Loops are used to repeat the same code multiple times. They are constructed using reserved words.

In R, there are three loop structures:

repeat {
  # Chunk of code to be repeated...
}

while (condition) {
  # Chunk of code to be repeated...
} 

for (variable in sequence) {
  # Chunk of code to be repeated...
}

mysum <- 0
for (i in seq(1,10)) {
  mysum <- mysum + i
}
mysum

## [1] 55

days <- c("Monday", "Tuesday", "Wednesday")
for (i in 1:length(days)) {
  cat("Day ", i, " of the week: ", days[i], "\n", sep="")
}

## Day 1 of the week: Monday
## Day 2 of the week: Tuesday
## Day 3 of the week: Wednesday

mysum <- 0
while (mysum < 10) {
  mysum <- mysum + 1
  
}
mysum

## [1] 10

Beware of infinite loops!

while (mysum < 100) {
  mysum <- mysum - 1
  
}

x <- 1
repeat {
  
  print(x)
  x <- x + 1
  if (x == 6) { 
    break
  }

}

## [1] 1
## [1] 2
## [1] 3
## [1] 4
## [1] 5

The if/else statement executes a block of code if a specified condition is TRUE. If the condition is FALSE, another block of code is executed.

temperature <- 39 #°C
fever_thresh <- 37.2

if (temperature > fever_thresh) {
  cat("Ouch, you have a fever. Stay in bed!\n")
  
}  else {
  cat("Don't panic! Your body temperature is normal.\n")
  
}

## Ouch, you have a fever. Stay in bed!

The if/else statement can be used to consider more than two conditions.

glycemia <- 100 # mg/dL
hypo_thresh <- 70
hyper_thresh <- 130

if (glycemia < hypo_thresh) {
  cat("Hypoglycemia\n")
}  else  if (glycemia > hyper_thresh) {
  cat("Hyperglycemia\n")
} else {
  cat("Normoglycemia\n")
}

## Normoglycemia

break can be used to interrupt a loop

x <- c(0, 1, 22, 100, 5, 8, 90, 6, 100)
wantedNum <- 100 # Number we are looking for
for (i in 1:length(x)) {
  if (x[i] == wantedNum) {
    cat(wantedNum, " found after ", i, " iterations.\n", sep="")
    break
  }
}

## 100 found after 4 iterations.

i

## [1] 4

next can be used to skip a loop iteration

x <- c(0, 1, 22, -9, 5, 8)
sumPosX <- 0 # Sum of all positive numbers in x
for (i in 1:length(x)) {
  
  if (x[i] > 0) {
    sumPosX <- sumPosX + x[i]
    
  } else {
    next
  }
}
sumPosX

## [1] 36