Loop with float increments (frange)?

[forum]

Hi!

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

Anton

 

[Dan Sommers]

On 14 Apr 2006 06:11:38 -0700,

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

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
final_value; see also <http://docs.python.org/tut/node16.html>.

Regards,
Dan

 

[Diez B. Roggisch]

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

 

[Tim Peters]

[[email protected]]
[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
final_value; see also <http://docs.python.org/tut/node16.html>.

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.