The syntax for the if condition is as follows:

    if (test expression) 
    { 
      Statement upon condition is true 
    } 

If the test expression is true, the statement is executed. If the statement is false, nothing happens.

In the following example, an atomic vector x is defined, and it is subjected to two conditions separately, that is, whether x is greater than 5 or less than 5. If either of the conditions gets satisfied, the value x gets printed in the console following the corresponding if statement:

> x <- 10
> if (x < 5) print(x)
> if (x > 5) print(x)
[1] 10

The syntax for the if...else condition is as follows:

    if(test expression) 
    { 
      Statement upon condition is true 
    } else { 
      Statement upon condition is false 
    } 

In this scenario, if the condition in the test expression is true, the statement under the if condition is executed, otherwise the statement under the else condition is executed.

The following is an example in which we define an atomic vector x with a value 10. Then, we verify whether x, when divided by 2, returns 1 (R reads 1 as a Boolean True). If it is True, the value x gets printed as an odd number, else as an even number:

> x=10
> if (x %% 2)
+ {
+   print(paste0(x, " :odd number"))
+ } else {
+   print(paste0(x, " :even number"))
+ }
[1] "10 :even number"

An ifelse() function is a condition used primarily for vectors, and it is an equivalent form of the if...else statement. Here, the conditional function is applied to each element in the vector individually. Hence, the input to this function is a vector, and the output obtained is also a vector. The following is the syntax of the ifelse condition:

    ifelse (test expression, statement upon condition is true,    
    statement upon condition is false)

In this function, the primary input is a test expression which must provide a logical output. If the condition is true, the subsequent statement is executed, else the last statement is executed.

The following is example in which a vector x is defined with integer values from 1 to 7. Then, a condition is applied on each element in vector x to determine whether the element is an odd number or an even number:

> x <- 1:6
> ifelse(x %% 2, paste0(x, " :odd number"), paste0(x, " :even  number"))
[1] "1 :odd number"  "2 :even number" "3 :odd number" 
[4] "4 :even number" "5 :odd number"  "6 :even number"

A for loop is primarily used to iterate a statement over a vector of values in a given sequence. The vector can be of any type, that is, numeric, character, Boolean, or complex. For every iteration, the statement is executed.

The following is the syntax of the for loop:

    for(x in sequence vector) 
    { 
      Statement to be iterated 
    } 

The loop continues till all the elements in the vector get exhausted as per the given sequence.

The following example details a for loop. Here, we assign a vector x, and each element in vector x is printed in the console using a for loop:

> x <- c("John", "Mary", "Paul", "Victoria")
> for (i in seq(x)) {
+   print(x[i])
+ }
[1] "John"
[1] "Mary"
[1] "Paul"
[1] "Victoria"

The nested for loop can be defined as a set of multiple for loops within each for loop as shown here:

    for( x in sequence vector) 
    { 
      First statement to be iterated 
      for(y in sequence vector) 
    { 
      Second statement to be iterated 
      ......... 
    } 
  } 

In a nested for loop, each subsequent for loop is iterated for all the possible times based on the sequence vector in the previous for loop. This can be explained using the following example, in which we define mat as a matrix (3x3), and our desired objective is to obtain a series of summations which will end up with the total sum of all the values within the matrix. Firstly, we initialize sum to 0, and then subsequently, sum gets updated by adding itself to all the elements in the matrix in a sequence. The sequence is defined by the nested for loop, wherein for each row in a matrix, each of the values in all the columns gets added to sum:

> mat <- matrix(1:9, ncol = 3)
> sum <- 0
> for (i in seq(nrow(mat))) 
+ {
+   for (j in seq(ncol(mat))) 
+   {
+     sum <- sum + mat[i, j]
+     print(sum)
+   }
+ }
[1] 1
[1] 5
[1] 12
[1] 14
[1] 19
[1] 27
[1] 30
[1] 36
[1] 45

In R, while loops are iterative loops with a specific condition which needs to be satisfied. The syntax of the while loop is as follows:

    while (test expression) 
   { 
     Statement upon condition is true (iteratively) 
   } 

Let's understand the while loop in detail with an example. Here, an object i is initialized to 1. The test expression which needs to be satisfied for every iteration is i<10. Since i = 1, the condition is TRUE, and the statement within the while loop is evaluated. According to the statement, i is printed on the console, and then increased by 1 unit. Now i increments to 2, and once again, the test expression, whether the condition (i < 10) is true or false, is checked. If TRUE, the statement is again evaluated. The loop continues till the condition becomes false, which, in our case, will happen when i increments to 10. Here, incrementing i becomes very critical, without which the loop can turn into infinite iterations:

> i <- 1
> while (i < 10) 
+ {
+   print(i)
+   i <- i + 1
+ }
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9

In R, the loops can be altered using break or next statements. This helps in inducing other conditions required within the statement inside the loop.

> for (i in 1:30) 
+ {
+   if (i < 8) 
+   {
+     print(paste0("Current value is ",i))
+   } else {
+     print(paste0("Current value is ",i," and the loop breaks"))
+     break
+   }
+ }
[1] "Current value is 1"
[1] "Current value is 2"
[1] "Current value is 3"
[1] "Current value is 4"
[1] "Current value is 5"
[1] "Current value is 6"
[1] "Current value is 7"
[1] "Current value is 8 and the loop breaks"

> for (i in 1:10) 
+ {
+   if (i %% 2) 
+   {
+     print(paste0(i, " is an odd number."))
+   } else {
+     next
+   }
+ }
[1] "1 is an odd number."
[1] "3 is an odd number."
[1] "5 is an odd number."
[1] "7 is an odd number."
[1] "9 is an odd number."

The repeat loop is an infinite loop which iterates multiple times without any inherent condition. Hence, it becomes mandatory for the user to explicitly mention the terminating condition, and use a break statement to terminate the loop.

The following is the syntax for a repeat statement:

    repeat 
    { 
      Statement to iterate along with explicit terminate condition    
      including break statement 
    } 

In the current example, i is initialized to 1. Then, a for loop iterates, within which an object cube is evaluated and verified using a condition whether the cube is greater than 729 or not. Simultaneously, i is incremented by 1 unit. Once the condition is met, the for loop is terminated using a break statement:

> i <- 1
> repeat 
+ {
+   cube <- i ** 3
+   i <- i + 1
+   if (cube < 729) 
+   {
+     print(paste0(cube, " is less than 729. Let's remain in the        
      loop."))
+   } else {
+     print(paste0(cube, " is greater than or equal to 729. Let's exit  
      the loop."))
+     break
+   }
+ }
[1] "1 is less than 729. Let's remain in the loop."
[1] "8 is less than 729. Let's remain in the loop."
[1] "27 is less than 729. Let's remain in the loop."
[1] "64 is less than 729. Let's remain in the loop."
[1] "125 is less than 729. Let's remain in the loop."
[1] "216 is less than 729. Let's remain in the loop."
[1] "343 is less than 729. Let's remain in the loop."
[1] "512 is less than 729. Let's remain in the loop."
[1] "729 is greater than 729. Let's exit the loop."