Round off

[Phat G5 (G3)]

Has anyone found a reliable way to force JS to round to a specific number of
places? Every time I try I get different results. For example, I'd need to
round 3.4589 to 2 places. What is the most reliable way to do it?

Thanks

-S

[Hal Rosser]

"Phat G5 (G3)" <no****@noone.c om> wrote in message
news:C05842D8.3 3E67%no****@noo ne.com...

Has anyone found a reliable way to force JS to round to a specific number of places? Every time I try I get different results. For example, I'd need to
round 3.4589 to 2 places. What is the most reliable way to do it?

Thanks

-S

** Here's latest version thanks to John's code-testing services
** removed some superfluous code and fixed the 1-digit bug

//**args ->nnn = the number to round and yyy= the number of places
function roundyv3(nnn,yy y){
//** the multiplier to use
var multy = "1e"+yyy;//*same as Math.pow(10, yyy) ***
nnn+=""; //** coerce nnn to a String
//make sure there's a dot and something after it
if (nnn.indexOf(". ") < 0){
nnn += ".0";
}
//** adding zeroes after the dot makes it behave when rounding
nnn = nnn + "00000";
//** now rounding works - but removes the dot if a whole number ***
var strOut = String(Math.rou nd(nnn * multy) / multy);
//add back the decimal if rounding removed it and yyy > 0
if (strOut.indexOf (".") < 0 && yyy > 0) {
strOut+= ".0";
}
var placesInStrOut = strOut.length - strOut.indexOf( ".") - 1;
var zeroesNeeded = yyy - placesInStrOut;
var strZeroes = String(Math.pow (10, zeroesNeeded));
if (zeroesNeeded > 0) {
strOut += strZeroes.subst r(1);
}
return strOut;
}

[Hal Rosser]

"Dr John Stockton" <jr*@merlyn.dem on.co.uk> wrote in message
news:EQ******** ******@merlyn.d emon.co.uk...

JRS: In article <UV************ ****@bignews5.b ellsouth.net>, dated Sun,
9 Apr 2006 15:08:45 remote, seen in news:comp.lang. javascript, Hal
Rosser <hm******@bells outh.net> posted :
FAQ 4.6 deals with cases that your code does not attempt to deal with;
so you may not see the need for some of it. ***
OK... right - but its not documented and its confusing code
Your code provides no way to choose the representation of the sign; 4.6
does, by modifying Sign. ***
Don't see a need for it negatives have a minus sign, (You want to add
parentheses, or what, for negatives?)
The code in the FAQ provides a routine, Stretch, potentially useful
elsewhere. If it's not used elsewhere, that part of the job can easily
be done within StrU. *******
but it is uncommented ,obfuscated code
You blasted my code (in a previous post) for not indenting-- but look at the
code in FAQ4.6 hehe
The code in the FAQ adds leading zeroes if required. ***
(For rounding backwards ??) (When would that be required?)
The code in the FAQ also shows how to implement a .toFixed for Number. **
I disagree - it confuses the reader unless he doesn't need to read the FAQ
**
By the way, although I used to use the code in the FAQ, I now use code
developed from it. ***
I would be interested in seeing *documented* code you're now using.
****
If the input nnn is a small integer, your code extends it, changing its
value. Try, for example, roundyv2(1, 2), which gives "10". ***
Fixed that - good catch - I forgot to reinsert the dot after rounding (whole
numbers).
For what conditions do you think that your lines between my added //
marks actually do something useful, and what do they then achieve? ***
Removed that - another decent (and constructive) catch - thanks.
Is using Math.pow an *efficient* way of generating trailing zeroes, in
comparison with others, averaged over probable usage?

***
The second hand on my clock didn't move much when that code executed, but I
changed one instance to the "1e" + x method. Purity of code is a good cause
I guess though.

I fixed it and reposted. Thanks for testing.
I took a 'software testing' course - but you have a real knack for it.
Hal

[John G Harris]

In article <h%************ *****@bignews6. bellsouth.net>, Hal Rosser
<hm******@bells outh.net> writes

<snip>

** Here's latest version thanks to John's code-testing services
** removed some superfluous code and fixed the 1-digit bug

//**args ->nnn = the number to round and yyy= the number of places
function roundyv3(nnn,yy y)
/* Preconditions :
abs(nnn) < 2^52 or thereabouts (you don't test for 'e' or 'E')
abs(yyy) < 20 or so (any more is pointless)
*/
{
//** the multiplier to use
var multy = "1e"+yyy;//*same as Math.pow(10, yyy) ***
nnn+=""; //** coerce nnn to a String
//make sure there's a dot and something after it
if (nnn.indexOf(". ") < 0){
nnn += ".0";
}
//** adding zeroes after the dot makes it behave when rounding
nnn = nnn + "00000";
//** now rounding works - but removes the dot if a whole number ***
I don't see why you converted nnn into a string when the next thing you
do is convert it back into the original number.
var strOut = String(Math.rou nd(nnn * multy) / multy);
//add back the decimal if rounding removed it and yyy > 0
if (strOut.indexOf (".") < 0 && yyy > 0) {
strOut+= ".0";
}
var placesInStrOut = strOut.length - strOut.indexOf( ".") - 1;
var zeroesNeeded = yyy - placesInStrOut;
var strZeroes = String(Math.pow (10, zeroesNeeded));
if (zeroesNeeded > 0) {
strOut += strZeroes.subst r(1);
}
return strOut;
}

John
--
John Harris

[Hal Rosser]

"John G Harris" <jo**@nospam.de mon.co.uk> wrote in message
news:Z4******** ******@jgharris .demon.co.uk...

In article <h%************ *****@bignews6. bellsouth.net>, Hal Rosser
<hm******@bells outh.net> writes

<snip>

** Here's latest version thanks to John's code-testing services
** removed some superfluous code and fixed the 1-digit bug

//**args ->nnn = the number to round and yyy= the number of places
function roundyv3(nnn,yy y)
/* Preconditions :
abs(nnn) < 2^52 or thereabouts (you don't test for 'e' or 'E')
abs(yyy) < 20 or so (any more is pointless)
*/

-----ok---

{
//** the multiplier to use
var multy = "1e"+yyy;//*same as Math.pow(10, yyy) ***
nnn+=""; //** coerce nnn to a String
//make sure there's a dot and something after it
if (nnn.indexOf(". ") < 0){
nnn += ".0";
}
//** adding zeroes after the dot makes it behave when rounding
nnn = nnn + "00000";
//** now rounding works - but removes the dot if a whole number ***
I don't see why you converted nnn into a string when the next thing you
do is convert it back into the original number.
*************** *************

*** to make sure a decimal point and zeroes are there
*** else rounding is inconsistent************** ************

var strOut = String(Math.rou nd(nnn * multy) / multy);
//add back the decimal if rounding removed it and yyy > 0
if (strOut.indexOf (".") < 0 && yyy > 0) {
strOut+= ".0";
}
var placesInStrOut = strOut.length - strOut.indexOf( ".") - 1;
var zeroesNeeded = yyy - placesInStrOut;
var strZeroes = String(Math.pow (10, zeroesNeeded));
if (zeroesNeeded > 0) {
strOut += strZeroes.subst r(1);
}
return strOut;
}

John
--
John Harris

[Dr John Stockton]

JRS: In article <h%************ *****@bignews6. bellsouth.net>, dated
Tue, 11 Apr 2006 00:14:15 remote, seen in news:comp.lang. javascript, Hal
Rosser <hm******@bells outh.net> posted :

"Phat G5 (G3)" <no****@noone.c om> wrote in message
news:C05842D8. 33E67%no****@no one.com...

Has anyone found a reliable way to force JS to round to a specific numberof

places? Every time I try I get different results. For example, I'd need to
round 3.4589 to 2 places. What is the most reliable way to do it?

Thanks

-S

** Here's latest version thanks to John's code-testing services
** removed some superfluous code and fixed the 1-digit bug

//**args ->nnn = the number to round and yyy= the number of places
function roundyv3(nnn,yy y){

BTW, it's a good convention IMHO to use, as in Fortran, names such as X
for floats and such as N for integers. Javascript does not make the
distinction, but it can help readability.
//** the multiplier to use
var multy = "1e"+yyy;//*same as Math.pow(10, yyy) ***
But "1e"+yyy gives a String; Math.pow(10, yyy) gives a Number.
nnn+=""; //** coerce nnn to a String
//make sure there's a dot and something after it
if (nnn.indexOf(". ") < 0){
nnn += ".0";
}
//** adding zeroes after the dot makes it behave when rounding
nnn = nnn + "00000";
When is that part beneficial, and how?
//** now rounding works - but removes the dot if a whole number ***
var strOut = String(Math.rou nd(nnn * multy) / multy);
//add back the decimal if rounding removed it and yyy > 0
if (strOut.indexOf (".") < 0 && yyy > 0) {
strOut+= ".0";
}
var placesInStrOut = strOut.length - strOut.indexOf( ".") - 1;
var zeroesNeeded = yyy - placesInStrOut;
var strZeroes = String(Math.pow (10, zeroesNeeded));
Why do that if zeroesNeeded is zero ? Also, it seems slower than using
a string constant of ten or twenty zeroes with substr.
if (zeroesNeeded > 0) {
strOut += strZeroes.subst r(1);
}
return strOut;
}

Your code gives funny results with large numbers and NaN .
Your code rounds 3.965 to 3.97 but -3.965 to -3.96 .

--
© John Stockton, Surrey, UK. ?@merlyn.demon. co.uk Turnpike v4.00 IE 4 ©
<URL:http://www.jibbering.c om/faq/> JL/RC: FAQ of news:comp.lang. javascript
<URL:http://www.merlyn.demo n.co.uk/js-index.htm> jscr maths, dates, sources.
<URL:http://www.merlyn.demo n.co.uk/> TP/BP/Delphi/jscr/&c, FAQ items, links.

[Dr John Stockton]

JRS: In article <Z4************ **@jgharris.dem on.co.uk>, dated Tue, 11
Apr 2006 21:16:17 remote, seen in news:comp.lang. javascript, John G
Harris <jo**@nospam.de mon.co.uk> posted :

In article <h%************ *****@bignews6. bellsouth.net>, Hal Rosser
<hm******@bell south.net> writes

<snip>

** Here's latest version thanks to John's code-testing services
** removed some superfluous code and fixed the 1-digit bug

//**args ->nnn = the number to round and yyy= the number of places
function roundyv3(nnn,yy y)

/* Preconditions :
abs(nnn) < 2^52 or thereabouts (you don't test for 'e' or 'E')
abs(yyy) < 20 or so (any more is pointless)
*/

You mean, I think, 2^53, the first integer that cannot be incremented
with ++. But the default conversion from Number to String does not use
e-format below almost 10^21.

Neither FAQ 4.6 nor my current code test for e/E as such; though they
detect them.

***

To use Bankers' Rounding sensibly, either it should be rounding to
integer (so x.5 is exact) or one should first round by a modest multiple
of the expected rounding error to an exact base-10 value (implying to a
string) and then do Bankers' on that.

--
© John Stockton, Surrey, UK. ?@merlyn.demon. co.uk Turnpike v4.00 IE 4 ©
<URL:http://www.jibbering.c om/faq/> JL/RC: FAQ of news:comp.lang. javascript
<URL:http://www.merlyn.demo n.co.uk/js-index.htm> jscr maths, dates, sources.
<URL:http://www.merlyn.demo n.co.uk/> TP/BP/Delphi/jscr/&c, FAQ items, links.

[John G Harris]

In article <sW************ **@merlyn.demon .co.uk>, Dr John Stockton
<jr*@merlyn.dem on.co.uk> writes

JRS: In article <Z4************ **@jgharris.dem on.co.uk>, dated Tue, 11
Apr 2006 21:16:17 remote, seen in news:comp.lang. javascript, John G
Harris <jo**@nospam.de mon.co.uk> posted :

In article <h%************ *****@bignews6. bellsouth.net>, Hal Rosser
<hm******@bel lsouth.net> writes

<snip>

** Here's latest version thanks to John's code-testing services
** removed some superfluous code and fixed the 1-digit bug

//**args ->nnn = the number to round and yyy= the number of places
function roundyv3(nnn,yy y)

/* Preconditions :
abs(nnn) < 2^52 or thereabouts (you don't test for 'e' or 'E')
abs(yyy) < 20 or so (any more is pointless)
*/

You mean, I think, 2^53, the first integer that cannot be incremented
with ++. But the default conversion from Number to String does not use
e-format below almost 10^21.

<snip>

Displaying something.75 when there are only 2 bits to the right of the
binary point would be thoroughly misleading. Displaying something.00
when there are no bits to the right of the binary point is also
misleading.

Deciding what should be written in the pre-conditions is not easy in
this case. My "thereabout s" had a much larger range than you might have
thought.

John
--
John Harris