weather <- data.frame(
month = a <- c('J','F','M','A','M','J','J','A','S','O','N','D'),
precip = b <- c(78,62,74,78,76, 83, 91, 93, 93,78, 93, 81),
max = c <- c(-6, -4, 2, 11, 19, 24, 26, 25, 20, 13, 5, -2),
min = d <- c(-15, -13, -7, 1, 8, 13, 16, 14, 9, 3, -2, -10) )
(weather$L.precip <- shift(weather$precip, -1) )
(weather$precip.L <- shift(weather$precip, 1) )
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