While Loop in R

Introduction to While Loop in R

The following article, While Loop in R provides a detailed outline for loops in R. Loops are important part of every programming language. It allows us to execute a set of statements/statements multiple times which is called iteration. Loops are used to repeat an action for several numbers of times or iterate when a condition is satisfied. Loops are also known as control structures. For both the above purposes, we have two different looping statements. We use FOR statement to iterate an action for a definite number of times and use While to iterate a specific action when a logical condition is met.

Control Flow

For Loop in R

Below is an example of using FOR loop statements.

Syntax:

For (variable in sequence)
Expression
Or
For (variable in sequence)
{
Expression 1
Expression 2
}

The expression above represents an action that is to be repeated.

Example

We use FOR loop to print 1 to 10 numbers in R programming.

for ( x in c(1:10) )
print(x)

Output:

[1] 1

[1] 2

[1] 3

[1] 4

[1] 5

[1] 6

[1] 7

[1] 8

[1] 9

[1] 10

Steps

1. In FOR statement, we have defined x from 1 to 10.
2. We print the values of x from 1 to 10

The expression print represents the action we want the program to do that is print the variable x. We have used FOR loop because we are definitive about the number of times a statement needs to be executed. In the above case, it is 10.

While Loop

Let’s say, we are not sure how many times we need to repeat an action or expression to be executed. In such cases, we make use of the While statement with the logical condition.

For the loop, we use curly brackets to wrap the expressions. If it is a single expression, curly brackets aren’t required.

Control Flow

Syntax:

While(condition)
expression
while(condition){
expression 1
expression 2
…
}

Example

a <- 1
b <- 2
while (b > 1){
c <- a + b
b <- 0
print(c)
}

Output:

[1] 3

Steps

1. In the above example, We have initialized a to 1 and b to 2.
2. In the while statement: We have a condition to check if b is greater than 1.
3. We then enter the loop as the condition (b>1) is true.
4. We add both a and b and store in resultant variable C.
5. Print c.

Infinite Loop Sequence

While using the while statement, we need to be cautious in defining the condition/statements. Otherwise, we may end up in an infinite loop.

Example 

while (b > 1)
{
c <- a + b
b <- 0
print(c)
}

If we remove the statement (B <- 0) from the program then, it will lead to an infinite loop because b is defined as 2 at the start and never changes its value through the program. Unless we change its value in the loop. (b <- 0) .This allows the program to print C only once and not infinite times.

Remember, all the statements in FOR and WHILE Loop are executed sequentially.

Loop Control Statements

There are two different types of loop control statements in R.

• Break
• Next

Break statement

The function of the break statement is to bring the execution out of the loop and execute the statements outside the loop if any.

Syntax:

While (condition)
{
Expression 1
Break
}

Example

a <- 1
b <- 2
while (b > 1)
{
c <- a + b
b <- 0
print(c)
break
}
print(b)

Output:

[1] 3

[1] 0

Steps

1. Using the same while example program, we have added a break statement after print statements.
2. We have used the break to come out of the loop and print b.

Next Statement

We use the NEXT statement to skip a statement in the loop.

Syntax:

While (condition)
{
Expression 1
next
skip statement
}

Example

a <- 1
b <- 2
while (b > 1){
c <- a + b
b <- 0
next
print(c)
}
print(b)

Output:

[1] 0

Steps

1. We used the NEXT statement after b <- 0
2. NEXT statement skips the statement (print(c)) and prints b.

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