Switch statement in R Programming

Introduction

A switch statement helps you choose one action from many options based on a value. It makes your code cleaner and easier to read than many if-else checks.

You want to run different code depending on a variable's value.

You have many possible cases to check and want to avoid long if-else chains.

You want to make your code easier to understand by grouping choices clearly.

You want to return different results based on user input or function arguments.

Syntax

R Programming

switch(expression, case1 = value1, case2 = value2, ..., default)

The expression is what you check against the cases.

If the expression matches a case name, the corresponding value is returned.

Examples

This checks the number 2 and returns the second value, which is "two".

R Programming

result <- switch(2, "one", "two", "three")
print(result)

This checks the string "b" and returns "banana" because it matches case b.

R Programming

result <- switch("b", a = "apple", b = "banana", c = "cherry")
print(result)

Since "x" does not match any case, it returns the default value "default fruit".

R Programming

result <- switch("x", a = "apple", b = "banana", c = "cherry", "default fruit")
print(result)

Sample Program

This program uses a switch statement to find the meaning of a color. It prints "go" because the choice is "green".

R Programming

choice <- "green"
color_meaning <- switch(choice,
                       red = "stop",
                       yellow = "caution",
                       green = "go",
                       "unknown color")
print(color_meaning)

Important Notes

If no case matches and no default is given, switch() returns NULL.

Case names can be numbers or strings, but they must match the expression type.

Switch is useful for simple choices but not for complex conditions.

Summary

Switch lets you pick one result from many options based on a value.

It makes code cleaner than many if-else statements.

Always include a default case to handle unexpected values.

Practice

1. What does the switch() function in R primarily do?

easy

A. Selects a value from multiple options based on a given expression

B. Repeats a block of code multiple times

C. Defines a new function

D. Creates a loop that runs until a condition is false

Solution

1. Step 1: Understand the purpose of switch()

   The switch() function evaluates an expression and returns a value based on matching cases.

2. Step 2: Compare with other options

   Options A, C, and D describe loops or function definitions, which are not the role of switch().

3. Final Answer:

   Selects a value from multiple options based on a given expression -> Option A

4. Quick Check:

   Switch chooses one option based on input [OK]

Hint: Switch picks one matching case value quickly [OK]

Common Mistakes:

• Confusing switch with loops like for or while
• Thinking switch defines functions
• Missing the idea of selecting based on expression

2. Which of the following is the correct syntax to use switch() in R?

easy

A. switch(expression, case1 = value1, case2 = value2, default = value3)

B. switch { expression; case1: value1; case2: value2; default: value3 }

C. switch(expression) { case1 => value1; case2 => value2; default => value3 }

D. switch(expression, case1: value1, case2: value2, default: value3)

Solution

1. Step 1: Recall R's switch syntax

   In R, switch() uses named arguments for cases like case1 = value1.

2. Step 2: Check each option

   switch(expression, case1 = value1, case2 = value2, default = value3) matches R syntax. Options A, B, and C use syntax styles from other languages or incorrect separators.

3. Final Answer:

   switch(expression, case1 = value1, case2 = value2, default = value3) -> Option A

4. Quick Check:

   R switch uses named arguments with = [OK]

Hint: Use = for cases inside switch in R [OK]

Common Mistakes:

• Using colon : instead of equals = for cases
• Trying to use curly braces {} with switch
• Confusing R syntax with other languages

3. What is the output of the following R code?

result <- switch("b", a = 10, b = 20, c = 30, 40)
print(result)

medium

A. 10

B. 20

C. 30

D. 40

Solution

1. Step 1: Identify the switch expression and cases

   The expression is "b". The cases are a=10, b=20, c=30, and a default 40.

2. Step 2: Match the expression to a case

   Since the expression matches case "b", the value 20 is returned.

3. Final Answer:

   20 -> Option B

4. Quick Check:

   Expression "b" matches case b = 20 [OK]

Hint: Match expression string to named case [OK]

Common Mistakes:

• Choosing default value instead of matching case
• Confusing case names with values
• Forgetting switch returns matched case value

4. Identify the error in this R switch statement:

switch(2,
  1 = "one",
  2 = "two",
  3 = "three"
)

medium

A. Switch expression should be a string, not a number

B. Missing a default case

C. Incorrect use of commas between cases

D. Using numeric cases with = instead of unnamed cases

Solution

1. Step 1: Understand switch with numeric expression in R

   When the expression is numeric, switch expects unnamed arguments, not named with =.

2. Step 2: Check the code

   The code uses named cases like 1 = "one", which is invalid syntax for numeric switch.

3. Final Answer:

   Using numeric cases with = instead of unnamed cases -> Option D

4. Quick Check:

   Numeric switch uses unnamed cases, not named with = [OK]

Hint: Use unnamed cases for numeric switch, named for string [OK]

Common Mistakes:

• Using = with numeric cases causes syntax error
• Assuming default case is mandatory
• Thinking switch expression must be string

5. You want to write a switch statement in R that returns "small", "medium", or "large" for numeric input 1, 2, or 3 respectively, and "unknown" for any other number. Which code correctly implements this?

hard

A. switch(as.character(x), '1' = "small", '2' = "medium", '3' = "large", "unknown")

B. switch(x, '1' = "small", '2' = "medium", '3' = "large", "unknown")

C. switch(x, "small", "medium", "large", "unknown")

D. switch(x, 1 = "small", 2 = "medium", 3 = "large", default = "unknown")

Solution

1. Step 1: Understand switch behavior with numeric input

   For numeric input matching positions 1,2,3, use unnamed positional arguments, placing "unknown" as the 4th for other small numbers.

2. Step 2: Analyze each option

   A, C use string literals like '1' = which cause syntax error (unexpected =). D uses numeric literals 1 = which cause syntax error (unexpected =). B uses unnamed arguments, correct for numeric positional matching.

3. Final Answer:

   switch(x, "small", "medium", "large", "unknown") -> Option C

4. Quick Check:

   Numeric switch uses unnamed positional args [OK]

Hint: Unnamed args for numeric switch positions [OK]

Common Mistakes:

• Using numeric cases with = causes syntax error
• Not providing a default value for unmatched cases
• Mixing numeric expression with string case names