R While Loop with Programming Examples

While Loop in R Programming

A while loop in R repeats a block of code for as long as its condition remains TRUE. The condition is tested before each pass, so a while loop can run zero times if the condition is FALSE from the start.

While Loop Syntax in R

Following is the syntax for While Loop in R programming:

while (condition) {
     Exp	
}

R While Loop Flowchart

Note: something inside the loop must eventually make the condition FALSE. If nothing does, the loop runs forever and you have to interrupt R with Esc or Ctrl+C.

while Loop vs for Loop vs repeat Loop in R

R offers three looping constructs, and choosing between them comes down to whether you know in advance how many iterations you need.

# repeat always runs at least once
count <- 1
repeat {
    print(count)
    count <- count + 1
    if (count > 5) break
}

The stock example above is a natural fit for while: you cannot know beforehand how many random draws it will take before the price falls to 45.

While Loop in R: Examples

Example 1

Start with the simplest possible R programming example. You create a counter and add 1 to it on every pass. The loop must be able to end, so the condition explicitly tells R to stop once the variable passes 10.

Note: If you want to see current loop value, you need to wrap the variable inside the function print().

#Create a variable with value 1
begin <- 1

#Create the loop
while (begin <= 10){

#See which loop we are on
cat('This is loop number', begin, '\n')

#add 1 to the variable begin after each loop
begin <- begin+1
print(begin)
}

Output:

## This is loop number 1[1] 2
## This is loop number 2[1] 3
## This is loop number 3[1] 4
## This is loop number 4[1] 5
## This is loop number 5[1] 6
## This is loop number 6[1] 7
## This is loop number 7[1] 8
## This is loop number 8[1] 9
## This is loop number 9[1] 10
## This is loop number 10[1] 11

Example 2

Suppose you bought a stock at 50 dollars. If the price falls to 45 or below you want to short it, otherwise you keep holding. Each pass of the loop moves the price randomly by up to 10 in either direction around 50:

set.seed(123)
# Set variable stock and price
stock <- 50
price <- 50

# Loop variable counts the number of loops 
loop <- 1

# Set the while statement
while (price > 45){

# Create a random price between 40 and 60
price <- stock + sample(-10:10, 1)

# Count the number of loop
loop <- loop + 1

# Print the number of loop
print(loop)
}

Output:

## [1] 2
## [1] 3
## [1] 4
## [1] 5
## [1] 6
## [1] 7

cat('it took',loop,'loop before we short the price. The lowest price is',price)

Output:

## it took 7 loop before we short the price. The lowest price is 40

How to Use break and next in a While Loop

Two keywords change the normal flow of any R loop.

• break exits the loop immediately, skipping every remaining iteration.
• next abandons only the current iteration and jumps straight to the next condition test.

# break: stop as soon as a condition is met
i <- 1
while (i <= 100) {
    if (i * i > 50) {
        cat('First square above 50 is', i * i, '\n')
        break
    }
    i <- i + 1
}

# next: skip the even numbers
i <- 0
while (i < 10) {
    i <- i + 1
    if (i %% 2 == 0) next
    print(i)
}

One trap worth knowing. In a while loop, next jumps back to the condition test without running anything below it. If your counter is incremented at the bottom of the body, next skips that increment and the loop hangs forever. That is why the second example increments before the next statement rather than after it.

Common While Loop Errors and Infinite Loops

Four mistakes account for almost every failed while loop.

• The counter is never updated. Forgetting the increment leaves the condition permanently TRUE. Press Esc in RStudio, or Ctrl+C in a terminal, to interrupt.
• The condition returns NA. If any value in the test is NA, R stops with “missing value where TRUE/FALSE needed”. Guard it with is.na() or with isTRUE().
• The condition is longer than one element. Since R 4.2, a condition of length greater than one is an error rather than a warning. Wrap it in any() or all().
• Floating-point comparison never becomes FALSE. while (x != 0.3) can loop forever because 0.1 + 0.2 is not exactly 0.3. Compare with a tolerance instead, using abs(x – 0.3) > 1e-8.

# Safety valve: cap the number of iterations
iter <- 0
while (price > 45 && iter < 1000) {
    price <- stock + sample(-10:10, 1)
    iter <- iter + 1
}

Adding a maximum iteration count is cheap insurance in any loop whose exit depends on random or external data.

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