Trevor Hunsaker

Salt Lake area Web developer

Pascal's Triangle in Python (with semi-decent printing)

Today, I ran across a Hacker News thread where someone mentioned that they often ask job applicants to whiteboard a function that would return a row of Pascal's Triangle. I couldn't resist, so and came up with this:

print "\n"
print "Printing Pascal's triangle."
num_rows = input("How many rows do you want: ")

def triangle(num_rows):
	width = 2*num_rows - 1
	prev_row = []
	triangle = []

	for row_num in range(num_rows):
		row_width = row_num + 1 # the number of items that will appear in this row
		if row_num == 0:
			row = [1]
			row = []
			for i in range(row_width):
				if i == 0 or i == row_width-1:
					# the '1's on either end of each row
					# Calculate the sum of the two numbers above this position
					row.append(prev_row[i-1] + prev_row[i])
		prev_row = row
	return triangle

# Actually generate the triangle		
triangle = triangle(num_rows)

# Now print it out nicely
# First, get the width of the last row in the triangle (printed as a string)
width = len(" ".join(["%s" % num for num in triangle[-1]]))

for line in triangle:
	print" ".join(["%s" % num for num in line]), width)

Obviously, I was less interested in returning the row as I was in building the triangle itself and printing it. Returning the actual row is left as an exercise to the reader.