R help - Jan 2006 - wilcox.test warnig message p-value where are the zeros in the data?

does anybody know why there are the two warnings in the example above?

Regards Knut

 > day_4
 [1] 540   1   1   1   1   1   1 300 720 480
 > day_1
 [1]  438  343    1  475    1  562  500  435 1045  890
> is.vector (day_1)
[1] TRUE
> is.vector (day_4)
[1] TRUE

 > wilcox.test(day_4 
,day_1,paired=TRUE,alternative="two.sided",exact=TRUE,conf.int=TRUE)

        Wilcoxon signed rank test with continuity correction

data:  day_4 and day_1
V = 1, p-value = 0.02086
alternative hypothesis: true mu is not equal to 0
95 percent confidence interval:
 -486.5 -120.0
sample estimates:
(pseudo)median
          -348

Warning messages:
1: cannot compute exact p-value with zeroes in: 
wilcox.test.default(day_4, day_1, paired = TRUE, alternative = 
"two.sided", 
2: cannot compute exact confidence interval with zeroes in: 
wilcox.test.default(day_4, day_1, paired = TRUE, alternative = 
"two.sided",

Knut Krueger <admin at biostatistic.de> writes:
> does anybody know why there are the two warnings in the example above?
> 
> Regards Knut
> 
>  > day_4
>  [1] 540   1   1   1   1   1   1 300 720 480
>  > day_1
>  [1]  438  343    1  475    1  562  500  435 1045  890
> 
> > is.vector (day_1)
> [1] TRUE
> > is.vector (day_4)
> 
> [1] TRUE
The paired Wilcoxon test depends on pairwise differences and as far as
I can see you have two of those being zero, in cases 3 and 5.
 
>  > wilcox.test(day_4 
>
,day_1,paired=TRUE,alternative="two.sided",exact=TRUE,conf.int=TRUE)
> 
>         Wilcoxon signed rank test with continuity correction
> 
> data:  day_4 and day_1
> V = 1, p-value = 0.02086
> alternative hypothesis: true mu is not equal to 0
> 95 percent confidence interval:
>  -486.5 -120.0
> sample estimates:
> (pseudo)median
>           -348
> 
> Warning messages:
> 1: cannot compute exact p-value with zeroes in: 
> wilcox.test.default(day_4, day_1, paired = TRUE, alternative = 
> "two.sided", 
> 2: cannot compute exact confidence interval with zeroes in: 
> wilcox.test.default(day_4, day_1, paired = TRUE, alternative = 
> "two.sided",
> 
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