Convert Strings Into Numbers (integer, float)
k p c
> Exists there a function or a macro in Common Lisp (Allegro CL 4.2)?
You can use read-from-string and parse-integer. read-from-string is
the comprehensive way to do it. Look at the documentation for
with-standard-io-syntax and *read-eval*.
Note that my profiling shows that Allegro CL 4.2 read-from-string is
slow.
Consider a freeware parse-float that is floating around somewhere. I
don't know if it is more efficient than read-from-string or not,
however.
By the way, please keep your line lengths below 77 characters.
If you post a followup to this article, I would appreciate a courtesy
verbatim copy by email to help work around potentially unreliable feeds.
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Klaus Berndl
How to convert a suitable string (i.e. the string contents only digits and perhaps one dot) into a number-format like integer (only digits allowed) or float (only digits and one dot allowed)?
Exists there a function or a macro in Common Lisp (Allegro CL 4.2)?
Ciao,
KLaus Berndl
e-mail: ber...@fmi.uni-passau.de
Michael Tselman
: My problem:
: Ciao,
: KLaus Berndl
: e-mail: ber...@fmi.uni-passau.de
In a standard Common Lisp you can use the function
-----------------------------------------------------------------------------
PARSE-INTEGER [Function]
Function in LISP package:
Args: (string
&key (start 0) (end (length string)) (radix 10) (junk-allowed nil))
Parses STRING for an integer and returns it.
-----------------------------------------------------------------------------
Using it you can also easily write a parse-float function for the floats in
your format.
--Misha
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-----------------------------------------------------------------------------+
Peter Norvig
> Klaus Berndl (ber...@vogelweide.uni-passau.de) wrote:
>
> : How to convert a suitable string (i.e. the string contents only digits and perhaps one dot) into a number-format like integer (only digits allowed) or float (only digits and one dot allowed)?
Here's a solution:
(defun parse-float (string)
"Return a float read from string, and the index to the remainder of string."
(multiple-value-bind (integer i)
(parse-integer string :junk-allowed t)
(multiple-value-bind (fraction j)
(parse-integer string :start (+ i 1) :junk-allowed t)
(values (float (+ integer (/ fraction (expt 10 (- j i 1))))) j))))
CL-USER> (parse-float " 123.456 ")
123.456
9
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Antonio Leitao
Klaus> My problem:
Klaus> How to convert a suitable string (i.e. the string contents only digits and perhaps one dot) into a number-format like integer (only digits allowed) or float (only digits and one dot allowed)?
Klaus> Exists there a function or a macro in Common Lisp (Allegro CL 4.2)?
'read-from-string' solves your problem
Hope this helps
Antonio Leitao
Antonio Leitao
I forgot to add some documentation:
FUNCTION
read-from-string - read an object from a string
Package LISP
Usage
read-from-string string [eof-error-p [eof-value [:start start]
[:end end] [:preserve-whitespace preserve]]]
DESCRIPTION
Returns two values: the LISP object read from successive characters in
string and the index of the first character in string that was not read.
You can read from a substring of string by specifying values for the
keyword arguments :start and :end. The keyword argument :start speci-
fies the index of the first character in string to read. Its value
defaults to 0, denoting the beginning of string. The keyword argument
:end, specifies an index one greater than the index of the last charac-
ter to read. Its value may be an integer greater than or equal to the
value of the :start argument and less than or equal to the length of
string, or nil. The value nil is the same as the default value, the
length of string.
If the value of the keyword argument :preserve-whitespace is specified
non-nil, white space will be preserved in the same manner as
read-preserving-whitespace.
The argument eof-error-p controls what happens when the end of string is
reached. If the value of the argument is t, the default, an error is
signaled. However, if the value of the argument is nil, then in most
situations an error is not signaled. Instead, the read-from-string
function terminates and returns the value of eof-value. eof-value
defaults to nil. The function read-from-string always signals an error
if the end of string is reached when a COMMON LISP object is partially
but not completely read.
EXAMPLES
(read-from-string "this is a test") => this 5
(read-from-string "this is a test" nil nil :preserve-whitespace t)
=> this 4
(read-from-string "this is a test" nil 'done :start 5) => is 8
(read-from-string "this is a test" nil 'done :start 5 :end 6)
=> i 6
(read-from-string "this is a test" nil 'done :start 14) => done 14
Sorry about the two-part answer.
Good Luck.
Antonio Leitao
Antonio Leitao
copyrighted documentation without permission.
I was trying to suggest the function read-from-string when I quoted
directly from Franz's CommonLisp documentation without even including
the copyright line.
Again, I am sorry. I never intended to get credit for the answer. I
just thought it would be helpful to include the function
documentation and completely forgot it was Franz's documentation and
not mine's.
Thanks for Erik Naggum for politely remind me of my fault (and suggest
a correction).
I will try to pay more attention in the future.
Antonio Leitao
Hillary Ryan
(defun parse-float (string)
"Return a float read from string, and the index to the remainder of string."
(multiple-value-bind (integer i)
(parse-integer string :junk-allowed t)
(values integer i)
(parse-integer string :start (+ i 1) :junk-allowed t)
(values integer i)
(values (float (+ integer (/ fraction (expt 10 (- j i 1))))) j))))))
Example:
CL-USER> (parse-float "1a b c")
1
1
Pascal J. Bourguignon
http://stackoverflow.com/questions/85223/how-to-manually-parse-a-floating-point-number-from-a-string
The "standard" algorithm for converting a decimal number to the best
floating-point approximation is William Clinger's How to read floating
point numbers accurately, downloadable from here. Note that doing this
correctly requires multiple-precision integers, at least a certain
percentage of the time, in order to handle corner cases.
Algorithms for going the other way, printing the best decimal number
from a floating-number, are found in Burger and Dybvig's Printing
Floating-Point Numbers Quickly and Accurately, downloadable here. This
also requires multiple-precision integer arithmetic
See also David M Gay's Correctly Rounded Binary-Decimal and
Decimal-Binary Conversions for algorithms going both ways.
--
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A bad day in () is better than a good day in {}.
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WJ
> (string->number "3.1415926536")
3.1415926536
WJ
>
> Racket:
>
> > (string->number "3.1415926536")
> 3.1415926536
gosh> (string->number "3.1415926536")
3.1415926536
dinesh bhardwaj
(defun parse-float (string)
"Return a float read from string, and the index to the remainder of string."
(multiple-value-bind (integer i)
(parse-integer string :junk-allowed t)
(parse-integer string :start (+ i 1) :junk-allowed t)
(defun parse-number (str)
(if (search "." str)
(parse-float str)
(parse-integer str)))
now it can parse both float and integer. This should not be used where speed is concern.
On Wednesday, 10 January 1996 13:30:00 UTC+5:30, k p c wrote:
> Quoth ber...@vogelweide.uni-passau.de (Klaus Berndl):
>
Robert L.
> From Peter Norvig :
>
> (defun parse-float (string)
> "Return a float read from string, and the index to the remainder of string."
> (multiple-value-bind (integer i)
> (parse-integer string :junk-allowed t)
> (multiple-value-bind (fraction j)
> (parse-integer string :start (+ i 1) :junk-allowed t)
> (values (float (+ integer (/ fraction (expt 10 (- j i 1))))) j))))
>
> and then add
>
> (defun parse-number (str)
> (if (search "." str)
> (parse-float str)
> (parse-integer str)))
>
> now it can parse both float and integer.
* (parse-number "23 45 junk.")
23.45
5
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Robert L.
> On 3/29/2018, dinesh bhardwaj wrote:
>
> > From Peter Norvig :
> >
> > (defun parse-float (string)
> > "Return a float read from string, and the index to the remainder of string."
> > (multiple-value-bind (integer i)
> > (parse-integer string :junk-allowed t)
> > (multiple-value-bind (fraction j)
> > (parse-integer string :start (+ i 1) :junk-allowed t)
> > (values (float (+ integer (/ fraction (expt 10 (- j i 1))))) j))))
> >
> > and then add
> >
> > (defun parse-number (str)
> > (if (search "." str)
> > (parse-float str)
> > (parse-integer str)))
> >
> > now it can parse both float and integer.
>
> No, it can't.
>
> * (parse-number "23 45 junk.")
>
> 23.45
> 5
debugger invoked on a SB-INT:SIMPLE-PARSE-ERROR in thread
#<THREAD "main thread" RUNNING {23EAC149}>:
junk in string "2345 junk"
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