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pascal triangle using lists

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Join Date: Feb 2009

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pascal triangle using lists

Feb 12th, 2009

I have to write a program which generates a pascal's triangle.
The user inputs the height of the triangle and it supposed to generate one.

This is what I have so far and I don't know how you would keep
adding rows after the second one. I'm so lost

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
numStr = raw_input("Please input the height of Pascal's Triangle: ")
row = int(numStr)
 
tri = []
 
row1 = [1]
row2 = [1, 1]
 
tri.append(row1)
tri.append(row2)
 
while len(tri) < row:
    print tri
    break
numStr = raw_input("Please input the height of Pascal's Triangle: ")
row = int(numStr)

tri = []

row1 = [1]
row2 = [1, 1]

tri.append(row1)
tri.append(row2)

while len(tri) < row:
    print tri
    break

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Re: pascal triangle using lists

Feb 12th, 2009

updated. Not sure what I'm doing, though.

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
numStr = raw_input("Please input the height of Pascal's Triangle: ")
height = int(numStr)
 
tri = []
 
row1 = [1]
row2 = [1, 1]
tri.append(row1)
tri.append(row2)
 
while len(tri) < height:
    newRow = [1]
    oldRow = tri[-1]
    start = 0
    while start < (3-2):
        newRow.append(oldRow[start] + oldRow[1])
        start += 1
    newRow.append(1)
 
 
print tri
numStr = raw_input("Please input the height of Pascal's Triangle: ")
height = int(numStr)

tri = []

row1 = [1]
row2 = [1, 1]
tri.append(row1)
tri.append(row2)

while len(tri) < height:
    newRow = [1]
    oldRow = tri[-1]
    start = 0
    while start < (3-2):
        newRow.append(oldRow[start] + oldRow[1])
        start += 1
    newRow.append(1)
    

print tri

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Re: pascal triangle using lists

Feb 12th, 2009

You should increment height, otherwise it's an infinite loop. In general you want to use a for() or while() loop and append to a list on each loop. Add print statements to print what you don't get (especially 'height' and 'start').

Last edited by woooee; Feb 12th, 2009 at 11:43 pm.

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Re: pascal triangle using lists

Feb 13th, 2009

I can see two way to solve the problem
1) use two dimentional list.
2) use two one dimentional list to store current row and previous row.
(that is what you are doing.)

in your case you use only previous row to calculate current row. so in start, your previous row will be row 1 which stores only 1
and you calculate current row by using currentrow[i] = previousrow [i] + previous row[i-1] in a loop. (first element and last element in list are special cases.) after the loop you add currentrow to triangle list and make copy currentrow to previousrow and calculate next row.
I hope that would help.

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