Why does not R round function round big numbers

Question

I need a R function that always returns same number of digits after the decimal point regardless of how big the argument is. I tried round() but it does not work this way. Here is my example:

Rweb:> round(111234.678912,4) # expect 111234.6789
[1] 111234.7 
Rweb:> round(111234.678912/10,4) # expect 11123.4679    
[1] 11123.47 
Rweb:> round(111234.678912/100,4) # expect 1112.3468      
[1] 1112.347 
Rweb:> round(111234.678912/1000,4)     
[1] 111.2347 
Rweb:> round(111234.678912/10000,4)     
[1] 11.1235

It does work if the argument is in exponential format but I need work with numbers in floating format.

This type of question gets asked a lot (for example [here](http://stackoverflow.com/q/5458729/1281189)). Perhaps worthy of FAQ status? — BenBarnes, Sep 19 '12 at 08:10
@BenBarnes I'm tempted to agree with you, but OTOH the same "gotcha" happens in M**lab and other languages, not to mention Ex**l . At some point users need some basic training in the difference between display and stored values, regardless of the language in question. — Carl Witthoft, Sep 19 '12 at 12:11
@BenBarnes: the linked question is about trailing zeroes, which isn't really the same thing. — David Robinson, Sep 19 '12 at 12:36
@DavidRobinson, you're right. The previously linked question wasn't exactly the same thing. I meant something like [this](http://stackoverflow.com/q/3443687/1281189) or [this](http://stackoverflow.com/q/2287616/1281189) or [this](http://stackoverflow.com/q/11228403/1281189) or [this](http://stackoverflow.com/q/8169406/1281189) — BenBarnes, Sep 19 '12 at 12:56
But for some reason, I thought I had copied [this](http://stackoverflow.com/q/3245862/1281189) link into the above comment, which is nearly exactly the question here. — BenBarnes, Sep 19 '12 at 12:59
For anyone else who, like me, thought the question was going to be about bignums :-), there's this: `Rgames> bfoo<-mpfr("1.234545678909887665453421") Rgames> bfoo 1 'mpfr' number of precision 84 bits [1] 1.234545678909887665453421 Rgames> round(bfoo,10) 1 'mpfr' number of precision 84 bits [1] 1.23454567889999999999999999` — Carl Witthoft, Sep 19 '12 at 13:18

David Robinson · Answer 1 · 2012-09-19T05:01:12.223

It does round the number to the correct number of digits. However, R has limits on the number of digits it displays of very large numbers. That is- those digits are there, they just aren't shown.

You can see this like so:

> round(111234.678912,4)
[1] 111234.7
> round(111234.678912,4) - 111234
[1] 0.6789

You can use formatC to display it with any desired number of digits:

> n = round(111234.678912,4)
> formatC(n, format="f")
[1] "111234.6789"
> formatC(n, format="f", digits=2)
[1] "111234.68"

As @mnel helpfully points out, you can also set the number of digits shown (including those to the left of the decimal point) using options:

> options(digits=6)
> round(111234.678912,4)
[1] 111235
> options(digits=10)
> round(111234.678912,4)
[1] 111234.6789

Might be worth noting can change number the digits by setting `options(digits)` — mnel, Sep 19 '12 at 04:57

score 3 · Answer 2 · answered Mar 16 '16 at 11:47

let x is a number with big decimal places.
x<-1111111234.6547389758965789345

Here x is a number with big decimal places , you can format decimal places as your wish. Such that we wish to take up to 8 decimal places of this number.

 x<-c(1111111234.6547389758965789345)
y<-formatC(x,digits=8,format="f")
 [1] "1111111234.65473890"

Here format="f" gives number in the usual decimal places say, xxx.xxx. But if you wanted to get a integer number from this object x you use format="d"

score 2 · Answer 3 · edited Sep 22 '12 at 13:49

2

For anyone else who, like me, thought the question was going to be about bignums :-), there's this to ponder :-)

 Rgames> bfoo<-mpfr("1.234545678909887665453421")
 Rgames> bfoo
 1 'mpfr' number of precision  84   bits 
  [1] 1.234545678909887665453421
 Rgames> round(bfoo,10)
 1 'mpfr' number of precision  84   bits 
 [1] 1.23454567889999999999999999`

edited Sep 22 '12 at 13:49

David Robinson

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answered Sep 19 '12 at 13:21

Carl Witthoft

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score 0 · Answer 4 · answered Nov 23 '13 at 18:55

About "bignums", @Carl Witthoft: Thanks, Carl. ... I did think about bignums, when I read it. Are you sure there 's a problem with the rounding? See this:

> mpfr("1.2345456789", prec=84)
1 'mpfr' number of precision  84   bits
[1] 1.23454567889999999999999999

and note that Rmpfr (I'm the maintainer) does stay close to the underlying MPFR library. For round(), I've applied the logic/principle of f(x) returning a result with the same formal precision as x. If you want rounding with decreased formal precision, you can conveniently use roundMpfr():

> roundMpfr(bfoo, 32)
1 'mpfr' number of precision  32   bits 
[1] 1.2345456788

Why does not R round function round big numbers

4 Answers4