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Loyack, Eric

2013-Nov-30 07:37 UTC

[R] beginner help: 10th index is being overwritten by 11th

I am using R Studio and writing code to determine p-values for statistical
t-tests.  Code is below.  When I print out the values in the loop for
PTMpvalMeans they are correct, but when I store them the 10th one
overwrites the 11th.  Can someone tell me what I am doing wrong? Thanks in
advance, Eric

rho <- 0.0
PTMpvalMeans <- numeric(11)
while (rho < 1.0){
x1 <- rnorm(25,0,1)
x2 <- rnorm(25,0,1)
y1 <- rnorm(25,0,1)
y2 <- rnorm(25,0,1)
w1 <- rho*x1 + sqrt(1-rho^2)*y1
w2 <- rho*x2 + sqrt(1-rho^2)*y2
controlPretest <- 50 + 10*x1
groupPretest <- 50 + 10*x2
controlPosttest <- 60 + 10*w1
groupPosttest <- 70 + 10*w2
postTestMethod <-
t.test(controlPosttest,groupPosttest,mu=0,alternative="two.sided")
PTMpval <- postTestMethod$p.value
for (i in 1:999){
x1 <- rnorm(25,0,1)
x2 <- rnorm(25,0,1)
y1 <- rnorm(25,0,1)
y2 <- rnorm(25,0,1)
w1 <- rho*x1 + sqrt(1-rho^2)*y1
w2 <- rho*x2 + sqrt(1-rho^2)*y2
controlPretestLoop <- 50 + 10*x1
groupPretestLoop <- 50 + 10*x2
controlPosttestLoop <- 60 + 10*w1
groupPosttestLoop <- 70 + 10*w2
controlPretest <- c(controlPretest,controlPretestLoop)
groupPosttest <- c(groupPosttest,groupPosttestLoop)
groupPretest <- c(groupPretest,groupPretestLoop)
controlPosttest <- c(controlPosttest,controlPosttestLoop)
postTestMethodLoop <-
t.test(controlPosttestLoop,groupPosttestLoop,mu=0,alternative="two.sided")
PTMpvalLoop <- postTestMethodLoop$p.value
PTMpval <- c(PTMpval,PTMpvalLoop)
}
PTMpvalMeans[rho*10 + 1] <- mean(PTMpval)
print(rho)
print(PTMpvalMeans[rho*10 + 1])
rho <- rho + 0.1
}
PTMpvalMeans

	[[alternative HTML version deleted]]

Berend Hasselman

2013-Nov-30 09:16 UTC

head link

[R] beginner help: 10th index is being overwritten by 11th

On 30-11-2013, at 08:37, Loyack, Eric <eloyack at arcadia.edu> wrote:
> I am using R Studio and writing code to determine p-values for statistical
> t-tests.  Code is below.  When I print out the values in the loop for
> PTMpvalMeans they are correct, but when I store them the 10th one
> overwrites the 11th.  Can someone tell me what I am doing wrong? Thanks in
> advance, Eric
> 
R FAQ 7.31
(http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f)

Insert print(formatC(10*rho+1,digits=16,format="f")) before the line
with print(PTMpvalMeans[rho*10 + 1]) in your code and you should see what?s
wrong.

Use an explicit counter variable k and increment with k <- k+1 to index your
vectors.

Berend
> rho <- 0.0
> PTMpvalMeans <- numeric(11)
> while (rho < 1.0){
> x1 <- rnorm(25,0,1)
> x2 <- rnorm(25,0,1)
> y1 <- rnorm(25,0,1)
> y2 <- rnorm(25,0,1)
> w1 <- rho*x1 + sqrt(1-rho^2)*y1
> w2 <- rho*x2 + sqrt(1-rho^2)*y2
> controlPretest <- 50 + 10*x1
> groupPretest <- 50 + 10*x2
> controlPosttest <- 60 + 10*w1
> groupPosttest <- 70 + 10*w2
> postTestMethod <-
>
t.test(controlPosttest,groupPosttest,mu=0,alternative="two.sided")
> PTMpval <- postTestMethod$p.value
> for (i in 1:999){
> x1 <- rnorm(25,0,1)
> x2 <- rnorm(25,0,1)
> y1 <- rnorm(25,0,1)
> y2 <- rnorm(25,0,1)
> w1 <- rho*x1 + sqrt(1-rho^2)*y1
> w2 <- rho*x2 + sqrt(1-rho^2)*y2
> controlPretestLoop <- 50 + 10*x1
> groupPretestLoop <- 50 + 10*x2
> controlPosttestLoop <- 60 + 10*w1
> groupPosttestLoop <- 70 + 10*w2
> controlPretest <- c(controlPretest,controlPretestLoop)
> groupPosttest <- c(groupPosttest,groupPosttestLoop)
> groupPretest <- c(groupPretest,groupPretestLoop)
> controlPosttest <- c(controlPosttest,controlPosttestLoop)
> postTestMethodLoop <-
>
t.test(controlPosttestLoop,groupPosttestLoop,mu=0,alternative="two.sided")
> PTMpvalLoop <- postTestMethodLoop$p.value
> PTMpval <- c(PTMpval,PTMpvalLoop)
> }
> PTMpvalMeans[rho*10 + 1] <- mean(PTMpval)
> print(rho)
> print(PTMpvalMeans[rho*10 + 1])
> rho <- rho + 0.1
> }
> PTMpvalMeans
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> 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.

Duncan Murdoch

2013-Nov-30 09:19 UTC

head link

[R] beginner help: 10th index is being overwritten by 11th

On 13-11-30 4:37 PM, Loyack, Eric wrote:> I am using R Studio and writing code to determine p-values for statistical
> t-tests.  Code is below.  When I print out the values in the loop for
> PTMpvalMeans they are correct, but when I store them the 10th one
> overwrites the 11th.  Can someone tell me what I am doing wrong? Thanks in
> advance, Eric
It looks like a fairly obscure case of FAQ 7.31.  It's a bad idea to use 
0.1 as a loop increment, because fractions like that can't be 
represented exactly, and rounding error accumulates.

Duncan Murdoch
>
> rho <- 0.0
> PTMpvalMeans <- numeric(11)
> while (rho < 1.0){
> x1 <- rnorm(25,0,1)
> x2 <- rnorm(25,0,1)
> y1 <- rnorm(25,0,1)
> y2 <- rnorm(25,0,1)
> w1 <- rho*x1 + sqrt(1-rho^2)*y1
> w2 <- rho*x2 + sqrt(1-rho^2)*y2
> controlPretest <- 50 + 10*x1
> groupPretest <- 50 + 10*x2
> controlPosttest <- 60 + 10*w1
> groupPosttest <- 70 + 10*w2
> postTestMethod <-
>
t.test(controlPosttest,groupPosttest,mu=0,alternative="two.sided")
> PTMpval <- postTestMethod$p.value
> for (i in 1:999){
> x1 <- rnorm(25,0,1)
> x2 <- rnorm(25,0,1)
> y1 <- rnorm(25,0,1)
> y2 <- rnorm(25,0,1)
> w1 <- rho*x1 + sqrt(1-rho^2)*y1
> w2 <- rho*x2 + sqrt(1-rho^2)*y2
> controlPretestLoop <- 50 + 10*x1
> groupPretestLoop <- 50 + 10*x2
> controlPosttestLoop <- 60 + 10*w1
> groupPosttestLoop <- 70 + 10*w2
> controlPretest <- c(controlPretest,controlPretestLoop)
> groupPosttest <- c(groupPosttest,groupPosttestLoop)
> groupPretest <- c(groupPretest,groupPretestLoop)
> controlPosttest <- c(controlPosttest,controlPosttestLoop)
> postTestMethodLoop <-
>
t.test(controlPosttestLoop,groupPosttestLoop,mu=0,alternative="two.sided")
> PTMpvalLoop <- postTestMethodLoop$p.value
> PTMpval <- c(PTMpval,PTMpvalLoop)
> }
> PTMpvalMeans[rho*10 + 1] <- mean(PTMpval)
> print(rho)
> print(PTMpvalMeans[rho*10 + 1])
> rho <- rho + 0.1
> }
> PTMpvalMeans
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> 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.
>

R help - Nov 2013 - beginner help: 10th index is being overwritten by 11th

[R] beginner help: 10th index is being overwritten by 11th

[R] beginner help: 10th index is being overwritten by 11th

[R] beginner help: 10th index is being overwritten by 11th