float / rounding question

[helen.m.flynn]

Hi I'm very much a beginner with Python.
I want to write a function to convert celcius to fahrenheit like this
one:

def celciusToFahren heit(tc):
tf = (9/5)*tc+32
return tf

I want the answer correct to one decimal place, so
celciusToFahren heit(12) would return 53.6.

Of course the function above returns 53.600000000000 001.

How do I format it correctly?

[sabatier]

On Feb 25, 10:44 am, helen.m.fl...@g mail.com wrote:

Hi I'm very much a beginner with Python.
I want to write a function to convert celcius to fahrenheit like this
one:

def celciusToFahren heit(tc):
tf = (9/5)*tc+32
return tf

I want the answer correct to one decimal place, so
celciusToFahren heit(12) would return 53.6.

Of course the function above returns 53.600000000000 001.

How do I format it correctly?

By the way, I tried this:

return '%2.1f' % tf but that returns a string instead of a number.

Any other suggestions?

[Jorge Godoy]

sabatier wrote:

On Feb 25, 10:44 am, helen.m.fl...@g mail.com wrote:

>Hi I'm very much a beginner with Python.
I want to write a function to convert celcius to fahrenheit like this
one:

def celciusToFahren heit(tc):
tf = (9/5)*tc+32
return tf

I want the answer correct to one decimal place, so
celciusToFahre nheit(12) would return 53.6.

Of course the function above returns 53.600000000000 001.

How do I format it correctly?

By the way, I tried this:

return '%2.1f' % tf but that returns a string instead of a number.

Any other suggestions?

But you are asking for a string on your format string above.

And also formatting make no sense in other context since a number is a
number and 53.600000 and 53.6 are the same number (besides precision).

You are concerned with how numbers are represented in binary. When
displaying the value use the format string you shown above and all will
work.

[Necmettin Begiter]

25 February 2008 Monday 12:44:46 tarihinde he***********@g mail.com şunları yazmıştı:

Hi I'm very much a beginner with Python.
I want to write a function to convert celcius to fahrenheit like this
one:

def celciusToFahren heit(tc):
tf = (9/5)*tc+32
return tf

I want the answer correct to one decimal place, so
celciusToFahren heit(12) would return 53.6.

Of course the function above returns 53.600000000000 001.

How do I format it correctly?

Use the round(number,di gits) function:

tf = round((9/5)*tc+32,1)

[Sion Arrowsmith]

Necmettin Begiter <ne************ ***@gmail.comwr ote:

>25 February 2008 Monday 12:44:46 tarihinde he***********@g mail.com =C5=9Fun=
lar=C4=B1 yazm=C4=B1=C5=9 Ft=C4=B1:

>Of course the function above returns 53.600000000000 001.
=20
How do I format it correctly?

Use the round(number,di gits) function:

tf =3D round((9/5)*tc+32,1)

>>53.6

53.600000000000 001

>>round(53.6, 1)

53.600000000000 001

--
\S -- si***@chiark.gr eenend.org.uk -- http://www.chaos.org.uk/~sion/
"Frankly I have no feelings towards penguins one way or the other"
-- Arthur C. Clarke
her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump

[Mel]

he***********@g mail.com wrote:

Hi I'm very much a beginner with Python.
I want to write a function to convert celcius to fahrenheit like this
one:

def celciusToFahren heit(tc):
tf = (9/5)*tc+32
return tf

I want the answer correct to one decimal place, so
celciusToFahren heit(12) would return 53.6.

Of course the function above returns 53.600000000000 001.

How do I format it correctly?

print celcisuToFahren heit (12)

will do fine.
Python 2.5.1 (r251:54863, Oct 5 2007, 13:36:32)
[GCC 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

>>def celciusToFahren heit(tc):

.... tf = (float(9)/5)*tc+32
.... return tf
....

>>celciusToFahr enheit (12)

53.600000000000 001

>>print celciusToFahren heit (12)

53.6
The straight value display from the interpreter pursues precision to
the bitter end, doing its formatting with the repr function. print
uses str formatting for a more expected result.

Mel.

[Terry Reedy]

<he***********@ gmail.comwrote in message
news:d4******** *************** ***********@z17 g2000hsg.google groups.com...
| Hi I'm very much a beginner with Python.
| I want to write a function to convert celcius to fahrenheit like this
| one:
|
| def celciusToFahren heit(tc):
| tf = (9/5)*tc+32
| return tf

Unless you are importing 'integer division' or using 3.0, that should be
9.0/5.0.

| I want the answer correct to one decimal place, so
| celciusToFahren heit(12) would return 53.6.

As written, running above on 2.x returns 44.

tjr

[John Machin]

On Feb 26, 7:14 am, "Terry Reedy" <tjre...@udel.e duwrote:

<helen.m.fl...@ gmail.comwrote in message

news:d4******** *************** ***********@z17 g2000hsg.google groups.com...
| Hi I'm very much a beginner with Python.
| I want to write a function to convert celcius to fahrenheit like this
| one:
|
| def celciusToFahren heit(tc):
| tf = (9/5)*tc+32
| return tf

Unless you are importing 'integer division' or using 3.0, that should be
9.0/5.0.

Has the syntax changed? I thought it was:
from __future__ import division

The OP may wish to avoid the confusion and the pointless division by
using:
tf = 1.8 * tc + 32

>
| I want the answer correct to one decimal place, so
| celciusToFahren heit(12) would return 53.6.

As written, running above on 2.x returns 44.

tjr

[Mark Dickinson]

On Mar 7, 5:12*pm, Piet van Oostrum <[e-mail] rote:

Python just uses the C library for printing, I presume, and the conversion
routines in the C library are rather simplistic. It is, however, possible
to do better, so that 53.6 -- although internally represented as something
that could be described as 53.600000000000 001 -- will actually be printed
as 53.6.

There are issues with doing this portably and reliably. See

http://bugs.python.org/issue1580

for a recent discussion.

Mark

[Steven D'Aprano]

On Fri, 07 Mar 2008 23:12:27 +0100, Piet van Oostrum wrote:

Sorry to come in so late in this discussion. Although it is correct to
say that many real numbers that have an exact decimal representation
cannot be exactly represented in binary, that is no excuse to print 53.6
as 53.600000000000 001. This is just lousy printing and the fact that
this kind of question comes up every week shows that it is confusing to
many people.

Good. That's a feature, not a bug.

Floats *are* confusing and unintuitive. Anyone with pretensions to be a
programmer should get over the illusion that floats are reals as soon as
possible, before they learn bad habits that have to be unlearned. If that
starts with them asking why they get 53.600000000000 001 instead of 53.6,
so be it. If they want to be insulated from the harsh reality, they can
do this:

>>print 53.6

53.6

--
Steven