Control Structures
Control structures in R allow you tocontrol 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 istrue
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 ininteractive 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 andassign it successive values from a sequence or vector. For loops are mostcommonly used for iterating over the elements of an object (list, vector, etc.)
for(iin 1:10) {
print(i)
}
This loop takes the i variable and in eachiteration of the loop gives it values 1, 2, 3, ..., 10, and then exits.
These following loops have the samebehavior:
x<- c("a", "b", "c", "d")
for(iin 1:4) {
print(x[i])
}
for(iin seq_along(x)) {
print(x[i])
}
for(letterin x) {
print(letter)
}
for(iin 1:4) print(x[i])
Nested for loops
for loops can be nested.
x<- matrix(1:6, 2, 3)
for(iin seq_len(nrow(x))) {
for(j in seq_len(ncol(x))) {
print(x[i, j])
}
}
Be careful with nesting though. Nestingbeyond 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 ininfinite loops if not written properly. Use with care!
Sometimes there will be more than onecondition 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 leftto right.
Repeat
Repeat initiates an infinite loop; theseare 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 bitdangerous because there’s no guarantee it will stop. Better to set a hard limiton the number of iterations (e.g. using a for loop) and then report whetherconvergence was achieved or not.
next, return
next is used to skip an iteration of a loop
for(iin 1:100) {
if(i <= 20) {
## Skip the first 20 iterations
next
}
## Do something here
}
return signals that a function should exitand return a given value
Summary
Control structures like if, while, and forallow 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 areprimarily useful for writing programs; for command-line interactive work, the*apply functions are more useful.
