# Prim's algorithm in CoffeeScript

I'm just starting to learn CoffeeScript and am trying to work through simple examples, this one being Prim's algorithm. I'd like feedback on everything but especially on making this script take advantage of the features of CoffeeScript most effectively.

nodes_dir = {
0 : [1],
1 : [0, 2, 3, 5],
2 : [1, 4],
3 : [1, 4, 5],
4 : [2, 3],
5: [1, 3]
}

edges_dir = {
0: {1:6}
1: {2:1, 3: 2, 5:0, 0:6},
2: {1:1, 4:4},
3: {1:2, 4:5, 5:3},
4:{2:4, 3:5},
5:{3:3, 1:0},
}

MAX_WEIGHT = 10000
NUM_NODES = 6
EDGE_WEIGHTS = [99, 2, 2, 1, 1, 5, 10]

get_min_key_value = (obj) ->
if Object.keys(obj).length < 1
return [-1, -1]
min_k = MAX_WEIGHT
min_v = MAX_WEIGHT
for k, v of obj
if v < min_v
min_v = v
min_k = k
delete obj[min_k]
[min_k, min_v]

# Prims algorithm as implemented in Skiena p 195
prim = (nodes_dir, edges_dir) ->
# record-keeping arrays
intree = []
distance = []
parent = []

# set default values for record-keeping arrays
for i in [1..NUM_NODES]
intree.push -1
distance.push MAX_WEIGHT
parent.push -1

current_node = 0

while intree[current_node] < 0
# add new node and new edge to record of edges/nodes
intree[current_node] = 1

# update any distances to tree that have become shorter with addition of new node
p = edges_dir[current_node]
for k,v of p
weight = EDGE_WEIGHTS[v]
if distance[k] > weight and intree[k] < 0
distance[k] = weight
parent[k] = current_node

# determine which node is now closest to tree and add it
dist = MAX_WEIGHT
for i in [0..NUM_NODES-1]
if intree[i] < 0 and dist > distance[i]
dist = distance[i]
current_node = i

f = prim(nodes_dir, edges_dir)


This already seems pretty good considering you just started learning coffeescript.

for k, v of obj
if v < min_v
min_v = v
min_k = k


Personally, I'm a huge fan of comprehensions. I would change the above to the following.

for k, v of obj when v < min_v
min_v = v
min_k = k


Your declarations at the top can be refined. No need for brackets/commas.

nodes_dir = {
0 : [1],
1 : [0, 2, 3, 5],
2 : [1, 4],
3 : [1, 4, 5],
4 : [2, 3],
5: [1, 3]
}


Can be simplified to just

nodes_dir =
0: [1]
1: [0, 2, 3, 5]
2: [1, 4]
3: [1, 4, 5]
4: [2, 3]
5: [1, 3]


Regarding the following

links.push link[current_node] if link[current_node] > 0


Don't lookup the value in the array based on the index twice.

linkNode = link[current_node]


Lastly,

intree = []
distance = []
parent = []

# set default values for record-keeping arrays
for i in [1..NUM_NODES]
intree.push -1
distance.push MAX_WEIGHT
parent.push -1


I would suggest creating an object that has these four arrays as properties, as well as putting that initialization code (the for loop) inside it's own method. This would add to clarity/reusability. Also, put all this code inside a coffeescript class declaration, which has a huge host of benefits.

for k, v of obj
if v < min_v
min_v = v
min_k = k


can be reduced to one line

[min_v, min_k] = [v, k] for k, v of obj when v < min_v


Also, if you can use ES6, this initialization code:

intree = []
distance = []
parent = []

for i in [1..NUM_NODES]
intree.push -1
distance.push MAX_WEIGHT
parent.push -1


can be shorter and more elegant:

distance = new Array(NUM_NODES).fill(MAX_WEIGHT)
intree = new Array(NUM_NODES).fill(-1)
parent = Array.from(intree)


Array.from clones an array so no references issue here.

min_k = MAX_WEIGHT
min_v = MAX_WEIGHT


you can write min_k = min_v = MAX_WEIGHT, we're dealing with numbers so no reference issues.

This return within an if

if Object.keys(obj).length < 1
return [-1, -1]


can be written in one line return [-1, -1] if Object.keys(obj).length < 1

• thanks for the additional comments. Quite useful especially to a CoffeeScript newbie like myself. – sunny Oct 11 '15 at 16:00
• perhaps I misunderstood your comments, but I think there is a mistake in your recommendation for finding the minimum value and its key. I tried the following: getMinKeyValuePair = (obj) -> return [-1, -1] if Object.keys(obj).length < 1 min_v = MAX_WEIGHT [min_k, min_v] = [k, v] for k, v of obj when v < min_v but this actually returns many key value pairs not just one. – sunny Oct 12 '15 at 15:03
• Actually, I didn't made any change to the logic of your code, just some tips for a shorter code. It's because you removed the [min_k, min_v] at the end of your function (compare the generated code with and without, there is an array created in the loop if you remove that line). Generally speaking, coffeescript can be tricky, when I started with coffeescript, I always generated the javascript to check if my code was still doing the same thing. – Shanoor Oct 12 '15 at 15:44

@Brad M already pointed out most of the stuff I'd have pointed out too. These are just some additional things

• CoffeeScript is still JavaScript, so it's typically written in camelCase - not snake_case.

• I'd avoid calling delete in get_min_key_value. Better to just let it return the min key, and let the caller worry about the rest, than to have a function with such side-effects.
However, it seems you're not actually using this function, so really it should probably just be deleted. Still, if you do need it, I'd suggest something like this:

getKeyForMinimumValue = (obj) ->
minValue = MAX_WEIGHT
for own key, value of obj when value < minValue # use own to only iterate instance properties
[minKey, minValue] = [key, value]
minKey


It's different, as it'll only return the key, or return undefined; not the key/value pair or [-1, -1]. But it's certainly shorter

• Instead of doing this [0..NUM_NODES-1], use a half-open range: [0...NUM_NODES] (three dots; excludes the end value).

• ... though, instead of having a separate, hard-coded value of the number of nodes, just use Object.keys(nodes_dir).length.

• many thanks for the thorough comments. I am not sure about avoiding the delete. I'm with you on avoiding side effects, so I suppose I can move it to the main function where I use it... – sunny Oct 11 '15 at 16:01
• @sunny That would be a better solution, yes. The point is really just to keep the function focussed on 1 task: Finding the minimum. Having it also delete properties goes beyond that – Flambino Oct 11 '15 at 16:55