VB.NET and Python equivalence

A [recent post on Hacker News](http://news.ycombinator.com/item?id=1885594) made me wonder if **<inflammatory>Python is just VB.NET without ‘End If’. </inflammatory>**

I thought I’d try to translate Peter Norvig’s masterful spellchecker program in Python into VB.NET 2008. Could his beautiful little program be converted line-by-line into VB? The answer is *yes.*

Here’s the [original program](http://norvig.com/spell-correct.html);

import re, collections

def words(text): return re.findall(‘[a-z]+’, text.lower())

def train(features):
model = collections.defaultdict(lambda: 1)
for f in features:
model[f] += 1
return model

NWORDS = train(words(file(‘big.txt’).read()))

alphabet = ‘abcdefghijklmnopqrstuvwxyz’

def edits1(word):
splits = [(word[:i], word[i:]) for i in range(len(word) + 1)]
deletes = [a + b[1:] for a, b in splits if b]
transposes = [a + b[1] + b[0] + b[2:] for a, b in splits if len(b)>1]
replaces = [a + c + b[1:] for a, b in splits for c in alphabet if b]
inserts = [a + c + b for a, b in splits for c in alphabet]
return set(deletes + transposes + replaces + inserts)

def known_edits2(word):
return set(e2 for e1 in edits1(word) for e2 in edits1(e1) if e2 in NWORDS)

def known(words): return set(w for w in words if w in NWORDS)

def correct(word):
candidates = known([word]) or known(edits1(word)) or known_edits2(word) or [word]
return max(candidates, key=NWORDS.get)

And here’s the equivalent VB;

Option Strict Off
Imports System.Text.RegularExpressions
Imports System.Collections.Generic

Module NorvigSpellChecker

Dim NWORDS

Dim alphabet As String = “abcdefghijklmnopqrstuvwxyz”

Function words(ByVal text)
Return From match In Regex.Matches(text.ToLower(), “[a-z]+”) Select match.Value
End Function

Function train(ByVal features)
Dim model = New DefaultDict(Function() 1)
For Each f In features
model(f) += 1
Next
Return model
End Function

Function edits1(ByVal word) As IEnumerable(Of Object)
Dim splits = From i In Enumerable.Range(0, len(word)) Let a = word.Substring(0, i), b = word.Substring(i) Select a, b
Dim deletes = From split In splits Where ifs(split.b) Select split.a + split.b.Substring(1)
Dim transposes = From split In splits Where len(split.b) > 1 Select split.a + split.b.Substring(1, 1) + split.b.Substring(0, 1) + split.b.Substring(2)
Dim replaces = From split In splits From c In alphabet Where ifs(split.b) Select split.a + c + split.b.Substring(1)
Dim inserts = From split In splits From c In alphabet Select split.a + c + split.b
Return deletes.Union(transposes).Union(replaces).Union(inserts).Distinct()
End Function

Function known_edits2(ByVal word)
Return edits1(word).SelectMany(Function(e1) edits1(e1)).Distinct().Where(Function(e) NWORDS.ContainsKey(e))
End Function

Function known(ByVal words As IEnumerable)
Return From w In words Where NWORDS.ContainsKey(w) Select w
End Function

Function correct(ByVal word)
Dim candidates = ORR(Function() known(New String() {word}), Function() known(edits1(word)), Function() known_edits2(word), Function() New String() {word})
Return candidates.OrderBy(Function(candidate) NWORDS(candidate)).FirstOrDefault()
End Function

End Module

Because I needed to define some python-equivalents, like the `len(string)` function, I ended up writing a bit more code, just to try to get a more exact match. Also, LINQ expressions need to know that certain things are enumerable, so I needed to litter a few type declarations through the code; about half a dozen. But overall I’m happy with the result. Take off the block terminators — ‘End Function’, ‘Next’, and ‘End Module’ — and you end up with a line-perfect equivalent.

Full source code [on github](https://github.com/stevecooperorg/Norvig-Spelling-Corrector-in-VB.NET/blob/master/NorvigSpellChecker.vb)

Advertisements