Compute the correlation by looping through factor levels in a dataframe

dataframe r aggregate correlation

You can use by in base R.

subdat <- dat[order(dat$T), c("y", "F", "Subj")]
by(subdat, subdat$Subj, function(x) with(x, cor(y[F == "f1"], y[F == "f2"])))

Output

subdat$Subj: S1
[1] -0.03755675
--------------------------------------------------------------------------------- 
subdat$Subj: S10
[1] -0.05481364
--------------------------------------------------------------------------------- 
subdat$Subj: S2
[1] 0.2822211
--------------------------------------------------------------------------------- 
subdat$Subj: S3
[1] 0.2671967
--------------------------------------------------------------------------------- 
subdat$Subj: S4
[1] 0.1268404
--------------------------------------------------------------------------------- 
subdat$Subj: S5
[1] 0.0374699
--------------------------------------------------------------------------------- 
subdat$Subj: S6
[1] 0.5655247
--------------------------------------------------------------------------------- 
subdat$Subj: S7
[1] 0.2141196
--------------------------------------------------------------------------------- 
subdat$Subj: S8
[1] 0.250178
--------------------------------------------------------------------------------- 
subdat$Subj: S9
[1] 0.1370734

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