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protentially serious R error

4 messages · liuxf, Sarah Goslee, arun +1 more

liuxf
# 
Hi guy,
I have recently encountered a problem while  I was just trying to generate
some random numbers with the function "rnorm", the problem is shown below:
########case 1############
[1] -1.2765922 -0.5732654 -1.2246126 -0.4734006
########case 2###########
*> rnorm(20*(1-0.8))
[1] -0.62036668  0.04211587 -0.91092165*
#########case 3############
[1]  0.1580288 -0.6545846  1.7672873  0.7167075
#############case 4#########
*> b<-1-0.8
[1] 0.9101742 0.3841854 1.6821761*

I was expecting the 4 cases should do the same job--generate 4 random
numbers. But in case 2 and 4 I only get 3. Has anyone else seen this problem
before? Thanks. (I have tried with other functions i.e "rchisq","rexp" ...)
#######################################################################
One of my colleague also have a problem that we think it might be related
with the problem I addressed above:
[1] 0.3868379 0.1587814 0.8140483 0.7796691 0.5357628 0.2431110 0.1782747
0.3906829 0.5262615 0.7440143
+   test2[i*100] <- sum(test1<i) 
+ }
[1]  0  0  0  0  0  0 *NA*  0  0  0  0  0  0  0  0  1  1  2  2  2  2  2  2
2  3  3  3  3  3  3  3  3  3  3  3  3  3  3  4  5  5  5  5  5
 [45]  5  5  5  5  5  5  5  5  6  7  7  7  7  7  7  7  7  7  7  7  7  7  7
7  7  7  7  7  7  7  8  8  8  9  9  9  9 10 10 10 10 10 10 10
 [89] 10 10 10 10 10 10 10 10 10 10 10 10

Every time he re-runs the code there always always a "NA"(highlighted). Does
any one know why?  Your help is greatly appreciated.   


Xiaofeng





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Sarah Goslee
# 
Potentially serious? Only to you: this looks like R FAQ 7.31 striking again.

Here's a hint:


identical(20*(1-0.8), 4)

and another

rnorm(round(20*(1-0.8)))

Sarah
On Fri, Nov 30, 2012 at 4:05 PM, liuxf <liuxf at math.mcmaster.ca> wrote:
--
Sarah Goslee
http://www.functionaldiversity.org
arun
# 
HI,
Use either ?round() or ?signif()
?rnorm(signif(20*(1-0.8)))
#[1]? 1.58079490 -0.03595492 -0.11561782? 0.12951975
?rnorm(signif(20*0.2))
#[1]? 1.70722614? 0.40898103 -2.94663960 -0.09284011
A.K.




----- Original Message -----
From: liuxf <liuxf at math.mcmaster.ca>
To: r-help at r-project.org
Cc: 
Sent: Friday, November 30, 2012 4:05 PM
Subject: [R] protentially serious R error

Hi guy,
I have recently encountered a problem while? I was just trying to generate
some random numbers with the function "rnorm", the problem is shown below:
########case 1############
[1] -1.2765922 -0.5732654 -1.2246126 -0.4734006
########case 2###########
*> rnorm(20*(1-0.8))
[1] -0.62036668? 0.04211587 -0.91092165*
#########case 3############
[1]? 0.1580288 -0.6545846? 1.7672873? 0.7167075
#############case 4#########
*> b<-1-0.8
[1] 0.9101742 0.3841854 1.6821761*

I was expecting the 4 cases should do the same job--generate 4 random
numbers. But in case 2 and 4 I only get 3. Has anyone else seen this problem
before? Thanks. (I have tried with other functions i.e "rchisq","rexp" ...)
#######################################################################
One of my colleague also have a problem that we think it might be related
with the problem I addressed above:
[1] 0.3868379 0.1587814 0.8140483 0.7796691 0.5357628 0.2431110 0.1782747
0.3906829 0.5262615 0.7440143
+?  test2[i*100] <- sum(test1<i) 
+ }
? [1]? 0? 0? 0? 0? 0? 0 *NA*? 0? 0? 0? 0? 0? 0? 0? 0? 1? 1? 2? 2? 2? 2? 2? 2
2? 3? 3? 3? 3? 3? 3? 3? 3? 3? 3? 3? 3? 3? 3? 4? 5? 5? 5? 5? 5
[45]? 5? 5? 5? 5? 5? 5? 5? 5? 6? 7? 7? 7? 7? 7? 7? 7? 7? 7? 7? 7? 7? 7? 7
7? 7? 7? 7? 7? 7? 7? 8? 8? 8? 9? 9? 9? 9 10 10 10 10 10 10 10
[89] 10 10 10 10 10 10 10 10 10 10 10 10

Every time he re-runs the code there always always a "NA"(highlighted). Does
any one know why?? Your help is greatly appreciated.? 


Xiaofeng





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______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
(Ted Harding)
# 
[See in-line below]
On 30-Nov-2012 21:05:23 liuxf wrote:
The key to this case can be seen in:

  20*0.2 - 4
  # [1] 0

  20*(1-0.8)-4
  # [1] -8.881784e-16

  0.2 - (1-0.8)
  # [1] 5.551115e-17

So you have fallen victim to the fact that, in general, floating-point
arithmetic is not exact (and this is not a "feature" only of R, but
of other packages that use standard floating-point CPU arithmetic.
This is effectovely the same issue:

  test1 <- runif(10,0,1)
  test1
  # [1] 0.3868379 0.1587814 0.8140483 0.7796691 0.5357628 0.2431110
  # 0.1782747 0.3906829 0.5262615 0.7440143
  test2 <- NULL
  for(i in seq(0.01,1,length=100)){
    test2[i*100] <- sum(test1<i) 
  }
  test2
  #   [1]  0  0  0  0  0  0 *NA*   0  0  0  0  0  0  0  0  1  1  2  2
  #  2  2  2  2  2  3  3  3  3  3  3  3  3  3  3  3  3  3  3  4  5  5
  #  5  5  5  5  5  5  5  5  5  5  5  6  7  7  7  7  7  7  7  7  7  7
  #  7  7  7  7  7  7  7  7  7  7  7  8  8  8  9  9  9  9 10 10 10 10
  # 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

  S <- seq(0.01,1,length=100))
  100*S[7] - 7
  # [1] -8.881784e-16

  which(100*S < (1:100))
  # [1] 7

Why not simply use expressions which evaluate to exact integers?
There is no obvious reason in the examples given to do otherwise.

But, if you must adopt your approach, then consider:

  round(100*S)

  100*S - (1:100)
  #  [1]  0.000000e+00  0.000000e+00  0.000000e+00  0.000000e+00
  #  [5]  0.000000e+00  8.881784e-16 -8.881784e-16  0.000000e+00
  #  [9]  0.000000e+00  0.000000e+00  0.000000e+00  0.000000e+00
  # [13]  0.000000e+00  1.776357e-15  1.776357e-15  0.000000e+00
  # [17]  0.000000e+00  3.552714e-15  0.000000e+00  0.000000e+00
  # [etc .... ]

  round(100*S) - (1:100)
  #  [1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
  # [26] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
  # [51] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
  # [76] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Hoping this helps,
Ted.

-------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at wlandres.net>
Date: 30-Nov-2012  Time: 23:26:41
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