View Problem
Maps
Algorithms

Create a histogram map from a list

Given the list [a,b,a,c,b,b], produce a map {a:2, b:3, c:1} which contains the count of each unique item in the list
DiskEdit
python
from collections import defaultdict
h = defaultdict(int)
for k in "abacbb":
h[k] += 1

h = {}
for k in "abacbb":
h[k] = h.setdefault(k, 0) + 1
DiskEdit
python 2.7+, 3.1+
from collections import Counter
h = Counter("abacbb")
print(h)
DiskEdit
csharp .NET 3.5
using System.Collections.Generic;
using System.Linq;

// This is a "functional" C# approach

// NOTE: In C# "maps" are of type Dictionary<Tkey, TValue>
// so our histogram map is of type Dictionary<object, int>
public class HistogramMap {
public Dictionary<object, int> FromList(List<object> list) {
// The "Aggregate" method works like "inject" in many other languages.
return list.Aggregate(
new Dictionary<object, int>(),
(map, obj) => {
// If this is the first time we've seen this obj, set the count to 0
if (!map.ContainsKey(obj)) map[obj] = 0;

// Increment the count
map[obj]++;

// Return the map for the next iteration.
// NOTE: This does NOT return from our "FromList" method
return map;
}
);
}

public static void Main() {
// Create our Histogram Map from a new list
var map = new HistogramMap().FromList(
new List<object>() { 'a', 'b', 'a', 'c', 'b', 'b' }
);

// This just prints the result
System.Console.WriteLine (
string.Join (", ",
// "Select" works like "map" or "collect" in many other languages
map.Select( kvp =>
string.Format("{0} : {1}", kvp.Key, kvp.Value)
).ToArray()
)
);
}
}
ExpandDiskEdit
csharp .Net 3.5/Mono 2.8
new[] {"a","b","a","c","b","b"}
.GroupBy(s => s)
.Select(s => new { Value = s.Key, Count = s.Count() })
.ToList()
.ForEach(e => Console.WriteLine("{0} : {1} ", e.Value, e.Count));
ExpandDiskEdit
erlang
% Imperative Solution
Histogram = histogram(List),
ExpandDiskEdit
erlang
% Functional (1) Solution
Histogram = histogram(List),
DiskEdit
erlang Using dict for a map.
lists:foldl(fun(Elem, OldDict) ->
dict:update_counter(Elem, 1, OldDict)
end,
dict:new(),
[a,b,a,c,b,b])).
ExpandDiskEdit
fantom
list := ["a","b","a","c","b","b"]
map := [Str:Int][:]
list.each |Str s, Int i| { if(!map.containsKey(s)) map.add(s,1); else map[s] = ++map[s] }
echo (map)
ExpandDiskEdit
groovy
histogram = [:]
list.each { item ->
if (!histogram.containsKey(item)) histogram[item] = 0
histogram[item]++
}
ExpandDiskEdit
groovy
histogram = [:]
list.each { histogram[it] = (histogram[it] ?: 0) + 1 }

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