List Operations

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Lua's cunning syntactic sugar for using tables as lists is just waiting to be exploited... Many of these functions are unashamedly functional in flavour, and very useful, though if you're not a functional programmer you may take a while to get used to them.

-- Map a function over a list

function map(f, l)

  local m = {}

  for i = 1, getn(l) do m[i] = f(l[i]) end

  return m

end



-- Map a function over a list of lists

function mapCall(f, l)

  local m = {}

  for i = 1, getn(l) do m[i] = call(f, l[i]) end

  return m

end



-- Apply a function to each element of a list

function apply(f, l)

  for i = 1, getn(l) do f(l[i]) end

end



-- Execute the members of a list as assignments (assumes the keys are

-- strings)

function assign(l)

  foreach(l, function (i, v) setglobal(i, v) end)

end



-- Turn a table into a list of lists

function listify(t)

  local l = {}

  foreach(t, function (i, v) tinsert(%l, {i,v}) end)

  return l

end



-- Call a function with values from 1..n, returning a list of results

function loop(n, f)

  local l = {}

  for i = 1, n do tinsert(l, f(i)) end

  return l

end



-- Concatenate two lists and return the result

function concat(l, m)

  local n = {}

  foreachi(l, function (i, v) tinsert(%n, v) end)

  foreachi(m, function (i, v) tinsert(%n, v) end)

  return n

end



-- Reverse a list and return the result

function reverse(l)

  local m = {}

  for i = getn(l), 1, -1 do tinsert(m, l[i]) end

  return m

end



-- Zip some lists together with a function

function zipWith(f, ls)

  local m, len = {}, getn(ls)

  for i = 1, call(max, map(getn, ls)) do

    local t = {}

    for j = 1, len do tinsert(t, ls[j][i]) end

    tinsert(m, call(f, t))

  end

  return m

end



-- Transpose a matrix (can be used to do unzip)

function transpose(ls)

  local ms, len = {}, getn(ls)

  for i = 1, call(max, map(getn, ls)) do

    ms[i] = {}

    for j = 1, len do

      tinsert(ms[i], ls[j][i])

    end

  end

  return ms

end

zip = transpose

unzip = transpose



-- Project a list of fields from a list of records

--   l: list of records

--   f: field to project

-- returns

--   l: list of f fields

function project(l, f)

  local p = {}

  for i = 1, getn(l) do

    p[i] = l[i][f]

  end

  return p

end



-- Make an index for a list on the given field

function makeIndex(f, l)

  local ind = {}

  for i = 1, getn(l) do

    ind[l[i][f]] = i

  end

  return ind

end