ch3

notes.R

@@ -48,3 +48,46 @@ decode
 decode[mat+1]
 
 skim(mat)
+
+`if`(FALSE,1,3:4) ## This is suposedly better than ifelse
+if (TRUE) 1 else 3:4
+
+## 3.4.5.1
+
+cv_split <- function(x, n){
+  indices <- sample(rep_len(seq_len(n),length(x)))
+  split(x,indices)
+}
+
+cv_split(sample(30),4)
+
+
+## 3.4.5.2
+
+set.seed(1);(x <- rnorm(10))
+
+mean(x)
+
+sample(100,replace = TRUE)
+
+s <- replicate(1000, {
+  x2 <- sample(x, replace = TRUE)
+  mean(x2)
+  })
+
+ci <- quantile(s,probs=c(.025,.975))
+
+hist(s)
+abline(v=ci)
+
+# 3.4.5.3
+my_mtcars <- mtcars[c("mpg", "hp")]
+my_mtcars$my_col <- sample(c("mpg", "hp"), size = nrow(my_mtcars), replace = TRUE)
+head(my_mtcars)
+
+
+# Selects only col 1:2 to keep as numeric when converted to matrix. 
+# Subsets by a matrix by match function. Much faster than for loop
+my_mtcars$my_val <- my_mtcars[1:2][cbind(seq_along(my_mtcars$my_col),
+                     match(my_mtcars$my_col,
+                           colnames(my_mtcars)[1:2]))]