Study R datasets and functions

Co-authored-by: Copilot <copilot@github.com>

Author: XuhuaHuang

R/README.md

@@ -1,6 +1,6 @@
 # Statistical Programming
 
-## Setup: Installing R and Tools
+## Setup: Installing `R` and Tools
 
 ```r
 install.packages("ggplot2")
@@ -68,14 +68,14 @@ A complete expression is any typed expression that falls into one of the followi
 - Control flow statements
 - Grouping statements
 
-## R Sessions
+## `R` Sessions
 `setwd` and `getwd`
 
 Use `?Syntax` to find operator syntax
 
 If unsure of operator precedence, use parentheses.
 
-## R Objects
+## `R` Objects
 
 All objects created in command line (`RStudio` calls it console) are saved
 in `.GlobalEnv`.
@@ -110,3 +110,112 @@ attributes(cars)
 names(cars)
 dim(cars)
 ```
+
+## `R` Data Objects
+
+- Vectors
+    - **Simplest** data object starting at index `1`
+    - **Ordered** set of values (numeric or character)
+    - `scan`, `c`, `rep`, `:`, `seq`
+    - `length`, `mode`, `class`, `names`
+    - conversion operations: `as.integer`, `as.double`
+    - verification operations: `is.integer`, `is.double`, `is.character`
+    - statistics: `max`, `min`, `mean`, `var`, `sd`
+    - mathematics: `sum`, `rank`, `order`, `round`, `floor`, `ceiling`, `abs`, `sqrt`, `exp`, `sin`, `sign`, `log`, `prod`
+
+- Matrices
+    - **Two-dimensional** data object
+    - `matrix`, `rbind`, `cbind`
+    - `length`, `dim`, `dimnames`, `nrow`, `ncol`
+    - indexing: `A[i, j]`, `x[1, 2:3]`, `x[1:2, 3]`, `x[1,]`, `x[, 2]`
+    - arithmetic: `+`, `-`, `*`, `/`
+    - matrix algebra: `%*%`, `t`, `solve`
+
+- `data.frame`
+    - **Tabular** data object (all objects have the same length)
+    - `data.frame`, `read.table`, `read.csv`, `as.data.frame`, `cbind`, `rbind`, `merge`
+    - `nrow`, `ncol`, `names`, `str`
+    - `is.data.frame`, `is.matrix`
+    - `length`, `mode`, `class`, `names`, `attributes`, `row.names`
+    - indexing: `df[i, j]`, `df$colname`, `df[["colname"]]`
+
+- Arrays
+    - **Multi-dimensional** homogeneous data object (all objects have the same type)
+    - `list`
+    - `array`, `as.array`
+    - `dim`, `dimnames`
+    - indexing: `A[i, j, k]`
+
+- Lists
+    - **Heterogeneous** data object (different types of objects)
+    - `list`, `as.list`
+    - `length`, `mode`, `class`, `attributes`
+    - `names`, `str`
+    - indexing: `L[[i]]`, `L$name`
+
+## Recycling Rule
+
+When performing operations on vectors of different lengths, `R` recycles the shorter vector until it matches the length of the longer vector. If the length of the longer vector is not a multiple of the shorter vector, `R` will issue a warning.
+
+```r
+x <- c(1, 2, 3)
+y <- c(10, 20)
+z <- x + y  # z will be c(11, 22, 13)
+```
+
+## Matrix Algebra
+
+- `+`, `-`, `*`, `/`: element-wise operations
+- `^`: element-wise exponentiation
+- module division: `%%`, `%/%`
+- `%*%`: matrix multiplication, `crossprod`
+- outer product: `outer`
+- `t`: transpose
+- `solve`: inverse of a matrix, or solution of linear equations
+
+### Indexing and Subsetting
+
+- `data[['name']] = NULL` to remove a column from a `data.frame`
+- `data$colname` to access a column in a `data.frame`
+
+### Logical indexing
+
+```r
+x <- c(1, 2, 3, 4, 5)
+x[x > 3]  # returns c(4, 5)
+x[c(T, F, T)]
+```
+
+### Missing values
+
+- `NA` represents missing values
+- `is.na(x)` to check for missing values
+- `x[!is.na(x)]` to remove missing values
+- `na.omit(x)` to remove missing values from a vector or data frame
+- `x[is.na(x)] <- 0` to replace missing values with a specific value (e.g., `0`)
+
+### Find Index(es) of a Specific Value within a Vector
+
+```r
+x <- c(1, 2, 3, 4, 5)
+which(x == 3)  # returns 3
+match(3, x)  # returns 3
+which(x %in% vals)
+which(x > 3)  # returns c(4, 5)
+```
+
+## Reading and Writing Datasets
+
+- `read.table`, `read.csv` to read data from files
+- `write.table`, `write.csv` to write data to files
+- `source` and `dump` to read and write `R` objects
+- `save` and `load` to save and load `R` objects in binary format
+- `sink("output.txt")` and `sink()` to redirect output to a file and stop redirecting output
+
+## `R` Functions
+
+Vectorized logical operations and logical indexes are extremely useful to compare, find, replace data elements.
+
+- Domain: arguments lists
+- Function body: expressions that define the function
+- Range: return values