```
## Daniel Gerlanc and Kris Kirby (2010-2012)
## Function for calculating Pearson's r
calcPearsonsR <- function(vals, freq, grps, lambdas) {
## Compute Pearson's r for one or more groups
##
## Args:
## vals: a numeric vector of values
## freq: a frequency vector of length equal to the number of records in
## 'data' indicating how many times an observation should be drawn
## from each sample.
## grps: a grouping vector of the same length as 'vals'
## lambdas: an optional named vector of contrast weights
##
## @pre: 'lambdas' is not optional and the sum of the lambdas must be 0
##
## Details:
## This function is meant to be passed as the 'statistic' argument
## to the 'boot' function. 'freq' should be a frequency vector of
## the type returned by 'boot'
## Get the integer indices of the different groups.
grp.idx = split(seq_along(vals), grps, drop=TRUE)
grp.nms = names(grp.idx)
## Calculate the mean and sum-of-squares for the bootstrap samples
## from each group.
ns = means = ss = numeric()
for (nm in grp.nms) {
this.grp.idx = grp.idx[[nm]]
sample.vals = rep(vals[this.grp.idx], times=freq[this.grp.idx])
means[nm] = mean(sample.vals) # stats for each group
ss[nm] = sum((sample.vals - means[nm])^2)
ns[nm] = length(sample.vals)
}
## Put means and lambdas in the same order
lambdas = lambdas[match(names(lambdas), grp.nms)]
C = sum(lambdas * means) # contrast
sumWts = sum(lambdas^2 / ns) # sum of the weights
SSw = sum(ss)
res = C / sqrt(C^2 + SSw * sumWts)
res
}
```

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