Python recursive function for Pascal's Triangle

Question

I'm trying to do a recursive function that generates a pascal's triangle up till the nth row, n being the user input. This is my code so far:

def printPascal(l,n):
    while n != 1:
        temp = [None]*(len(l)+1)
        temp[0] = 1
        temp[len(l)] = 1
        for i in range(1,len(temp)-1):
            temp[i] = l[i] + l[i-1]
            l = temp
            print(temp)
        n = n-1
        printPascal(l,n)


n = int(input("Enter a value for n:"))
l = [1,1]
printPascal(l,n)

And this is the error it gives me:

Traceback (most recent call last):
  File "C:\Users\User\Desktop\test.py", line 16, in <module>
    printPascal(l,n)
  File "C:\Users\User\Desktop\test.py", line 11, in printPascal
    printPascal(l,n)
  File "C:\Users\User\Desktop\test.py", line 7, in printPascal
    temp[i] = l[i] + l[i-1]
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

The thing is i kinda understand the issue and have tried tracing it to no avail. I know that somehow in the temp[i] = l[i] + l[i-1] code either the l[i] or l[i -1] is a "None" and i don't know why.

Thanks for your time and help in this little predicament of mine.

Daniel Martin · Accepted Answer · 2016-05-02 11:36:56Z

1

You have too much of your code inside the "for" loop. You replace the value of l with the value of temp before you've finished filling in all the values in temp.

The for loop should only contain the first statement.

answered May 2, 2016 at 11:36

Daniel Martin

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1 Comment

M.Raaj Over a year ago

Thanks alot, that fixed up my problem. I didn't realize that the loop included the other lines as well.

Keiwan · Accepted Answer · 2016-05-02 11:38:42Z

1

There was a little indentation error in your code.

def printPascal(l,n):
    while n != 1:
        temp = [0]*(len(l)+1)
        temp[0] = 1
        temp[len(l)] = 1
        for i in range(1,len(temp)-1):
            temp[i] = l[i] + l[i-1]
        l = temp
        print(temp)
        n = n-1
        printPascal(l,n)

edited May 2, 2016 at 11:38

answered May 2, 2016 at 11:33

Keiwan

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2 Comments

hoyland Over a year ago

It's the indentation error that's the culprit, if I'm reading properly. Initializing with 0 just hides the problem.

Keiwan Over a year ago

Yes you're right about that. But that was what enabled me to actually realize the error :)

Byte Commander · Accepted Answer · 2016-05-02 12:01:21Z

Instead of fixing your code, I'll show you an optimized approach:

def pascal_triangle(n, triangle=[[1]]):
    if n > len(triangle):
        last_row = triangle[-1]
        next_row = [a+b for (a, b) in zip([0] + last_row, last_row + [0])]
        return pascal_triangle(n, triangle + [next_row])
    return triangle

How you can run it:

n = int(input("Pascal's triangle size: "))
print(*pascal_triangle(n), sep="\n")

Example output for input 9:

[1]
[1, 1]
[1, 2, 1]
[1, 3, 3, 1]
[1, 4, 6, 4, 1]
[1, 5, 10, 10, 5, 1]
[1, 6, 15, 20, 15, 6, 1]
[1, 7, 21, 35, 35, 21, 7, 1]
[1, 8, 28, 56, 70, 56, 28, 8, 1]

Or using a prettier printing command:

print(*[" ".join(map(str, line)).center(40) for line in pascal_triangle(9)], sep="\n")

It would look like this:

                    1                         
                   1 1                        
                  1 2 1                       
                 1 3 3 1                      
                1 4 6 4 1                     
              1 5 10 10 5 1                   
             1 6 15 20 15 6 1                 
           1 7 21 35 35 21 7 1                
          1 8 28 56 70 56 28 8 1

See this code running on ideone.com

Sadikov · Accepted Answer · 2018-08-03 15:59:14Z

0

You get this error because you not calculate and you not return sufficient information to the function.

try to debug or to follow your code..

You will fined the problem easily :)

answered Aug 3, 2018 at 15:59

Sadikov

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