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.