How to use Ifelse() Function in Vectors of R Programming

A Vectorized if-then-else : The ifelse() Statement
Like the if-then-else construct found in most languages, R also includes a vectorized version, the ifelse() function. This the short form of the traditional IF Else statement.
>ifelse(b,u,v)
where b is a Boolean vector, and u and v are vectors.
The return value is itself a vector; element i is u[i] if b[i] is true, or v[i] if b[i] is false.
Example:
> x <- 1:10
# %% is the mod operator
> y <- ifelse(x %% 2 == 0,5,12) 
> y

[1] 12 5 12 5 12 5 12 5 12 5

Here, we wish to produce a vector in which there is a 5 wherever x is even or a 12 wherever x is odd. So, the actual argument corresponding to the formal argument b is (F,T,F,T,F,T,F,T, F,T). The second actual argument,5, corresponding to u, is treated as (5,5,...)(ten 5s) by recycling. The third argument, 12, is also recycled, to (12,12,...).

Here is another example:
> x <- c(5,2,9,12)
> ifelse(x > 6,2*x,3*x)

[1] 15 6 18 24
We return a vector consisting of the elements of x, either multiplied by 2 or 3, depending on whether the element is greater than 6. The expression x > 6 is a vector of Booleans. If the ith component is true, then the ith element of the return value will be set to the ith element of 2*x; otherwise,it will be set to 3*x[i], and so on.
The advantage of ifelse() over the standard if-then-else construct is that it is vectorized, thus potentially much faster.

if…else statement
The syntax of if…else statement is:
if (test_expression) {
statement1
} else {
statement2
}
The else part is optional and is only evaluated if test_expression is FALSE.
It is important to note that else must be in the same line as the closing braces of the if statement. Example :
x <- -7
if(x > 0){
print("Non-negative number")
} else {
print("Negative number")
}

Output :
[1] "Negative number"

The above conditional can also be written in a single line as follows.
if(x > 0) print("Non-negative number") else print("Negative number")
This feature of R allows us to write construct as shown below.
> x <- -7
> y <- if(x > 0) 5 else 6
> y
[1] 6

nested if…else
The if…else ladder (if…else…if) statement allows you execute a block of code among more than 2 alternatives
The syntax of if…else statement is:
if ( test_expression1) {
statement1
} else if ( test_expression2) {
statement2
} else if ( test_expression3) {
statement3
} else {
statement4
}
Only one statement will get executed depending upon the test_expressions.Example :
x <- 0
if (x < 0) {
print("Negative number")
} else if (x > 0) {
print("Positive number")
} else
print("Zero")

Output
[1] "Zero"

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