Performance Awareness

Preallocate Vector

Fill Known Size

When the output size is known, preparing the vector shape before filling it keeps intent clear.

Program

Play the script to change the planned length and watch the filled total update.

item_count
preallocate_vector.R
Replay: real traced execution (multi-file project)
item_count <- 3
result <- numeric(item_count)
result <- seq_len(item_count) * 2
total <- sum(result)
label <- paste("sum", total, sep = ":")
cat(label, "\n", sep = "")

item_count <- 2
result <- numeric(item_count)
result <- seq_len(item_count) * 2
total <- sum(result)
label <- paste("sum", total, sep = ":")
cat(label, "\n", sep = "")

item_count <- 4
result <- numeric(item_count)
result <- seq_len(item_count) * 2
total <- sum(result)
label <- paste("sum", total, sep = ":")
cat(label, "\n", sep = "")

  1. item_count ← 3

    1item_count <- 32result <- numeric(item_count)
    values this step3item_count

  2. result ← 0, 0, 0

    1item_count <- 32result <- numeric(item_count)3result <- seq_len(item_count) * 2
    values this step0, 0, 0result3item_count

  3. result ← 2, 4, 6

    2result <- numeric(item_count)3result <- seq_len(item_count) * 24total <- sum(result)
    values this step2, 4, 6result3item_count

  4. total ← 12

    3result <- seq_len(item_count) * 24total <- sum(result)5label <- paste("sum", total, sep = ":")
    values this step12total2, 4, 6result

  5. label ← sum:12

    4total <- sum(result)5label <- paste("sum", total, sep = ":")6cat(label, "\n", sep = "")
    values this stepsum:12label12total

  6. cat(label, " ", sep = "")

    5label <- paste("sum", total, sep = ":")6cat(label, "\n", sep = "")
    outputsum:12
    values this stepsum:12label

  1. item_count ← 2

    1item_count <- 22result <- numeric(item_count)
    values this step2item_count

  2. result ← 0, 0

    1item_count <- 22result <- numeric(item_count)3result <- seq_len(item_count) * 2
    values this step0, 0result2item_count

  3. result ← 2, 4

    2result <- numeric(item_count)3result <- seq_len(item_count) * 24total <- sum(result)
    values this step2, 4result2item_count

  4. total ← 6

    3result <- seq_len(item_count) * 24total <- sum(result)5label <- paste("sum", total, sep = ":")
    values this step6total2, 4result

  5. label ← sum:6

    4total <- sum(result)5label <- paste("sum", total, sep = ":")6cat(label, "\n", sep = "")
    values this stepsum:6label6total

  6. cat(label, " ", sep = "")

    5label <- paste("sum", total, sep = ":")6cat(label, "\n", sep = "")
    outputsum:6
    values this stepsum:6label

  1. item_count ← 4

    1item_count <- 42result <- numeric(item_count)
    values this step4item_count

  2. result ← 0, 0, 0, 0

    1item_count <- 42result <- numeric(item_count)3result <- seq_len(item_count) * 2
    values this step0, 0, 0, 0result4item_count

  3. result ← 2, 4, 6, 8

    2result <- numeric(item_count)3result <- seq_len(item_count) * 24total <- sum(result)
    values this step2, 4, 6, 8result4item_count

  4. total ← 20

    3result <- seq_len(item_count) * 24total <- sum(result)5label <- paste("sum", total, sep = ":")
    values this step20total2, 4, 6, 8result

  5. label ← sum:20

    4total <- sum(result)5label <- paste("sum", total, sep = ":")6cat(label, "\n", sep = "")
    values this stepsum:20label20total

  6. cat(label, " ", sep = "")

    5label <- paste("sum", total, sep = ":")6cat(label, "\n", sep = "")
    outputsum:20
    values this stepsum:20label
known size The selected count defines the output length before values are filled.
preallocation `numeric(item_count)` creates a vector of the intended size.
filled vector Replacing the zeros with computed values models a predictable fill step.