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Merged
merged 13 commits into from
Oct 10, 2024
Merged

Added countingsort.lua #38

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b77dacd
Create countingsort.lua
icemberg Oct 5, 2024
593d66b
Update countingsort.lua
icemberg Oct 5, 2024
2be6f6b
Update countingsort.lua
icemberg Oct 7, 2024
c08bc1a
Update sort_spec.lua
icemberg Oct 7, 2024
a3c6be4
Update src/sorting/countingsort.lua
icemberg Oct 8, 2024
fbd5c22
Update countingsort.lua
icemberg Oct 8, 2024
9f471ae
Update src/sorting/countingsort.lua
icemberg Oct 8, 2024
39e5284
Update sort_spec.lua
icemberg Oct 8, 2024
495b01c
Update sort_spec.lua
icemberg Oct 8, 2024
dbf28e9
Update sort_spec.lua
icemberg Oct 9, 2024
b73ddb6
Update countingsort.lua
appgurueu Oct 10, 2024
b6c98e9
Update sort_spec.lua
appgurueu Oct 10, 2024
585cf1d
Fix formatting
appgurueu Oct 10, 2024
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58 changes: 58 additions & 0 deletions src/sorting/countingsort.lua
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Original file line number Diff line number Diff line change
@@ -0,0 +1,58 @@
function counting_sort(arr, key_function)
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  1. This function should be returned directly for require to work with this as expected rather than polluting globals;
  2. I would rename arr to list for consistency with the rest of the code.
  3. key_function should have a comment explaining what it does and what it defaults to. Especially since this sorting algorithm has special requirements (keys must be integers).

-- If no key_function is provided, use identity function
key_function = key_function or function(x) return x end

-- Step 1: Find the range of keys (min_key and max_key)
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Code is self-explanatory, no comment needed. You can also simplify this to

local min_key, max_key = math.huge, -math.huge
for _, elem in ipairs(list) do
    local key = key_function(elem)
    min_key = math.min(min_key, key)
    max_key = math.max(max_key, key)
end

local min_key, max_key = math.huge, -math.huge
for i = 1, #arr do
local key = key_function(arr[i])
if key < min_key then
min_key = key
end
if key > max_key then
max_key = key
end
end

-- Step 2: Initialize the count array
local count = {}
for i = min_key, max_key do
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You get a problem here when arr is empty. Please add a special case to return early in that case.
Furthermore, as it currently is, this may (unnecessarily) populate the hash part if min_key is not 1, so please shift accordingly to prevent that.

count[i] = 0
end

-- Step 3: Count the occurrences of each key. In this case key is same as arr[i]
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Unnecessary comment, also slightly misleading (keys are produced by the key function).

for i = 1, #arr do
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Again use ipairs

local key = key_function(arr[i])
count[key] = count[key] + 1
end

-- Step 4: Compute cumulative counts to get final positions
for i = min_key + 1, max_key do
count[i] = count[i] + count[i - 1]
end

-- Step 5: Build the output array (in stable order)
local output = {}
for i = #arr, 1, -1 do
local element = arr[i]
local key = key_function(element)
output[count[key]] = element
count[key] = count[key] - 1
end

-- Step 6: Copy the output array back to the original array
for i = 1, #arr do
arr[i] = output[i]
end
end

-- Sample array
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Please add proper tests rather than some example code at the end of the file. This can essentially use the same tests as Radix Sort: check_sort(countingsort, nil, true), see https://github.com/TheAlgorithms/Lua/blob/main/.spec/sorting/sort_spec.lua.

local arr = {100, 2, 2, 8, 3, 10000000, 1}

-- Simple usage with identity key function
counting_sort(arr)

-- Print sorted array
for i, v in ipairs(arr) do
print(v)
end