Floyd's triangle in Python

Floyd's triangle fills a right-angled triangle with the numbers 1, 2, 3, ... in order. Row 1 has one number, row 2 has two, and so on.

The program

rows = int(input("Number of rows: "))

number = 1
for i in range(1, rows + 1):
    row = []
    for _ in range(i):
        row.append(str(number))
        number += 1
    print(" ".join(row))

Typed in this example: 5. Press Run to type your own.

Number of rows: 5
1
2 3
4 5 6
7 8 9 10
11 12 13 14 15

How it works

• number is created once, before the loops, and is never reset. Each new row continues from where the last row stopped.
• The inner loop runs i times, so row i gets i numbers. _ is the usual name for a loop variable that the code does not use.
• The numbers are collected as strings and joined with spaces, so no line ends with a space.
• The last number is 1 + 2 + ... + rows. For 5 rows that is 15, and the formula rows * (rows + 1) // 2 gives the same.

Lining up the columns

When the numbers have different lengths, the columns do not line up. Pad every number to the width of the largest one:

rows = 5

last = rows * (rows + 1) // 2
width = len(str(last))

number = 1
for i in range(1, rows + 1):
    row = []
    for _ in range(i):
        row.append(f"{number:>{width}}")
        number += 1
    print(" ".join(row))

 1
 2  3
 4  5  6
 7  8  9 10
11 12 13 14 15

• The largest number is 15, so width is 2.
• f"{number:>{width}}" right-aligns each number in width characters. The inner {width} puts the variable's value into the format, so it becomes >2.
• The padding goes in front of each number, so the lines still have no spaces at the end.