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

[Wildemar Wildenburger]

Ben Finney wrote:

You most likely do *not* want floating-point numbers for currency,
since they rely on the operating system's binary floating point
support which cannot accurately represent decimal fractions.

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?

/W

[Robert Kern]

Wildemar Wildenburger wrote:

Ben Finney wrote:

>You most likely do *not* want floating-point numbers for currency,
since they rely on the operating system's binary floating point
support which cannot accurately represent decimal fractions.

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 many, many situations where one is *not* trying to represent numbers
that have nice decimal fractions or the error induced is insignificant to the
problem. For example, I might take temperature readings good to 0.1 degrees
Celcius. I'll type those numbers in decimal even though when I do my
calculations I'll use floating point math because the error (about 1e-15 or so)
is so far below the error in my measurement (about 0.1) that I won't have problems.

Binary floating point has the advantage of being widely implemented and quite
fast compared to decimal floating point.

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

[Ben Finney]

Wildemar Wildenburger <la*********@kl apptsowieso.net writes:

But what use is there for floats, then? When is it OK to use them?

When one is willing to sacrifice decimal precision for speed of
calculation, and doesn't need the numbers to stay precise. E.g. when
performing millions of calculations on real-world (analogue)
measurements.

--
\ "Too many Indians spoil the golden egg." -- Sir Joh |
`\ Bjelke-Petersen |
_o__) |
Ben Finney

[Steve Holden]

Ben Finney wrote:

Wildemar Wildenburger <la*********@kl apptsowieso.net writes:

>But what use is there for floats, then? When is it OK to use them?

When one is willing to sacrifice decimal precision for speed of
calculation, and doesn't need the numbers to stay precise. E.g. when
performing millions of calculations on real-world (analogue)
measurements.

Traditionally, mainframe computers used BCD arithmetic to handle
currency, and the Decimal module is probably the best way to proceed if
your Python is recent enough to include it.

That isn't to say that currencies can't be handled using floating-point.
The important thing then becomes to round after each calculation rather
than allowing the errors to build up until they become significant
enough to make a difference to the final result.

regards
Steve
--
Steve Holden [phone] [phone]
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
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[sturlamolden]

On 31 Aug, 02:12, Wildemar Wildenburger
<lasses_w...@kl apptsowieso.net wrote:

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. 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
calculations is mandatory.

Floats are written as decimal fractional numbers in program code, and
they are printed as decimal fractional numbers on the screen. A novice
programmer may for this reason easily mistake floats for being
decimals. But inside the computer floats are something very different.
Floats do not represent decimal fractional numbers. Floats represent
binary fractional numbers. Decimal numbers are base-10, binary numbers
are base-2. Floats and decimals are similar but different.

It is much easier to manipulate for computers than decimals. Digital
electronics by convention only has two states, TTL voltage levels 0.0
V and 5.0 V, which are taken to mean 0 and 1 in binary (or false and
true in boolean) respectively. Computers work internally with binary
numbers. Floats are preferred to decimals in e.g. numerical maths and
other scientific computing for their better performance speed wise.
Knowledge of the behaviour of floats, e.g. how they cause rounding and
truncation errors, is pivotal in that field of computer science. For
most purposes, we could replace floats with decimals. But then we
would suffer a major speed penalty.

Decimals are only "precise" if the input values can be represented
precisely by decimal fractions. If we typed 1.1 and took that to mean
"one dollar and 10 cents", then certainly "exactly 1.1" was what we
meant. But if we ment "1.1 centigrades read from a mercury
thermometer", we would actually (by convention) mean "something
between 1.05 and 1.14 centigrades". Decimals also fail to be exact if
we try to compute things like "1.0/3.0". 1/3 does not have an exact
representation in decimal, and we get an approximation from the
computer.

For purposes where exact precision is not needed, decimals are neither
better nor worse than floats. This accounts for 99% of the cases where
fractional numbers are used on a computer.

More about floating point numbers here:
http://www.math.byu.edu/~schow/work/...atingPoint.htm
Sturla Molden

[Steve Holden]

sturlamolden wrote:

On 31 Aug, 02:12, Wildemar Wildenburger
<lasses_w...@kl apptsowieso.net wrote:

>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
calculations 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.

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.

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
Many services currently offer free registration
----------- Thank You for Reading -------------

[Chris Mellon]

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

sturlamolden wrote:

On 31 Aug, 02:12, Wildemar Wildenburger
<lasses_w...@kl apptsowieso.net wrote:

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
calculations 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.

[MRAB]

On Aug 31, 5:28 pm, "Chris Mellon" <arka...@gmail. comwrote:

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

sturlamolden wrote:

On 31 Aug, 02:12, Wildemar Wildenburger
<lasses_w...@kl apptsowieso.net wrote:

>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
calculations 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.

FYI, 1 pound 4 shillings was 1.20 pounds; 1.333... pounds was 1 pound
6 shillings 8 pence. I think...

[David H Wild]

In article <ma************ *************** ***********@pyt hon.org>,
Chris Mellon <ar*****@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.

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