diff --git a/sose2021/r/w02/P02-1-exerc.R b/sose2021/r/w02/P02-1-exerc.R
new file mode 100644
index 0000000..9a0ad24
--- /dev/null
+++ b/sose2021/r/w02/P02-1-exerc.R
@@ -0,0 +1,31 @@
+NUMERIC_TYPES <- c("double", "integer")
+
+lsq <- function(X, y) {
+    # TODO
+    stopifnot("X may not be empty" = length(X) > 0)
+    stopifnot("y may not be empty" = length(y) > 0)
+    stopifnot("X must be numeric" = typeof(X) %in% NUMERIC_TYPES)
+    stopifnot("y must be numeric" = typeof(y) %in% NUMERIC_TYPES)
+    stopifnot("X must be matrix" = is.matrix(X))
+    stopifnot("y must be vector or matrix with one column" = is.vector(y) || (is.matrix(y) && ncol(y) == 1))
+    stopifnot("dimensions of X and y do not fit" = (is.vector(y) && nrow(X) == length(y)) || (is.matrix(y) && nrow(y) == nrow(X)))
+    stopifnot("y may not contain NA" = all(!is.na(y)))
+    stopifnot("X may not contain NA" = all(!is.na(X)))
+    A <- t(X) %*% X
+    stopifnot("det(t(X) %*% X) must not be zero" = det(A) != 0)
+    solve(A, t(X) %*% y) 
+}
+
+lsq(matrix(1:6, nrow=3), 1:3)
+lsq(matrix(runif(6), nrow=3), matrix(runif(3), ncol=1))
+lsq(matrix(letters[1:6] , nrow=2), 1:3)
+lsq(matrix(1:6, nrow=3), list(1,2,3))
+lsq(1:6, 1:3)
+lsq(matrix(1:6, nrow=3), array(1:3, dim=c(1,1,3)))
+lsq(matrix(1:6, nrow=3), 1:4)
+lsq(matrix(1:6, nrow=3), matrix(1:3, nrow=1))
+lsq(matrix(1:6, nrow=3), matrix(1:6, nrow=3))
+lsq(matrix(double(0), nrow=0, ncol=0), matrix(double(0), nrow=0, ncol=0))
+lsq(matrix(1:6, nrow=3), c(1,NA,3))
+lsq(matrix(c(1:5, NA), nrow=3), 1:3)
+lsq(matrix(c(1,1,2,1,1,2), nrow=3), 1:3)
diff --git a/sose2021/r/w02/P02-2-exerc.R b/sose2021/r/w02/P02-2-exerc.R
new file mode 100644
index 0000000..a0e25b8
--- /dev/null
+++ b/sose2021/r/w02/P02-2-exerc.R
@@ -0,0 +1,35 @@
+my_matrix <- function(vec, nrow=NULL, ncol=NULL, colnames=NULL, rownames=NULL) {
+    stopifnot("at least one of nrow or ncol has to be specified" = !is.null(nrow) || !is.null(ncol))
+    if (is.null(nrow)) {
+        stopifnot("incompatible length" = length(vec) %% ncol == 0)
+        nrow <- length(vec) / ncol
+    } else if (is.null(ncol)) {
+        stopifnot("incompatible length" = length(vec) %% nrow == 0)
+        ncol <- length(vec) / nrow
+    } else if (length(vec) == 1) {
+        vec <- rep(vec, nrow * ncol)
+    } else stopifnot("incompatible length" = length(vec) == nrow * ncol)
+
+    dim(vec) <- c(nrow, ncol)
+    stopifnot("lenght of colnames must be ncol" = is.null(colnames) || length(colnames) == ncol)
+    stopifnot("lenght of rownames must be nrow" = is.null(rownames) || length(rownames) == nrow)
+    dimnames(vec) <- list(rownames, colnames)
+    return(vec)
+}
+
+my_matrix(1:6)
+my_matrix(1:6, ncol=1)
+my_matrix(1:6, ncol=2)
+my_matrix(1:6, ncol=3)
+my_matrix(1:6, ncol=6)
+my_matrix(1:6, ncol=4)
+my_matrix(1:6, nrow=2)
+my_matrix(1:6, nrow=7)
+my_matrix(1:6, ncol=2, nrow=2)
+my_matrix(1:6, ncol=2, nrow=3)
+my_matrix(1:6, ncol=2, nrow=1)
+my_matrix(0, ncol=3, nrow=2)
+my_matrix(1:6, ncol=3, colnames=LETTERS[1:3])
+my_matrix(1:6, ncol=3, colnames=LETTERS[1:2])
+my_matrix(1:6, ncol=3, rownames=letters[24 + 1:2])
+my_matrix(1:6, ncol=3, colnames=LETTERS[1:3], rownames=letters[24 + 1:2])
diff --git a/sose2021/r/w02/P02-3-exerc.R b/sose2021/r/w02/P02-3-exerc.R
new file mode 100644
index 0000000..1467fe6
--- /dev/null
+++ b/sose2021/r/w02/P02-3-exerc.R
@@ -0,0 +1,22 @@
+my_tibble <- function(data) {
+    attr(data, "row.names") <- (1:lengths(data)[1])
+    attr(data, "class") <- c("tbl_df", "tbl", "data.frame")
+    return(data)
+}
+
+library(tibble)
+my_tb <- my_tibble(list(x=1:3, y=letters[1:3]))
+tb <- tibble(x=1:3 , y=letters[1:3])
+identical(tb, my_tb)
+
+my_factor <- function(data) {
+    lvls <- unique(data)
+    vec <- sapply(data, function(x) match(x, lvls), USE.NAMES=FALSE)
+    attr(vec, "levels") <- lvls
+    attr(vec, "class") <- "factor"
+    return(vec)
+}
+
+my_fac <- my_factor (c("a", "b", "a", "a", "c", "c"))
+fac <- factor(c("a", "b", "a", "a", "c", "c"))
+identical(fac, my_fac)
diff --git a/sose2021/r/w02/P02-4-exerc.R b/sose2021/r/w02/P02-4-exerc.R
new file mode 100644
index 0000000..496ed94
--- /dev/null
+++ b/sose2021/r/w02/P02-4-exerc.R
@@ -0,0 +1,56 @@
+collatz <- function(raw_x, max_iter) {
+    stopifnot("Length of x must be 1" = length(raw_x) == 1)
+    x <- as.integer(raw_x)
+    stopifnot("Cannot interprete x as integer" = !is.na(x))
+    sequ <- c(x)
+    len <- NA
+    for (n in (1:(max_iter-1))) {
+        if (x == 1) {
+            len <- n
+            break
+        } else if (x %% 2 == 0) {
+            x <- x / 2
+        } else {
+            x <- 3*x+1
+        }
+        sequ <- c(sequ, x)
+    }
+    return(list(seq=sequ, len=len))
+}
+
+str(collatz(1, 1e4))
+## List of 2
+##  $ seq: int 1
+##  $ len: int 1
+str(collatz(2, 1e4))
+## List of 2
+##  $ seq: int [1:2] 2 1
+##  $ len: int 2
+str(collatz(3, 1e4))
+## List of 2
+##  $ seq: int [1:8] 3 10 5 16 8 4 2 1
+##  $ len: int 8
+str(collatz(3, 5))
+## List of 2
+##  $ seq: int [1:5] 3 10 5 16 8
+str(collatz("4", 1e4))
+## List of 2
+##  $ seq: int [1:3] 4 2 1
+##  $ len: int 3
+str(collatz("four", 1e4))
+## Warning in collatz("four", 10000): NAs introduced by coercion
+## Error in collatz("four", 10000): Cannot interprete x as integer
+str(collatz(1:5, 1e4))
+## Error in collatz(1:5, 10000): Length of x must be 1
+str(collatz(5.0, 1e4))
+## List of 2
+##  $ seq: int [1:6] 5 16 8 4 2 1
+##  $ len: int 6
+str(collatz(5.1, 1e4))
+## List of 2
+##  $ seq: int [1:6] 5 16 8 4 2 1
+##  $ len: int 6
+str(collatz(5.9, 1e4))
+## List of 2
+##  $ seq: int [1:6] 5 16 8 4 2 1
+##  $ len: int 6