strings (dollar.cents) into floats

[luca bertini]

Hi,

i have strings which look like money values (ie 34.45)
is there a way to convert them into float variables?
everytime i try I get this error: "numb = float(my_line) ValueError:
empty string for float()"
"
here's the code

************

import sys
import re

for line in sys.stdin.readl ines():
my_line = line.rstrip()
numb = float(my_line)
print numb

[Steve Holden]

Chris Mellon wrote:

On 8/31/07, Steve Holden <st***@holdenwe b.comwrote:

>sturlamolden wrote:

>>On 31 Aug, 02:12, Wildemar Wildenburger
<lasses_w...@ klapptsowieso.n etwrote:

I've heard (ok, read) that several times now and I understand the
argument. But what use is there for floats, then? When is it OK to use them?
There are fractions that can be exactly represented by floats that
cannot be exactly represented by decimals.

Would you care to give an example?

>>There are fractions that
can be exactly represented by decimals that cannot be exactly
represented by floats.

Which one is better? Which do we prefer?

What a float cannot do is to represent a decimal fractional number
(e.g. 1.1) exactly. If we need that, we cannot use floats. A notable
example is monetary computations, it covers 99% of the use for decimal
numbers in computers. For this reason, we should never use floats to
add 10 cents to a dollar. The use of decimals for monetary
calculation s is mandatory.

That last sentence is patent nonsense, and completely untrue. Many
satisfactory financial applications have been written using only
floating-point arithmetic. Indeed I believe the accountant's Swiss army
knife, the Excel spreadsheet, uses floating-point numbers exclusively.

This is true, although Excel munges it's FP to provide "expected" results.

It depends on what you consider a "financial" application though.
Excel, while in extremely broad use, is not used to implement any of
the systems which actually "define" money, like the back end financial
systems at banks and credit unions.

In my experience, by far the most common method of calculating
financial numbers is actually using integer amounts, and then applying
well-defined rounding rules which I can't be bothered to look up the
name for.

For what it's worth, the work that I do with money (which is
middleware doing data transport, not calculations or billing) uses
either string representations or fixed point.

>What you say about floating-point have speed advantages is true, but you
go too far in claiming that decimal arithmetic is mandatory for monetary
calculations . That's about as sensible as saying that base 12 and base
20 arithmetic units were required to calculate in pounds, shillings and
pence.

I believe that to the degree that "real" accounting was done in those
currencies it did in fact use non-decimal bases. Just as people don't
use decimal time values (except us crazy computer folk), you're write
1 pound 4 shillings, not 1.333... pounds.

There is much in what you say (which, I am happy to note, doesn't really
contradict much of what I wrote). While I agree that Excel isn't a
fundamental component of back-end financial systems you might be
surprised at just how many people rely on its "currency" cells for
financial applications, albeit of a more lightweight kind.

You let yourself down just a little right at the end by not knowing the
intricacies of the old sterling currency - since there were twenty
shillings to the pound and twelve pennies to a shilling that would have
been £1.2. However, you are correct in assuming that nobody tried to
represent £1 3s. 4d. as 23.333... shillings.

regards
Steve
--
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[MRAB]

On Aug 31, 5:39 pm, David H Wild <dhw...@talktal k.netwrote:

In article <mailman.218.11 88577696.28954. [e-mail] >,
Chris Mellon <arka...@gmail. comwrote:

I believe that to the degree that "real" accounting was done in those
currencies it did in fact use non-decimal bases. Just as people don't
use decimal time values (except us crazy computer folk), you're write
1 pound 4 shillings, not 1.333... pounds.

When I worked on the British Railways National Payroll system, about 35
years ago, we, in common with many large users, wrote our system to deal
with integer amounts of pennies, and converted to pounds, shillings and
pence in the output part of the system.

So you never handled halfpennies?

[Steve Holden]

MRAB wrote:

On Aug 31, 5:39 pm, David H Wild <dhw...@talktal k.netwrote:

>In article <mailman.218.11 88577696.28954. [e-mail] >,
Chris Mellon <arka...@gmail. comwrote:

>>I believe that to the degree that "real" accounting was done in those
currencies it did in fact use non-decimal bases. Just as people don't
use decimal time values (except us crazy computer folk), you're write
1 pound 4 shillings, not 1.333... pounds.

When I worked on the British Railways National Payroll system, about 35
years ago, we, in common with many large users, wrote our system to deal
with integer amounts of pennies, and converted to pounds, shillings and
pence in the output part of the system.

So you never handled halfpennies?

Nobody was paid to the ha'penny. You could spend them in the shops, but
payroll systems didn't use them. I am happy to say that I didn't become
involved in financial programming until after decimalization (February
1971, IIRC - I was in Sweden at the time).

regards
Steve
--
Steve Holden [phone] [phone]
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
--------------- Asciimercial ------------------
Get on the web: Blog, lens and tag the Internet
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[David H Wild]

In article <11************ **********@o80g 2000hse.googleg roups.com>,
MRAB <go****@mrabarn ett.plus.comwro te:

When I worked on the British Railways National Payroll system, about
35 years ago, we, in common with many large users, wrote our system to
deal with integer amounts of pennies, and converted to pounds,
shillings and pence in the output part of the system.

So you never handled halfpennies?

Halfpennies had disappeared from the BR accounting system before the first
use of computers for accounting work.

--
David Wild using RISC OS on broadband
www.davidhwild.me.uk

[John Machin]

On Sep 1, 4:58 am, MRAB <goo...@mrabarn ett.plus.comwro te:

On Aug 31, 5:39 pm, David H Wild <dhw...@talktal k.netwrote:In article <mailman.218.11 88577696.28954. [e-mail] >,

Chris Mellon <arka...@gmail. comwrote:

I believe that to the degree that "real" accounting was done in those
currencies it did in fact use non-decimal bases. Just as people don't
use decimal time values (except us crazy computer folk), you're write
1 pound 4 shillings, not 1.333... pounds.

When I worked on the British Railways National Payroll system, about 35
years ago, we, in common with many large users, wrote our system to deal
with integer amounts of pennies, and converted to pounds, shillings and
pence in the output part of the system.

So you never handled halfpennies?

Or farthings?

[Lawrence D'Oliveiro]

In message <11************ *********@50g20 00hsm.googlegro ups.com>,
sturlamolden wrote:

There are fractions that can be exactly represented by floats that
cannot be exactly represented by decimals.

There are no such.

[Grant Edwards]

On 2007-09-01, Lawrence D'Oliveiro <ld*@geek-central.gen.new _zealandwrote:

In message <11************ *********@50g20 00hsm.googlegro ups.com>,
sturlamolden wrote:

>There are fractions that can be exactly represented by floats that
cannot be exactly represented by decimals.

There are no such.

In that statement does "float" mean finite-length base-2 FP and
"decimal" mean finite-length base-10 FP?

--
Grant Edwards grante Yow! My LESLIE GORE record
at is BROKEN...
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