How do I bound a number between two constants using the modulus operator?

Question

def boundThis(x):
    min = 20
    max= 100
    boundX = int(x)
    if (boundX > max):
        boundX = boundX % max + min-1
        if (boundX > max):
          boundX-=max-1
    else:
        while(boundX < min):
            boundX += max-min
    return boundX

I'm trying to bound x between two numbers, 20 and 100 (exclusive). That said, once it hits 100, it should loop back to 20.

I get how it works with the while loop, (while boundX < min) but am having trouble with the modulus operator and writing the correct expression with that.

Ex. boundThis(201) should give me like 21, and boundThis(100) giving me 20.

possible duplicate of [How to limit a number to be within a specified range? (Python)](http://stackoverflow.com/questions/5996881/how-to-limit-a-number-to-be-within-a-specified-range-python) — Steve Howard, Mar 13 '14 at 02:25
Although actually looking at this closer.... Are you trying to have the number wrap around, rather than clamp? I.E. `min = 10, max = 15` and `number = 9, output = 15`, `number = 16, output = 10`? — aruisdante, Mar 13 '14 at 02:31

metatoaster · Accepted Answer · 2014-03-13T02:56:34.427

Do note that min and max are already builtin functions, so don't name variables with those identifiers as you can't do something like the following (assuming you are not trying to generate a list of numbers within the specified bounds):

>>> def bound(low, high, value):
...     return max(low, min(high, value))
... 
>>> bound(20, 100, 1000)
100
>>> bound(20, 100, 10)
20
>>> bound(20, 100, 60)
60

Edit:

For the wrap around condition, think of this as a math problem. As you know the modulus operator essentially finds the remainder of what is left over from a division operation on a divisor, so we can use that. So the base case is very simple if you have an offset of 0 to n.

>>> 99 % 100
99
>>> 101 % 100
1

But you want it offset from some base value (low), so you have to apply basic mathematics and reduce that problem to the one above. I would suggest you try to figure this out on your own before reading the solution below.

>>> def bound(value, low=20, high=100):
...     diff = high - low
...     return (((value - low) % diff) + low)
...
>>> bound(0)
80
>>> bound(19)
99
>>> bound(20)
20
>>> bound(100)
20
>>> bound(99)
99

Even if you're trying to generate a list it would work fine, you'd just use a list comprehension like `list = [bound(20, 100, value) for value in numbers]` — aruisdante, Mar 13 '14 at 02:28

How do I bound a number between two constants using the modulus operator?

1 Answers1