How to Use the if and if...else Statements in R

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Jesse John Feb 02, 2024
R R Function
  1. Syntax of the if Statement in R
  2. Correct Form of the else Statement in R
  3. Use Multiple if and else Statements in R
How to Use the if and if...else Statements in R

This article describes the correct syntax of R’s if and if-else conditional constructs.

Note that only the first element is used in the if construct if it is based on rows of a dataframe or vector elements.

Syntax of the if Statement in R

The valid forms of the if construct are as follows:

if(condition)
    statement to execute

if(condition){
    statement 1
    statement 2
}

Without parentheses, the if block will only evaluate one statement or expression if the condition is true. The others will get executed even if the condition is false.

Example code:

j = 15
if(j < 12)
    print(j+1)
    print("This should not print if j >= 12.")
print(j)

Output:

> j = 15
> if(j < 12)
+   print(j+1)
>   print("This should not print if j >= 12.")
[1] "This should not print if j >= 12."
> print(j)
[1] 15

The second statement after if got evaluated because we did not use parentheses.

We need to use parentheses to evaluate multiple statements if the condition is true.

Example code:

j = 53
if(j < 46){
    print(j+1)
    print("This should not print if j >= 46.")}
print(j)

Output:

> j = 53
> if(j < 46){
+   print(j+1)
+   print("This should not print if j >= 46.")}
> print(j)
[1] 53

Correct Form of the else Statement in R

The else keyword must be placed on the same line as the end of the expression that follows if. The following code raises an error.

Example code:

j = 18
if(j < 28)
    print(j)
else
    print("Not lower than 28.")

Output:

> j = 18
> if(j < 28)
+   print(j)
[1] 18
> else
Error: unexpected 'else' in "else"
>   print("Not lower than 28.")
[1] "Not lower than 28."

The correct form is demonstrated below.

Example code:

j = 8
if(j < 15)
    print(j) else
        print("Not less than 15.")

Output:

> j = 8
> if(j < 15)
+   print(j) else
+     print("Not less than 15.")
[1] 8

Using parentheses helps make our code more readable and avoids mistakes. Now, place the else keyword on the same line as the closing parentheses following the if block.

Example code:

k = 101
if(k <= 100){
    print("k is not more than 100.")
} else {
    print("k is more than 100.")
}

Output:

> k = 101
> if(k <= 100){
+   print("k is not more than 100.")
+ } else {
+   print("k is more than 100.")
+ }
[1] "k is more than 100."

Use Multiple if and else Statements in R

There are several points to consider when using a sequence of conditional constructs. These are common in most languages.

  • When using multiple conditions, the sequence in which we write them is significant.

  • If we use a chain of if and else statements, ending with an else statement, the statements corresponding to the first true condition or the else condition will get executed.

  • If we use a sequence of if statements, more than one condition may evaluate to true.

  • If we do not use a last else statement, none of the statements may be executed.

Example code:

jk  = 155

# Wrong Order
if(jk < 300){
    print("Less than 300.")
} else if (jk < 200) {
    print("Less than 200.")
} else if (jk < 100) {
    print("Less than 100.")
}

# Correct Order
if(jk < 100){
    print("Less than 100.")
} else if (jk < 200) {
    print("Less than 200.")
} else if (jk < 300) {
    print("Less than 300.")
}

# Multiple conditions satisfied. No else statements.
if(jk < 100){
    print("Less than 100.")
}
if (jk < 200) {
    print("Less than 200.")
}
if (jk < 300) {
    print("Less than 300.")
}

# Last else to cover remaining cases.
if(jk < 15){
    print("Less than 15.")
} else if (jk < 25) {
    print("Less than 25.")
} else if (jk < 35) {
    print("Less than 35.")
} else {
    print("Not less than 35.")
}

# NO last else; no condition may be met.
if(jk < 15){
    print("Less than 15.")
} else if (jk < 25) {
    print("Less than 25.")
} else if (jk < 35) {
    print("Less than 35.")
}

Output:

> jk  = 155
>
> # Wrong Order
> if(jk < 300){
+   print("Less than 300.")
+ } else if (jk < 200) {
+   print("Less than 200.")
+ } else if (jk < 100) {
+   print("Less than 100.")
+ }
[1] "Less than 300."
>
> # Correct Order
> if(jk < 100){
+   print("Less than 100.")
+ } else if (jk < 200) {
+   print("Less than 200.")
+ } else if (jk < 300) {
+   print("Less than 300.")
+ }
[1] "Less than 200."
>
> # Multiple conditions satisfied. No else statements.
> if(jk < 100){
+   print("Less than 100.")
+ }
> if (jk < 200) {
+   print("Less than 200.")
+ }
[1] "Less than 200."
> if (jk < 300) {
+   print("Less than 300.")
+ }
[1] "Less than 300."
>
> # Last else to cover remaining cases.
> if(jk < 15){
+   print("Less than 15.")
+ } else if (jk < 25) {
+   print("Less than 25.")
+ } else if (jk < 35) {
+   print("Less than 35.")
+ } else {
+   print("Not less than 35.")
+ }
[1] "Not less than 35."
>
> # NO last else; no condition may be met.
> if(jk < 15){
+   print("Less than 15.")
+ } else if (jk < 25) {
+   print("Less than 25.")
+ } else if (jk < 35) {
+   print("Less than 35.")
+ }

To use an if condition on the values of dataframe columns, read about the Vectorized ifelse() Function of R.

Author: Jesse John
Jesse John avatar Jesse John avatar

Jesse is passionate about data analysis and visualization. He uses the R statistical programming language for all aspects of his work.

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