Python. Pascal's triangle

Question

I am a beginner in Python. Now I am trying to create an algorithm that will build a Pascal's triangle.

My first step is to create a basic iteration that will use a list called current level and create the next level list based on it as follows:

1. Add 0 to the first number to create the left number and insert it into a list.
2. Create the inner row of numbers and insert them into a list
3. Add 0 to the last number to create the right number and insert it into a list.

My code includes only one iteration, but iteration is not a problem now. I do not understand how to address the elements of the list by their index correctly.

Can somebody give me a hint?

Thanks!

currentlevel = [1]

nextlevel = []
leftnumber = 0 + currentlevel[0]
nextlevel.append(leftnumber)

for item[index] in currentlevel:
    if index > 1 and index < len(currentlevel)-1:
        item[index] = item[index] + item[index+1]
        nextlevel.append(item[index])
    else:
        rightnumber = currentlevel[-1] + 0
        nextlevel.append(rightnumber)

currentlevel = nextlevel
print(currentlevel)

Answer 1

1. This does not work:

   for item[index] in currentlevel:
   

   Use this:

   for index, item in enumerate(currentlevel):
   

   and then wherever you are using item[index] use either item or currentlevel[index]. Except...

2. Except here:

   item[index] = item[index] + item[index+1]
   nextlevel.append(item[index]) 
   

   Here it would be best if you used another variable:

   next_item = currentlevel[index] + currentlevel[index+1]
   nextlevel.append(next_item) 
   

   Although item would work, it is slightly confusing; but if you use currentlevel[index] where next_item is, you'd be destroying the previous row. Not a problem if you want to print-and-forget, but it's nice to have a choice and be flexible.

3. This will skip two elements unnecesarily:

   if index > 1 and index < len(currentlevel)-1:
   

   Remove the first clause:

   if index < len(currentlevel)-1:
   

That's all I see for now, can't test.

EDIT: Not an error as such, but 0 + whatever is same as whatever.

Answer 2

Unless I'm missing some Python novelty, the line for item[index] in currentlevel: is a syntax error (edit: it's not a syntax error but a name error; see Tutleman's comment).

Try the following alternative to get both the list element and its index:

for index, item in enumerate(currentlevel):

Or use the following one to get just the indexes and access the elements through them:

for index in range(len(currentlevel)):
    item = currentlevel[index]

Check this answer for more information.

Answer 3

I don't normally post full answers but...

maxlevel = 5
currentlevel = [1]

for levels in range(maxlevel):
    nextlevel = [1]
    for index,item in enumerate(currentlevel[0:-1]):
        if (len(currentlevel) == 1):
            break

        nextlevel.append(item + currentlevel[index + 1])

    nextlevel.append(1)

    print(nextlevel)

    currentlevel = nextlevel