Switch statement in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a switch statement in R?

A switch statement in R lets you choose one action from many based on a value. It helps run different code depending on the input.

beginner

How do you write a basic switch statement in R?

Use switch(value, case1 = result1, case2 = result2, ...). R checks the value and runs the matching case.

intermediate

What happens if no case matches in a switch statement?

If no case matches, switch returns NULL or the default if provided as the last unnamed argument.

intermediate

Can switch in R use numeric values as cases?

Yes, switch can use numeric values. When the first argument is numeric, it picks the case by position number.

beginner

Example: What does switch('b', a = 1, b = 2, c = 3) return?

It returns 2 because the value 'b' matches the case named 'b'.

In R, what does switch(2, 'apple', 'banana', 'cherry') return?

A 'banana'

B 'apple'

C 'cherry'

D NULL

When the first argument is numeric, switch returns the case at that position. Position 2 is 'banana'.

What does switch('x', a = 1, b = 2) return if 'x' is not a case?

A NULL

B 2

C 1

D Error

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

Which of these is the correct way to add a default case in switch?

A switch(x, a=1, b=2, default=3)

B switch(x, a=1, b=2, 3)

C switch(x, a=1, b=2, else=3)

D switch(x, a=1, b=2, default())

The last unnamed argument acts as the default if no named case matches.

If you want to select the third case by position, what should the first argument be?

A NULL

B '3'

C 'c'

D 3

When the first argument is numeric, switch picks the case by position number.

What type of values can the first argument of switch be?

A Only numeric

B Only character strings

C Either numeric or character

D Only logical

The first argument can be numeric (to pick by position) or character (to pick by name).

Explain how the switch statement works in R and how it differs when the first argument is numeric versus character.

Think about how you pick an option from a list by number or by name.

Describe how to provide a default case in an R switch statement and what happens if no case matches and no default is given.

What happens if you ask for a flavor not on the menu?

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