Chapter6 - Chapter 6 R Chapter 6 Advanced Programming in R...

Chapter 6 R
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Chapter 6 Advanced Programming in R
Now that you have learned the elementary commands in R and many ways of applying
them, it is time to discover its advanced functionalities. This chapter introduces grouped
expressions, conditional execution, loops, and deals more intensively with writing
functions.
6.1 Grouped expressions
R is an expression language in the sense that its only command type is a function or
expression which returns a result. Even an assignment is an expression whose result is the
value assigned, and it may be used wherever any expression may be used; in particular
multiple assignments are possible.
Commands may be grouped together in braces,
{expr1 ; expr2 ;...; exprm}
in which case the value of the group is the result of the last expression in the group
evaluated. Since such a group is also an expression it may, for example, be itself included
in parentheses and used a part of an even larger expression, and so on.
6.2 Conditional execution: if statements
1. The language has available a conditional construction of the form
> if (expr_1) expr_2 else expr_3
where expr_1 must evaluate to a single logical value and the result of the entire
expression is then evident.
2. Here is an example if an “if” statement:
>
if (x < 3) print("x less than 3") else print ("x not less than 3")

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3. The “short-circuit” operators && and || are often used as part of the condition in an if
statement. Whereas & and | apply element-wise to vectors, && and || apply to vectors of
length one, and only evaluate their second argument if necessary.
4. If you want statement 1 and / or statement 2 to consist of more than one statement, then
the if construct looks like this:
> if (condition) {
statement 1a
statement 1b
statement 1c
} else {
statement 2a
statement 2b
statement 2c
}
The group of statements between a
{
and a
}
are treated as one statement by the if and
else .
6. There is a vectorized version of the if/else construct, the ifelse() function. This has
the form ifelse(condition, a, b) and returns a vector of the length of its longest
argument, with elements a[i] if condition[i] is true, otherwise b[i] .
x <- c(1,2,3)
y <- c(4,5,2)
(x>2)&(y>1)
(x>2)&&(y>1)

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6.3 Iterations
An iteration is, in principle, a loop or repeatedly executed instruction cycle, with only a
few changes in each cycle. In programming languages that are not matrix or array-oriented,
like C, Fortran, even a simple matrix multiplication needs three nested loops (over rows,
columns, and the indices). Since R is matrix-oriented, these operations are much more
efficient and easy to formulate in mathematical terms. This means they are faster than
loops and the code is much easier to read and write.