Switch statement in R Programming - Step-by-Step Execution

Concept Flow - Switch statement

Start

↓

Evaluate expression

↓

Match case?

No→Default case if exists

Yes↓

Execute matched case

↓

End

The switch statement evaluates an expression, matches it to a case, executes that case, or the default if no match.

Execution Sample

R Programming

x <- "b"
switch(x,
       a = "Apple",
       b = "Banana",
       c = "Cherry",
       "Unknown fruit")

This code uses switch to return a fruit name based on the value of x.

Execution Table

Step	Expression Value	Case Checked	Match?	Action	Output
1	"b"	a	No	Check next case
2	"b"	b	Yes	Return "Banana"	Banana
3	N/A	Stop	N/A	Exit switch	Banana

💡 Match found at case 'b', switch returns 'Banana' and stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	"b"	"b"	"b"	"b"

Key Moments - 2 Insights

What happens if the expression value does not match any case?

If no case matches, the switch returns the default value (last unnamed argument). See execution_table exit_note for no-match scenario.

Does switch check all cases even after a match?

No, switch stops checking after the first match, as shown in execution_table step 2 and 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 2?

A "Cherry"

B "Banana"

C "Apple"

D "Unknown fruit"

💡 Hint

Check the 'Output' column at step 2 in the execution_table.

At which step does the switch stop checking cases?

A Step 3

B Step 1

C Step 2

D It checks all cases

💡 Hint

Look at the 'Action' column in steps 2 and 3 in execution_table.

If x was "d", what would the output be?

A "Cherry"

B "Banana"

C "Unknown fruit"

D "Apple"

💡 Hint

Refer to key_moments about default case when no match is found.

Concept Snapshot

switch(expression, case1 = value1, case2 = value2, ..., default)
- Evaluates expression once
- Matches expression to cases by name or position
- Executes first matching case
- Returns default if no match
- Stops after first match

Full Transcript

The switch statement in R evaluates an expression and compares it to each case in order. When it finds a matching case, it executes that case and stops checking further cases. If no case matches, it returns the default value, which is the last unnamed argument. For example, if x is "b", switch returns "Banana" because it matches the case named "b". If x does not match any case, switch returns the default value like "Unknown fruit". This behavior helps select one value from many options based on a single expression.

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