R for Computational Science

COSC/MATH 201 · Fall 2026

Author: Beau Christ

Basics

Basic arithmetic operators:

1 + 4    # addition
5 - 1    # subtraction
4 * 2    # multiplication
9 / 3    # division
4 ^ 2    # exponentiation
4 %% 2   # modulus

To assign values to variables:

x <- 3
"hello" -> name

Simple data types:

w <- 2.5634  # numeric
x <- TRUE    # logical
y <- "Hi!"   # character
z <- 500L    # integer

To check a data type: class()

Vectors

A vector is a one-dimensional array with a single type

# create a new vector
u <- c(1, 10, 49)

# name the elements
names(v) <- c("A", "B", "C")

# add two vectors (result is vector)
r <- c(1, 5, 7) + c(2, 4, 5)

# sum the values
sum(v)

# compare two vectors (result is logical vector)
a > b

# select elements
a[3]
a[c(1,5)]
a[2:5]

 # average the values in a vector
mean(v)

# logical selection
vect > 0            # returns logical indices
vect[vect > 0]  # returns actual values

Matrices

A matrix is a two-dimensional array with a single data type

# create a new matrix
matrix(1:9, byrow = TRUE, nrow = 3)

# name the rows/columns
colnames(mat) <- c("A", "B", "C")
rownames(mat) <- c("A", "B", "C")
matrix(..., dimnames = list(c("a", "b"), c("c", "d")))

# sum the rows/columns
colSums(mat)
rowSums(mat)

# bind (attach) a new row or column
cbind(mat, new)
rbind(mat, new)

# select values
mat[1,2]
mat[3,]
mat[c(1,6), 4:7]

# perform arithmetic (element-wise)
m / 5
mat1 * mat2

Conditionals and Control Flow

TRUE evaluates to 1, FALSE evaluates to 0

Relational (comparison) operators: ==, !=, <, >, <=, >=

To obtain the first or last few elements:

head(values, 4)   # get the first 4 elements
tail(values, 10)  # get the last 10 elements

Useful tip: sum() can count the number of TRUE values.

Logical (boolean) operators: &, |, !

Conditional (if else) statements:

if (number < 10) {
  if (number < 5) {
    result <- "extra small"
  } else {
    result <- "small"
  }
} else if (number < 100) {
  result <- "medium"
} else {
  result <- "large"
}

Loops

# while loop
while (speed > 30) {
  print("Slow down!")
  speed <- speed - 7
}

# to stop a for/while loop
break

# for loop (version 1)
for (item in vec) {
  print(item)
}

# for loop (version 2)
for (i in 1:length(vec)) {
  print(vec[i])
}

# to continue to the next iteration
next

To concatenate things together: paste() To split a string: strsplit(some_string, split = "")

Functions

Useful built-in functions include:

# mean/average
mean(x, trim = 0.2, na.rm = TRUE)

# standard deviation
sd()

# getting help, can also use '?'
help(sd)
?sd

# for viewing function arguments
args()

# absolute value
abs()

# load a package
library()

# install new packages
install.packages()

# round up or down (and optionally specify the number of digits after the decimal point)
round()
round(3.14159, digits = 2)

# sum values
sum()

# reverse items
rev()

# sort low to high
sort(, decreasing=)

# repeat a value
rep(, times=)

# create a sequence
seq(, , by=)

To create a new function (return statement optional):

sum_abs <- function(x, y) {
  abs(x) + abs(y)
}

Plotting

To do a simple point plot, you provide the x and y values (these will be the x-y points):

x <- seq(0, 2*pi, 0.1)
y <- sin(x)
plot(x, y)
plot(x, y, col = "red", pch = 3)  # Or optionally change the point character and color
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