R Built-in Functions

R ships with hundreds of built-in functions covering math, statistics, string manipulation, data structure handling, and more. These come from the base R installation — no packages needed. Knowing the most useful built-in functions saves you from writing code that already exists.

Math Functions

abs(-15)          # 15       absolute value
sqrt(81)          # 9        square root
ceiling(4.2)      # 5        round up
floor(4.8)        # 4        round down
round(3.567, 2)   # 3.57     round to 2 decimal places
trunc(7.9)        # 7        truncate (drop decimal)
exp(1)            # 2.718... e to the power 1
log(100)          # 4.605... natural log
log10(1000)       # 3        log base 10
log2(8)           # 3        log base 2
factorial(6)      # 720
choose(5, 2)      # 10       combinations (5 choose 2)

Statistical Functions

x <- c(12, 45, 23, 67, 34, 56, 78, 29)

mean(x)      # 43
median(x)    # 39.5
var(x)       # 444.57
sd(x)        # 21.08
sum(x)       # 344
prod(x)      # product of all values
cumsum(x)    # running total
cumprod(x)   # running product
cummax(x)    # running maximum
cummin(x)    # running minimum
diff(x)      # differences between consecutive pairs
range(x)     # c(12, 78)
quantile(x, 0.25)  # 1st quartile
cor(x, y)    # correlation coefficient

Sequence and Vector Functions

seq(1, 10, by=2)        # 1 3 5 7 9
seq(0, 1, length.out=5) # 0.00 0.25 0.50 0.75 1.00
rep(c(1,2), times=3)    # 1 2 1 2 1 2
rep(c(1,2), each=3)     # 1 1 1 2 2 2
rev(1:5)                # 5 4 3 2 1
sort(c(3,1,4,1,5))      # 1 1 3 4 5
order(c(3,1,4,1,5))     # 2 4 1 3 5 (sort indices)
unique(c(1,2,2,3,3,3))  # 1 2 3
duplicated(c(1,2,2,3))  # FALSE FALSE TRUE FALSE
table(c("A","B","A","C","B","A"))  # frequency table
which(c(T,F,T,F,T))     # 1 3 5 (positions of TRUE)

String Functions

toupper("hello")          # "HELLO"
tolower("WORLD")          # "world"
nchar("R is great")       # 10
paste("a","b","c")        # "a b c"
paste0("x", 1:3)          # "x1" "x2" "x3"
substr("Hello World",1,5) # "Hello"
gsub("o","0","food")      # "f00d"
trimws("  hi  ")          # "hi"
strsplit("a,b,c",",")     # list("a","b","c")
sprintf("%.2f", 3.14159)  # "3.14"

Type and Check Functions

class(42L)          # "integer"
typeof(3.14)        # "double"
is.numeric(5)       # TRUE
is.character("hi")  # TRUE
is.na(NA)           # TRUE
is.null(NULL)       # TRUE
is.vector(c(1,2))   # TRUE
is.data.frame(df)   # TRUE
exists("myvar")     # TRUE/FALSE

Apply Family

sapply(1:5, sqrt)              # 1.00 1.41 1.73 2.00 2.24
lapply(list(1,4,9), sqrt)      # list of square roots
apply(matrix(1:9,3,3), 1, sum) # row sums
tapply(scores, groups, mean)   # mean per group

Environment Functions

ls()              # list all variables
rm(x)             # remove variable x
gc()              # garbage collection (free memory)
getwd()           # current working directory
setwd("/path")    # set working directory
system.time({     # time how long code takes
  Sys.sleep(0.5)
})

Memorizing every built-in function is impossible — and unnecessary. The goal is knowing what categories of tools exist so you search for the right function when you need it. Use ?function_name for documentation and help.search("topic") to find functions by description.