Scripting Loops In R

June 1, 2016
By

(This article was first published on R-exercises, and kindly contributed to R-bloggers)

450px-InvertigoloopAn R programmer can determine the order of processing of commands, via use of the control statements; repeat{}, while(), for(), break, and next

Answers to the exercises are available here.

Exercise 1
The repeat{} loop processes a block of code until the condition specified by the break statement, (that is mandatory within the repeat{} loop), is met.

The structure of a repeat{} loop is:
repeat {
commands
if(condition) {
break
}
}

For the first exercise, write a repeat{} loop that prints all the even numbers from 210, via incrementing the variable, “i <- 0“.

Exercise 2
Using the following variables:

msg <- c("Hello")
i <- 1

Write a repeat{} loop that breaks off the incrementation of, “i“, after 5 loops, and prints “msg” at every increment.

Exercise 3
while() loop will repeat a group of commands until the condition ceases to apply. The structure of a while() loop is:

while(condition) {
commands
}

With, i <- 1, write a while() loop that prints the odd numbers from 1 through 7.

Exercise 4
Using the following variables:

msg <- c("Hello")
i <- 1

Write a while() loop that increments the variable, “i“, 6 times, and prints “msg” at every iteration.

Exercise 5
The for() loop repeats commands until the specified length of the condition is met. The structure of a for() loop is:
for(condition) { commands }

For example:
for(i in 1:4) {
print("variable"[i])
}

for(i in seq("variable")) {
print(i)
}

for(i in seq_along("variable")) {
print("variable"[i])
}

for(letter in "variable") {
print(letter)
}

For this exercise, write a for() loop that prints the first four numbers of this sequence: x <- c(7, 4, 3, 8, 9, 25)

Exercise 6
For the next exercise, write a for() loop that prints all the letters in y <- c("q", "w", "e", "r", "z", "c").

Exercise 7
The break statement is used within loops to exit from the loop. If the break statement is within a nested loop, the inner loop is exited, and the outer loop is resumed.

Using i <- 1, write a while() loop that prints the variable, “i“, (that is incremented from 15), and uses break to exit the loop if “i” equals 3.

Exercise 8
Write a nested loop, where the outer for() loop increments “a3 times, and the inner for() loop increments “b3 times. The break statement exits the inner for() loop after 2 incrementations. The nested loop prints the values of variables, “a” and “b“.

Exercise 9
The next statement is used within loops in order to skip the current evaluation, and instead proceed to the next evaluation.

Therefore, write a while() loop that prints the variable, “i“, that is incremented from 25, and uses the next statement, to skip the printing of the number 3.

Exercise 10
Finally, write a for() loop that uses next to print all values except “3” in the following variable: i <- 1:5

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