#' Compute a correlation matrix with stars to indicate significant correlations
#'
#' A function to compute a correlation matrix with stars
#' to indicate significant correlations (p<.05 = *,
#' p<.01 = **, p<.001).
#' Return a data frame.
#'
#' @author
#' Guillaume T. Vallet \email{gtvallet@@gmail.com}, University of de Montreal (Canada);
#'
#' @param x A data frame with only the variables to correlate.
#' @return Return a data frame representing the correlation matrix.
#' @keywords correlation, matrix, significance
#' @export
corstars <- function(x){
require(Hmisc)
x <- as.matrix(x)
R <- rcorr(x)$r
p <- rcorr(x)$P
## define notions for significance levels; spacing is important.
mystars <- ifelse(p < .001, "***", ifelse(p < .01, "** ", ifelse(p < .05, "* ", " ")))
## trunctuate the matrix that holds the correlations to two decimal
R <- format(round(cbind(rep(-1.11, ncol(x)), R), 2))[,-1]
## build a new matrix that includes the correlations with their apropriate stars
Rnew <- matrix(paste(R, mystars, sep=""), ncol=ncol(x))
diag(Rnew) <- paste(diag(R), " ", sep="")
rownames(Rnew) <- colnames(x)
colnames(Rnew) <- paste(colnames(x), "", sep="")
## remove upper triangle
Rnew <- as.matrix(Rnew)
Rnew[upper.tri(Rnew, diag = TRUE)] <- ""
Rnew <- as.data.frame(Rnew)
## remove last column and return the matrix (which is now a data frame)
Rnew <- cbind(Rnew[1:length(Rnew)-1])
return(Rnew)
}
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