A.12 Basic Math Functions
abs # absolute value
round # 반올림반버림, round to even number (IMPORTANT !!!)
floor # 버림
ceiling # 올림
exp # exponential
log # logarithmic
sqrt # square root
min # minimum
max # maximum
mean # average
median # median
sum # summation
cumsum # cumulative summation
colSums # sums by columns
colMeans # means by columns
rowSums # sums by rows
rowMeans # means by rows
choose # combination
scale # normalize, standardize
crossprod # cross product of matrix, X'X or X'Y
det # determiant of a matrix
diag # diagonal vector of a matrix or construct a diagonal matrix
eigen # eigenvalues and eigenvectors by spectral decomposition
solve # inverse of a matrix
qr # QR decomposition
svd # singular value decomposition
수학적 표현들을 어떻게 R로 표현하는가 하는 것은 제 1장을 참고한다.