test_whyR_for.R

x <- matrix(rnorm(10^7), ncol=4)
dim(x)

## [1] 2500000       4

# for
mx1 <- rep(NA, nrow(x))
system.time(for(i in 1:nrow(x)) mx1[i] <- max(x[i,]))

##    user  system elapsed 
##   1.544   0.004   1.550

# or apply?
system.time(mx2 <- apply(x, 1, max))

##    user  system elapsed 
##   4.052   0.032   4.085

identical(mx1,mx2)

## [1] TRUE

# why's that??
# 'for' loop is not slow... that memory allocation and object's copying is slow:
mx4 <- NULL
system.time(for(i in 1:nrow(x)) mx4 <- c(mx4, max(x[i,])))

##     user   system  elapsed 
## 6281.944  151.892 6434.270