# List Operations

[!] VersionNotice: The below code pertains to an older Lua version, Lua 4. It does not run as is under Lua 5. However, similar list utility functions are available for Lua 5 in std.list in [stdlib].

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
```

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Last edited January 2, 2007 1:37 am GMT (diff)