

What do you think is the problem? It's doing exactly what you ask it to - what should it be doing instead? Note that `list2 = list1` means both names reference *the same object*, which probably isn't what you want. — jonrsharpe, Sep 02 '14 at 10:49

score 4 · Accepted Answer · edited May 23 '17 at 11:46

4

With list2 = list1 you are saying that the two names list1 and list2 are referencing the same list.
To really copy the list, you can use list2 = list1[:] (or a module like copy. See also this question ("python list by value not by reference"))

for a in range(inpt):
    list1.append(1)
    for x in range(a+1):
        if (x == 0 or x == a):
            list1[x]
        elif (x > 0 or x < a):
            list1[x] = list2[x] + list2[x-1]
    print list1
    list2 = list1[:]

edited May 23 '17 at 11:46

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answered Sep 02 '14 at 10:48

fredtantini

15,966
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Oh my gosh you are my savior. Thanks so much everything already works like a charm ^^ – edrichhans Sep 02 '14 at 10:53
don't need parens in your if/elif statements and what does `list1[x]` do? – Padraic Cunningham Sep 02 '14 at 11:48
@JoeSmart, `list1[x]` is essentially doing nothing – Padraic Cunningham Sep 02 '14 at 13:33
@PadraicCunningham apologies, being awake for 24 hours doesn't help my awareness, and I totally missed the "standalone" `list1[x]`, I thought you meant the use of `list1[x]` after the `elif` statement. Sorry for coming across cocky and patronizing. – Joe Smart Sep 02 '14 at 13:43

Padraic Cunningham · Answer 2 · 2014-09-02T13:54:58.407

You can simplify your code.

for x in range(a+1) starts at 0 and goes all the way to a because you are using a+1, you use if checks to avoid 0 and a so it would make more sense to start your range from 1 and loop over the range(a) thus removing the need for any if/elif checks:

for a in xrange(inpt):
    list1.append(1)
    for x in xrange(1, a):
        list1[x] = list2[x] + list2[x-1]
    print list1
    list2 = list1[:]

score -1 · Answer 3 · edited May 23 '17 at 12:00

-1

I believe you need to deep copy or clone your lists instead of list2=list1 as this will make both variables point to the same list. Refer to How to clone or copy a list?

edited May 23 '17 at 12:00

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answered Sep 02 '14 at 10:49

Ranald Lam

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Pascal's triangle in python?

3 Answers3