# Loop with float increments (frange)?

Discussion in 'Python' started by forum@anton.e4ward.com, Apr 14, 2006.

1. ### 4ward.comGuest

Hi!

what's the standard way for a "for" loop with float increments?

Anton

4ward.com, Apr 14, 2006

2. ### Dan SommersGuest

On 14 Apr 2006 06:11:38 -0700,
4ward.com wrote:

> what's the standard way for a "for" loop with float increments?

f = initial_value
while f <= final_value:
process(f)
f = f + increment

Note that there is no general guarantee that f will actually be

Regards,
Dan

--
Dan Sommers
<http://www.tombstonezero.net/dan/>
"I wish people would die in alphabetical order." -- My wife, the genealogist

Dan Sommers, Apr 14, 2006

3. ### Diez B. RoggischGuest

4ward.com wrote:

> Hi!
>
> what's the standard way for a "for" loop with float increments?

AFAIK there is no, but you should be easily able to write an frange
yourself:

def frange(from, to, step):
while from < to:
yield from
from += step

for x in frange(10.5, 23.4, 0.3):
print x

Diez

Diez B. Roggisch, Apr 14, 2006
4. ### Tim PetersGuest

[4ward.com]
>> what's the standard way for a "for" loop with float increments?

[Dan Sommers]
> Use a while loop instead:
>
> f = initial_value
> while f <= final_value:
> process(f)
> f = f + increment
>
> Note that there is no general guarantee that f will actually be

There's no guarantee that the `while` loop will execute "the expected"
number of times either, and it's generally a Bad Idea to do "f +=
increment" inside the loop: the value of f suffers an additional
rounding error on each iteration that way, potentially making it drift
away from "the expected" value more & more as the loop goes on.

Standard careful numeric practice is this way:

n = compute the number of iterations you want to make in total
for i in xrange(n):
process(initial_value + i * increment)

Then each value computed suffers a total of only two rounding errors
(one for the multiply, one for the add), independent of how large `n`
may be, and doesn't get worse as the loop goes on.

Tim Peters, Apr 14, 2006