Rounding positive/negative numbers to nearest "whole" number

Question

I'm trying to round numbers derived from cmath's divide function to a "whole" number, the results often being negative due to the nature of the program.

Example code:

strength = input("Input Strength Stat: ")
str_mod = round(strength/2)

However, the result of this is that it, due to an oddity in python, always returns closer to zero rather than further if, say str_mod prior to rounding ends up at something like -1.5 (resulting in -1 rather than -2 as desired)

This, since I'm trying to create an automatic derived stat calc script for a custom Pen and Paper RPG system, is not the desired behavior. What I desire is for the script to return, at -1.5, -2.0 upon rounding. I need to be able to do this while still rounding positive numbers up similarly.

score 4 · Answer 1 · answered Jul 13 '12 at 22:01

4

this is because you are getting a(truncated) int from strength/2

try strength/2.0

this is not an oddity with python but simply how most languages cast types for numbers

5/2 = 2

5/2.0 = 2.5

answered Jul 13 '12 at 22:01

Joran Beasley

110,522
12
160
179

1

or `from __future__ import division` – mgilson Jul 13 '12 at 22:03
1

yeah ... but still its better he understands some bit of numeric casting for any other language he might deal with – Joran Beasley Jul 13 '12 at 22:04
(+1) Nice work figuring out what the actual problem was. Sometimes these problems with `input` are a little difficult because of the type differences between python2 and python3. You nailed this one pretty fast though. – mgilson Jul 13 '12 at 22:17
This was ultimately the best answer, but unfortunately less useful than the one just under it that points out a major security flaw in my program, and how to fix it. Thank you. – user1524705 Jul 14 '12 at 23:57

score 3 · Accepted Answer · answered Jul 13 '12 at 22:08

3

You could do this the safe way:

strength = float(raw_input("Input Strength Stat:"))
str_mod = round(strength/2)

In this case, since strength is guaranteed to be a float, you can safely divide by 2 without worrying about truncation or importing division from __future__. As a bonus, you remove a huge security liability in your game as well.

Using input, a user could do some serious damage to your system:

RPG console> Input Strength Stat: __import__('os').system('rm -r ~')
                                 #^ User input (don't try this!)

answered Jul 13 '12 at 22:08

mgilson

300,191
65
633
696

Thank you very much, sir. I'll have to remember this, as I'm just learning python. – user1524705 Jul 14 '12 at 23:47
wouldnt this only apply if you were to `eval` it? – Joran Beasley Jul 15 '12 at 01:13
1

@JoranBeasley -- The OP is using python 2.x as evidenced by the lack of true division and that the return value of `input` is apparently an `int`. In python 2.x, `input` is equivalent to `eval(raw_input(...))` – mgilson Jul 15 '12 at 15:18
ahhh thats good to know. I never used input and didnt realize that it eval'd the input – Joran Beasley Jul 15 '12 at 18:43

score 0 · Answer 3 · edited May 23 '17 at 11:50

0

You may need to use Decimal to get the same rounding as RPG. With Decimal, you can set the rounding mode to up, down, half up, etc.

edited May 23 '17 at 11:50

Community

1
1

answered Jul 13 '12 at 22:03

dawg

98,345
23
131
206

score 0 · Answer 4 · answered Jul 13 '12 at 22:05

0

Alternatively, you could try:

from __future__ import division

in order to use py3k division. (e.g. result will be always float)

answered Jul 13 '12 at 22:05

jermenkoo

643
5
20

score 0 · Answer 5 · answered Dec 14 '22 at 10:16

If I understand your problem right, you'd like something that will round half away from zero, as:

-0.5 -> -1

0.5 -> 1

These folks have some good answers. In particular, I based my answer below on AGN Gazer's answer:

from math import floor, ceil

strength = float(raw_input("Input Strength Stat:"))
str_mod  = floor((strength/2) + 0.5) if strength  >= 0.0 else ceil((strength/2)  - 0.5)

Thanks AGN Gazer!

Rounding positive/negative numbers to nearest "whole" number

5 Answers5