There are a lot of builtin function in R. R matches your input parameters with its function arguments, either by value or by position, then executes the function body. Function arguments can have default values: if you do not specify these arguments, R will take the default value.
 General function
 Maths function
 Statistical function
General functions
diff() function
If you work on time series, you need to stationary the series by taking their lag values. A stationary process allows constant mean, variance and autocorrelation over time. This mainly improves the prediction of a time series. It can be easily done with the function diff(). We can build a random timeseries data with a trend and then use the function diff() to stationary the series. The diff() function accepts one argument, a vector, and return suitable lagged and iterated difference.
length() function
In many cases, we want to know the length of a vector for computation or to be used in a for loop. The length() function counts the number of rows in vector x. The following codes import the cars dataset and return the number of rows.
Math functions
R has an array of mathematical functions.
Operator 
Description 
abs(x) 
Takes the absolute value of x 
log(x,base=y) 
Takes the logarithm of x with base y; if base is not specified, returns the natural logarithm 
exp(x) 
Returns the exponential of x 
sqrt(x) 
Returns the square root of x 
factorial(x) 
Returns the factorial of x (x!)

Statistical functions
Operator 
Description 
mean(x) 
Mean of x 
median(x) 
Median of x 
var(x) 
Variance of x 
sd(x) 
Standard deviation of x 
scale(x) 
Standard scores (zscores) of x 
quantile(x) 
The quartiles of x 
summary(x) 
Summary of x: mean, min, max etc.. 