My answer is changing with the same code

Question

I am a complete python beginner and I am trying to solve this problem :

A number is called triangular if it is the sum of the first n positive integers for some n For example, 10 is triangular because 10 = 1+2+3+4 and 21 is triangular because 21 = 1+2+3+4+5+6. Write a Python program to find the smallest 6-digit triangular number. Enter it as your answer below.

I have written this program:

n = 0
trinum = 0
while len(str(trinum)) < 6:
    trinum = n*(n+1)/2
    n += 1
print(trinum)

And it only works in the python I have installed on my computer if I say while len(str(trinum)) < 8: but it is supposed to be while len(str(trinum)) < 6:. So I went to http://www.skulpt.org/ and ran my code there and it gave me the right answer with while len(str(trinum)) < 6: like it's supposed to. But it doesn't work with 6 with the python i have installed on my computer. Does anyone have any idea what's going on?

Answer 1

Short Answer

In Python 3, division is always floating point division. So on the first pass you get something like str(trinum) == '0.5'. Which isn't what you want.

You're looking for integer division. The operator for that is //.

Long Answer

The division operator changed in Python 2.x to 3.x. Previously, the type of the result was dependent on the arguments. So 1/2 does integer division, but 1./2 does floating point division.

To clean this up, a new operator was introduced: //. This operator will always do integer division.

So in Python 3.x, this expression (4 * 5)/2 is equal to 10.0. Note that this number is less than 100, but it has 4 characters in it.

If instead, we did (4*5)//2, we would get the integer 10 back. Which would allow your condition to hold true.