Vector recycling behavior in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of vector recycling in addition

What is the output of this R code?

c(1, 2, 3, 4) + c(10, 20)

R Programming

c(1, 2, 3, 4) + c(10, 20)

A [1] 11 22 13 24

B [1] 11 22 33 44

C [1] 11 22 31 42

D [1] 11 12 13 14

💡 Hint

Remember that R recycles the shorter vector to match the length of the longer one.

The shorter vector c(10, 20) is repeated to become c(10, 20, 10, 20). Then element-wise addition happens: 1+10=11, 2+20=22, 3+10=13, 4+20=24.

❓ Predict Output

intermediate

Output of vector recycling with logical vectors

What is the output of this R code?

c(TRUE, FALSE, TRUE) & c(FALSE, TRUE)

R Programming

c(TRUE, FALSE, TRUE) & c(FALSE, TRUE)

A [1] FALSE FALSE FALSE

B [1] FALSE FALSE TRUE

C [1] FALSE TRUE FALSE

D [1] TRUE TRUE TRUE

💡 Hint

The shorter vector is recycled to match the longer vector length before element-wise AND.

The shorter vector c(FALSE, TRUE) is recycled to c(FALSE, TRUE, FALSE). Then element-wise AND: TRUE & FALSE = FALSE, FALSE & TRUE = FALSE, TRUE & FALSE = FALSE.

🔧 Debug

advanced

Error caused by vector recycling with incompatible lengths

What error does this R code produce?

c(1, 2, 3) + c(10, 20, 30, 40)

R Programming

c(1, 2, 3) + c(10, 20, 30, 40)

A Error: object lengths differ and cannot be recycled

B Error: non-numeric argument to binary operator

C No error, output is [1] 11 22 33 44

D Warning: longer object length is not a multiple of shorter object length

💡 Hint

R recycles shorter vectors but warns if lengths are not multiples.

The shorter vector length 3 does not divide the longer vector length 4 evenly. R recycles but gives a warning about length mismatch.

🧠 Conceptual

advanced

Understanding recycling with named vectors

Given these named vectors in R:

a <- c(x=1, y=2, z=3)
b <- c(y=10, z=20)

What is the output of a + b?

R Programming

a <- c(x=1, y=2, z=3)
b <- c(y=10, z=20)
a + b

A Error: non-numeric argument to binary operator

B Warning about recycling and output: 1 NA NA

C Numeric vector: 11 22 23

D Warning about recycling and output: NA 12 23

💡 Hint

Named vectors do not recycle by matching names in arithmetic operations.

R ignores names in arithmetic and recycles by position. b is shorter, so recycled to length 3: c(10, 20, 10). Then addition: 1+10=11, 2+20=22, 3+10=13. But since lengths differ and names exist, a warning is issued and names are dropped, resulting in numeric vector without names.

❓ Predict Output

expert

Output length after recycling with complex vectors

What is the length of the vector produced by this R code?

length(c(1, 2, 3, 4, 5) + c(10, 20))

R Programming

length(c(1, 2, 3, 4, 5) + c(10, 20))

A 2

B Error: lengths differ and cannot be recycled

C 5

D 10

💡 Hint

The result length matches the longer vector length after recycling.

The shorter vector c(10, 20) is recycled to length 5: c(10, 20, 10, 20, 10). The addition produces a vector of length 5.

Practice

1. What happens in R when you add two vectors of different lengths, like c(1, 2, 3) + c(4, 5)?

easy

A. R throws an error because vector lengths differ.

B. Only the first two elements are added, and the rest are ignored.

C. The shorter vector is repeated to match the longer vector's length before addition.

D. The result is a vector of length 2 with NA values.

Solution

1. Step 1: Understand vector lengths

   The first vector has length 3, the second has length 2.

2. Step 2: Apply recycling rule

   The shorter vector c(4, 5) is repeated to become c(4, 5, 4) to match length 3.

3. Final Answer:

   The shorter vector is repeated to match the longer vector's length before addition. -> Option C

4. Quick Check:

   Vector recycling repeats shorter vector [OK]

Hint: Short vector repeats to match longer vector length [OK]

Common Mistakes:

• Thinking R throws an error for different lengths
• Assuming vectors must be same length without recycling
• Ignoring recycling and expecting NA values

2. Which of the following is the correct syntax to add two vectors a and b in R, using vector recycling?

easy

A. a + b

B. add(a, b)

C. a & b

D. sum(a, b)

Solution

1. Step 1: Identify vector addition syntax in R

   In R, adding vectors element-wise is done with the + operator.

2. Step 2: Check other options

   add() is not a base R function; & is logical AND; sum() sums all elements, not element-wise.

3. Final Answer:

   a + b -> Option A

4. Quick Check:

   Use + for element-wise vector addition [OK]

Hint: Use + operator for vector addition in R [OK]

Common Mistakes:

• Using sum() which sums all elements, not element-wise
• Using & which is logical AND, not addition
• Assuming add() is a base R function

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

v1 <- c(10, 20, 30, 40)
v2 <- c(1, 2)
v1 + v2

medium

A. [11, 22, 33, 44]

B. [11, 22, 31, 42]

C. [11, 22, 31, 41]

D. Error due to length mismatch

Solution

1. Step 1: Check vector lengths

   v1 has length 4, v2 has length 2.

2. Step 2: Apply recycling

   v2 repeats as c(1, 2, 1, 2). Adding element-wise:
   10+1=11, 20+2=22, 30+1=31, 40+2=42.

3. Final Answer:

   [11, 22, 31, 42] -> Option B

4. Quick Check:

   Recycle shorter vector and add element-wise [OK]

Hint: Repeat shorter vector to match length, then add [OK]

Common Mistakes:

• Adding only first two elements and ignoring recycling
• Expecting error due to different lengths
• Misaligning recycled elements

4. Consider this R code:

x <- c(5, 10, 15)
y <- c(2, 4, 6, 8)
z <- x + y

What is the problem and how to fix it?

medium

A. Lengths differ and 3 does not divide 4; fix by making lengths compatible.

B. No problem; code runs fine with recycling.

C. Syntax error due to missing parentheses; add parentheses.

D. Vectors must be same length; truncate longer vector.

Solution

1. Step 1: Check vector lengths

   x length is 3, y length is 4.

2. Step 2: Understand recycling warning

   3 does not divide 4 evenly, so R will recycle x but give a warning about length mismatch.

3. Step 3: Fix the problem

   Make vector lengths compatible, e.g., use length 6 or 12, or truncate longer vector.

4. Final Answer:

   Lengths differ and 3 does not divide 4; fix by making lengths compatible. -> Option A

5. Quick Check:

   Unequal lengths without exact multiple cause warnings [OK]

Hint: Lengths must be multiples to avoid warnings [OK]

Common Mistakes:

• Ignoring recycling warning
• Assuming code runs without warnings
• Trying to fix with syntax changes

5. You have two vectors:

v1 <- c(3, 6, 9, 12, 15)
v2 <- c(1, 2)

You want to multiply them element-wise without warnings and get a vector of length 7 by repeating both vectors appropriately. Which code achieves this?

hard

A. rep(v1, each=2) * rep(v2, times=5)

B. v1 * v2

C. v1 + v2

D. rep(v1, length.out=7) * rep(v2, length.out=7)

Solution

1. Step 1: Understand desired output length

   You want a length 7 vector, so both vectors must be length 7.

2. Step 2: Use rep() with length.out

   Using rep(v1, length.out=7) repeats v1 to length 7; similarly for v2.

3. Step 3: Multiply element-wise

   Multiplying these repeated vectors gives length 7 without warnings.

4. Final Answer:

   rep(v1, length.out=7) * rep(v2, length.out=7) -> Option D

5. Quick Check:

   Repeat vectors to same length before operation [OK]

Hint: Use rep() with length.out to match lengths [OK]

Common Mistakes:

• Using direct multiplication causing warnings
• Using rep() with each or times incorrectly
• Adding vectors instead of multiplying