Control Structures
Control structures in R allow you to control the flow of execution of the program, depending on
runtime conditions. Common structures are：
if, else: testing a condition
for: execute a loop a fixed number of times
while: execute a loop while a condition is true
repeat: execute an infinite loop
break: break the execution of a loop
next: skip an interation of a loop
return: exit a function
Most control structures are not used in interactive sessions, but rather when writing functions or
longer expresisons
Control Structures: if
if(<condition>) { ## do something
} else { ## do something else
}
if(<condition1>) { ## do something
} else if(<condition2>) { ## do something different
} else { ## do something different
}
例:
if(x > 3) {
y <- 10
} else {
y <- 0
}
Of course, the else clause is not necessary
if(<condition1>) {
}
if(<condition2>) {
}
for
for loops take an interator variable and assign it successive values from a sequence or vector. For loops are most commonly used for iterating over the elements of an object (list, vector, etc.)
for(i in 1:10) {
print(i)
}
This loop takes the i variable and in each iteration of the loop gives it values 1, 2, 3, ..., 10, and then exits.
These following loops have the same behavior:
x <- c("a", "b", "c", "d")
for(i in 1:4) {
print(x[i])
}
for(i in seq_along(x)) {
print(x[i])
}
for(letter in x) {
print(letter)
}
for(i in 1:4) print(x[i])
Nested for loops
for loops can be nested.
x <- matrix(1:6, 2, 3)
for(i in seq_len(nrow(x))) {
for(j in seq_len(ncol(x))) {
print(x[i, j])
}
}
Be careful with nesting though. Nesting beyond 2–3 levels is often very difficult to read/understand
While
While loops begin by testing a condition. If it is true, then they execute the loop body. Once the loop body is executed, the condition is tested again, and so forth
count <- 0
while(count < 10) {
print(count)
count <- count + 1
}
While loops can potentially result in infinite loops if not written properly. Use with care!
Sometimes there will be more than one condition in the test
z <- 5
while(z >= 3 && z <= 10) {
print(z)
coin <- rbinom(1, 1, 0.5)
if(coin == 1) { ## random walk
z <- z + 1
} else {
z <- z - 1
}
}
Conditions are always evaluated from left to right.
Repeat
Repeat initiates an infinite loop; these are not commonly used in statistical applications but they do have their uses. The only way to exit a repeat loop is to call break.
x0 <- 1
tol <- 1e-8
repeat {
x1 <- computeEstimate()
if(abs(x1 - x0) < tol) {
break
} else {
x0 <- x1
}
}
The loop in the previous slide is a bit dangerous because there’s no guarantee it will stop. Better to set a hard limit on the number of iterations (e.g. using a for loop) and then report whether convergence was achieved or not.
next, return
next is used to skip an iteration of a loop
for(i in 1:100) {
if(i <= 20) {
## Skip the first 20 iterations
next
}
## Do something here
}
return signals that a function should exit and return a given value
Summary
Control structures like if, while, and for allow you to control the flow of an R program
Infinite loops should generally be avoided, even if they are theoretically correct.
Control structures mentiond here are primarily useful for writing programs; for command-line interactive work, the *apply functions are more useful.