Standard Deviation in R [closed] - r

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I am trying to calculate the standard deviation from scratch in R (I do know R has a build-in function for that), but I'm not getting the correct results, so I don't know what I'm doing wrong.
My.SD<-function(X) {
M<-(sum(X)/length(X))
my.var<-sum((X-M)^2)
StanDev<-sqrt(my.var/length(X))
print(StanDev)
}


I guess you might need to use (length(X)-1) rather than length(x), i.e.,
My.SD<-function(X) {
M<-(sum(X)/length(X))
my.var<-sum((X-M)^2)
StanDev<-sqrt(my.var/(length(X)-1))
print(StanDev)
}
since here it should be sample standard deviation.

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I have a task a bit complicated for my knowledge in R. I need to reproduce this graphic of the figure in R, I performed several searches and could not find anything. The main thing is to be able to reproduce the graphic (it doesn't have to be identical), subtitles are not so important. Any ideas on how to do it or just using another program? Thanks!!

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I am struggling to create a variable based on a condition in my data frame.
I think the code is quite self-explanatory.
DT$xp_ratio_y<- apply(DT,1,function(x)
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I think you're making it too complicated. Just calculate for all then remove those you don't want:
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How can I pass a matrix by reference in R [closed]

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I am using a function recursively and the function has to update the matrix. I passed the function as an argument, but it seems matrix can only be passed as the pass by value, because the moment program comes out of the scope of function matrix doesn't hold updated values. Since, I have to do multiple updates in the matrix recursively, I have to pass it as a parameter.
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Residuals against fitted values show pattern [closed]

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I am going to fit a model using linear and quadratic model.
If the graphs of residuals against fitted values show pattern(not random pattern),what is the cause and how can i fix it?

This should be a comment, but I have insufficient rep for that.
The cause of the pattern is that your model doesn't account for something.
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All, I wrote this function but it has flaws:
X=cbind(rep(1:5,4),rep(c(1,2,4,8,16),2),c(3.4,4,45,6,4,2,36,4,34,7,8,0,2,4,5,7,9,12,23,1))
getXbarl<-function(Y,l){
xbar=tapply(Y,l,mean)
return(matrix(xbar[l]))#***
}
#It works for the first row:
getXbarl(X[,3],X[,1])
#but not the second row, because the factors are no longer 1:5 here.
getXbarl(X[,3],X[,2])
Please help me write a fix. The issue is that for xbar[l]***, it no longer corresponds to the index.

I think you need ave
ave(X[,3], X[,2])
As a function
getXbarl2 <- function(Y,l) matrix(ave(Y,l))
identical(getXbarl(X[,3], X[,1]), getXbarl2(X[,3], X[,1]))
#[1] TRUE

getXbarl<-function(Y,l){
xbar=tapply(Y,l,mean)
l2=factor(l,labels=1:length(xbar))
return(matrix(xbar[l2]))
}
X=cbind(rep(1:5,4),rep(c(1,2,4,8,16),2),c(3.4,4,45,6,4,2,36,4,34,7,8,0,2,4,5,7,9,12,23,1))
getXbarl(X[,3],X[,1])
getXbarl(X[,3],X[,2])

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