General

builtins() # List all built-in functions
options()  # Set options to control how R computes & displays results

?NA        # Help page on handling of missing data values
abs(x)     # The absolute value of "x"
append()   # Add elements to a vector
c(x)       # A generic function which combines its arguments
cat(x)     # Prints the arguments
cbind()    # Combine vectors by row/column (cf. "paste" in Unix)
diff(x)    # Returns suitably lagged and iterated differences
gl()       # Generate factors with the pattern of their levels
grep()     # Pattern matching
identical()  # Test if 2 objects are *exactly* equal
jitter()     # Add a small amount of noise to a numeric vector
julian()     # Return Julian date
length(x)    # Return no. of elements in vector x
ls()         # List objects in current environment
mat.or.vec() # Create a matrix or vector
paste(x)     # Concatenate vectors after converting to character
range(x)     # Returns the minimum and maximum of x
rep(1,5)     # Repeat the number 1 five times
rev(x)       # List the elements of "x" in reverse order
seq(1,10,0.4)  # Generate a sequence (1 -> 10, spaced by 0.4)
sequence()     # Create a vector of sequences
sign(x)        # Returns the signs of the elements of x
sort(x)        # Sort the vector x
order(x)       # list sorted element numbers of x
tolower(),toupper()  # Convert string to lower/upper case letters
unique(x)      # Remove duplicate entries from vector
system("cmd")  # Execute "cmd" in operating system (outside of R)
vector()       # Produces a vector of given length and mode

formatC(x)     # Format x using 'C' style formatting specifications
floor(x), ceiling(x), round(x), signif(x), trunc(x)   # rounding functions

Sys.getenv(x)  # Get the value of the environment variable "x"
Sys.putenv(x)  # Set the value of the environment variable "x"
Sys.time()     # Return system time
Sys.Date()     # Return system date
getwd()        # Return working directory
setwd()        # Set working directory
?files         # Help on low-level interface to file system
list.files()   # List files in a give directory
file.info()    # Get information about files

# Built-in constants:
pi,letters,LETTERS   # Pi, lower & uppercase letters, e.g. letters[7] = "g"
month.abb,month.name # Abbreviated & full names for months

Maths

log(x),logb(),log10(),log2(),exp(),expm1(),log1p(),sqrt() # Fairly obvious 
cos(),sin(),tan(),acos(),asin(),atan(),atan2() # Usual stuff 
cosh(),sinh(),tanh(),acosh(),asinh(),atanh() # Hyperbolic functions 
union(),intersect(),setdiff(),setequal() # Set operations 
+,-,*,/,^,%%,%/% # Arithmetic operators 
<,>,=,==,!= # Comparison operators 
eigen() # Computes eigenvalues and eigenvectors 
deriv() # Symbolic and algorithmic derivatives of simple expressions 
integrate() # Adaptive quadrature over a finite or infinite interval. 
sqrt(),sum() 

?Control # Help on control flow statements (e.g. if, for, while) 
?Extract # Help on operators acting to extract or replace subsets of vectors 
?Logic # Help on logical operators 
?Mod # Help on functions which support complex arithmetic in R 
?Paren # Help on parentheses 
?regex # Help on regular expressions used in R 
?Syntax # Help on R syntax and giving the precedence of operators 
?Special # Help on special functions related to beta and gamma functions

Graphical

log(x),logb(),log10(),log2(),exp(),expm1(),log1p(),sqrt() # Fairly obvious 
cos(),sin(),tan(),acos(),asin(),atan(),atan2() # Usual stuff 
cosh(),sinh(),tanh(),acosh(),asinh(),atanh() # Hyperbolic functions 
union(),intersect(),setdiff(),setequal() # Set operations 
+,-,*,/,^,%%,%/% # Arithmetic operators 
<,>,=,==,!= # Comparison operators 
eigen() # Computes eigenvalues and eigenvectors 
deriv() # Symbolic and algorithmic derivatives of simple expressions 
integrate() # Adaptive quadrature over a finite or infinite interval. 
sqrt(),sum() 

?Control # Help on control flow statements (e.g. if, for, while) 
?Extract # Help on operators acting to extract or replace subsets of vectors 
?Logic # Help on logical operators 
?Mod # Help on functions which support complex arithmetic in R 
?Paren # Help on parentheses 
?regex # Help on regular expressions used in R 
?Syntax # Help on R syntax and giving the precedence of operators 
?Special # Help on special functions related to beta and gamma functions

Fitting / regression / optimisation

lm # Fit liner model 
glm # Fit generalised linear model 
nls # non-linear (weighted) least-squares fitting 
lqs # "library(MASS)" resistant regression 
optim # general-purpose optimisation 
optimize # 1-dimensional optimisation 
constrOptim # Constrained optimisation 
nlm # Non-linear minimisation 
nlminb # More robust (non-)constrained non-linear minimisationR

Statistical

help(package=stats) # List all stats functions 
?Chisquare # Help on chi-squared distribution functions 
?Poisson # Help on Poisson distribution functions 
help(package=survival) # Survival analysis 
cor.test() # Perform correlation test 
cumsum(); cumprod(); cummin(); cummax() # Cumuluative functions for vectors 
density(x) # Compute kernel density estimates 
ks.test() # Performs one or two sample Kolmogorov-Smirnov tests 
loess(), lowess() # Scatter plot smoothing 
mad() # Calculate median absolute deviation 
mean(x), weighted.mean(x), median(x), min(x), max(x), quantile(x) 
rnorm(), runif() # Generate random data with Gaussian/uniform distribution 
splinefun() # Perform spline interpolation 
smooth.spline() # Fits a cubic smoothing spline 
sd() # Calculate standard deviation 
summary(x) # Returns a summary of x: mean, min, max etc. 
t.test() # Student's t-test 
var() # Calculate variance 
sample() # Random samples & permutations 
ecdf() # Empirical Cumulative Distribution Function 
qqplot() # quantile-quantile plot

Other Useful Links :

HTTP://WWW.SR.BHAM.AC.UK/~AJRS/R/R-FUNCTION_LIST.HTML

http://www.columbia.edu/~cjd11/charles_dimaggio/DIRE/resources/R/rFunctionsList.pdf https://www.statmethods.net/management/functions.html