r - How to iterate through parameters to analyse

Question

Is there a better way to iterate through a set of parameters of a given dataset? Obviously, I try to get a table of correlation coefficients: columns are "CI, CVP, mean PAP, mean SAP", rows are "ALAT, ASAT, GGT, Bili, LDH, FBG". For each combination I′d like to get the correlation coefficient and the significance level (p=...). Below You see "the hard way". But is there a more elegant way, possibly with a printable table?

attach(Liver)
cor.test(CI, ALAT, method = "spearman")
cor.test(CI, ASAT, method = "spearman")
cor.test(CI, GGT, method = "spearman")
cor.test(CI, Bili, method = "spearman")
cor.test(CI, LDH, method = "spearman")
cor.test(CI, FBG, method = "spearman")

cor.test(CVP, ALAT, method = "spearman")
cor.test(CVP, ASAT, method = "spearman")
cor.test(CVP, GGT, method = "spearman")
cor.test(CVP, Bili, method = "spearman")
cor.test(CVP, LDH, method = "spearman")
cor.test(CVP, FBG, method = "spearman")

cor.test(meanPAP, ALAT, method = "spearman")
cor.test(meanPAP, ASAT, method = "spearman")
cor.test(meanPAP, GGT, method = "spearman")
cor.test(meanPAP, Bili, method = "spearman")
cor.test(meanPAP, LDH, method = "spearman")
cor.test(meanPAP, FBG, method = "spearman")

cor.test(meanSAP, ALAT, method = "spearman")
cor.test(meanSAP, ASAT, method = "spearman")
cor.test(meanSAP, GGT, method = "spearman")
cor.test(meanSAP, Bili, method = "spearman")
cor.test(meanSAP, LDH, method = "spearman")
cor.test(meanSAP, FBG, method = "spearman")

detach("Liver")

Answer 1

The trick is to get all possible combinations. Here, I create a data.frame with 10 columns and get all combinations using the combn function. Then its pretty straightforward to obtain the corrleation- and p- values.

set.seed(12)
x <- as.data.frame(matrix(rnorm(100), nrow=10))
combinations <- combn(ncol(x), 2)

out <- apply(combinations, 2, function(idx) {
    t <- cor.test(x[, idx[1]], x[, idx[2]], method = "spearman")
    c(names(x)[idx[1]], names(x)[idx[2]], t$estimate, t$p.value)
})
# more formatting if necessary
out <- as.data.frame(t(out))
names(out) <- c("col.idx1", "col.idx2", "cor", "pval")

Edit: An even more compact code by utilizing FUN argument within combn (as per Greg's suggestion)

set.seed(12)
x <- as.data.frame(matrix(rnorm(100), nrow=10))
out <- as.data.frame(t(combn(ncol(x), 2, function(idx) {
    t <- cor.test(x[, idx[1]], x[, idx[2]], method = "spearman")
    c(names(x)[idx[1]], names(x)[idx[2]], t$estimate, t$p.value)
})))
names(out) <- c("col.idx1", "col.idx2", "cor", "pval")