Functional Programming Patterns

Map Values

Apply One Function

Mapping applies the same transformation to each value and returns the transformed vector.

Program

Play the script to change the multiplier used by the mapped function.

multiplier
map_values.R
Replay: real traced execution (multi-file project)
multiplier <- 2
values <- c(2, 4, 6)
mapped <- vapply(values, function(x) x * multiplier, numeric(1))
total <- sum(mapped)
label <- paste("map", total, sep = ":")
cat(label, "\n", sep = "")

multiplier <- 1
values <- c(2, 4, 6)
mapped <- vapply(values, function(x) x * multiplier, numeric(1))
total <- sum(mapped)
label <- paste("map", total, sep = ":")
cat(label, "\n", sep = "")

multiplier <- 3
values <- c(2, 4, 6)
mapped <- vapply(values, function(x) x * multiplier, numeric(1))
total <- sum(mapped)
label <- paste("map", total, sep = ":")
cat(label, "\n", sep = "")

  1. multiplier ← 2

    1multiplier <- 22values <- c(2, 4, 6)
    values this step2multiplier

  2. values ← 2, 4, 6

    1multiplier <- 22values <- c(2, 4, 6)3mapped <- vapply(values, function(x) x * multiplier, numeric(1))
    values this step2, 4, 6values

  3. mapped ← 4, 8, 12

    2values <- c(2, 4, 6)3mapped <- vapply(values, function(x) x * multiplier, numeric(1))4total <- sum(mapped)
    values this step4, 8, 12mapped2, 4, 6values2multiplier

  4. total ← 24

    3mapped <- vapply(values, function(x) x * multiplier, numeric(1))4total <- sum(mapped)5label <- paste("map", total, sep = ":")
    values this step24total4, 8, 12mapped

  5. label ← map:24

    4total <- sum(mapped)5label <- paste("map", total, sep = ":")6cat(label, "\n", sep = "")
    values this stepmap:24label24total

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

    5label <- paste("map", total, sep = ":")6cat(label, "\n", sep = "")
    outputmap:24
    values this stepmap:24label

  1. multiplier ← 1

    1multiplier <- 12values <- c(2, 4, 6)
    values this step1multiplier

  2. values ← 2, 4, 6

    1multiplier <- 12values <- c(2, 4, 6)3mapped <- vapply(values, function(x) x * multiplier, numeric(1))
    values this step2, 4, 6values

  3. mapped ← 2, 4, 6

    2values <- c(2, 4, 6)3mapped <- vapply(values, function(x) x * multiplier, numeric(1))4total <- sum(mapped)
    values this step2, 4, 6mapped2, 4, 6values1multiplier

  4. total ← 12

    3mapped <- vapply(values, function(x) x * multiplier, numeric(1))4total <- sum(mapped)5label <- paste("map", total, sep = ":")
    values this step12total2, 4, 6mapped

  5. label ← map:12

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

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

    5label <- paste("map", total, sep = ":")6cat(label, "\n", sep = "")
    outputmap:12
    values this stepmap:12label

  1. multiplier ← 3

    1multiplier <- 32values <- c(2, 4, 6)
    values this step3multiplier

  2. values ← 2, 4, 6

    1multiplier <- 32values <- c(2, 4, 6)3mapped <- vapply(values, function(x) x * multiplier, numeric(1))
    values this step2, 4, 6values

  3. mapped ← 6, 12, 18

    2values <- c(2, 4, 6)3mapped <- vapply(values, function(x) x * multiplier, numeric(1))4total <- sum(mapped)
    values this step6, 12, 18mapped2, 4, 6values3multiplier

  4. total ← 36

    3mapped <- vapply(values, function(x) x * multiplier, numeric(1))4total <- sum(mapped)5label <- paste("map", total, sep = ":")
    values this step36total6, 12, 18mapped

  5. label ← map:36

    4total <- sum(mapped)5label <- paste("map", total, sep = ":")6cat(label, "\n", sep = "")
    values this stepmap:36label36total

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

    5label <- paste("map", total, sep = ":")6cat(label, "\n", sep = "")
    outputmap:36
    values this stepmap:36label
function argument The anonymous function receives one value at a time.
map `vapply` applies the function to every input value.
typed result `numeric(1)` states that each mapped result is one number.