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---
title: Jaro distance
id: 5a23c84252665b21eecc7ec2
challengeType: 5
forumTopicId: 302292
---
## Description
<section id='description'>
The Jaro distance is a measure of similarity between two strings. The higher the Jaro distance for two strings is, the more similar the strings are. The score is normalized such that <code>0</code> equates to no similarity and <code>1</code> is an exact match.
<strong>Definition</strong>
The Jaro distance \( d_j \) of two given strings \(s_1\) and \(s_2\) is
\begin{align}d_j = \begin{cases}0&amp; & \text{if }m=0 \\\\{\frac {1}{3}}\left({\frac {m}{|s_{1}|}}+{\frac {m}{|s_{2}|}}+{\frac {m-t}{m}}\right)&amp; & \text{otherwise}\end{cases}\end{align}
Where:
<ul>
<li>\(m\) is the number of <i>matching characters</i>;</li>
<li> \(t\) is half the number of <i>transpositions</i>.</li>
</uL>
Two characters from \(s_1\) and \(s_2\) respectively, are considered <i>matching</i> only if they are the same and not farther than \(\left\lfloor\frac{\max(|s_1|,|s_2|)}{2}\right\rfloor-1\).
Each character of \(s_1\) is compared with all its matching characters in \(s_2\) . The number of matching (but different sequence order) characters divided by 2 defines the number of <i>transpositions</i>.
<strong>Example</strong>
Given the strings \(s_1\) <i>DWAYNE</i> and \(s_2\) <i>DUANE</i> we find:
<ul>
<li>\(m = 4\)</li>
<li>\(|s_1| = 6\)</li>
<li>\(|s_2| = 5\)</li>
<li>\(t = 0\)</li>
</ul>
We find a Jaro score of: \(d_j = \frac{1}{3}\left(\frac{4}{6} + \frac{4}{5} + \frac{4-0}{4}\right) = 0.822\).
</section>
## Instructions
<section id='instructions'>
Write a function a that takes two strings as parameters and returns the associated Jaro distance.
</section>
## Tests
<section id='tests'>
```yml
tests:
- text: <code>jaro</code> should be a function.
testString: assert(typeof jaro=='function');
- text: <code>jaro("MARTHA", "MARHTA")</code> should return a number.
testString: assert(typeof jaro('MARTHA', 'MARHTA')=='number');
- text: <code>jaro("MARTHA", "MARHTA")</code> should return <code>0.9444444444444445</code>.
testString: assert.equal(jaro('MARTHA', 'MARHTA'), 0.9444444444444445);
- text: <code>jaro("DIXON", "DICKSONX")</code> should return <code>0.7666666666666666</code>.
testString: assert.equal(jaro('DIXON', 'DICKSONX'), 0.7666666666666666);
- text: <code>jaro("JELLYFISH", "SMELLYFISH")</code> should return <code>0.8962962962962964</code>.
testString: assert.equal(jaro('JELLYFISH', 'SMELLYFISH'), 0.8962962962962964);
- text: <code>jaro("HELLOS", "CHELLO")</code> should return <code>0.888888888888889</code>.
testString: assert.equal(jaro('HELLOS', 'CHELLO'), 0.888888888888889);
- text: <code>jaro("ABCD", "BCDA")</code> should return <code>0.8333333333333334</code>.
testString: assert.equal(jaro('ABCD', 'BCDA'), 0.8333333333333334);
```
</section>
## Challenge Seed
<section id='challengeSeed'>
<div id='js-seed'>
```js
function jaro(s, t) {
}
```
</div>
</section>
## Solution
<section id='solution'>
```js
function jaro(s, t) {
var s_len = s.length;
var t_len = t.length;
if (s_len == 0 && t_len == 0) return 1;
var match_distance = Math.max(s_len, t_len) / 2 - 1;
var s_matches = new Array(s_len);
var t_matches = new Array(t_len);
var matches = 0;
var transpositions = 0;
for (var i = 0; i < s_len; i++) {
var start = Math.max(0, i - match_distance);
var end = Math.min(i + match_distance + 1, t_len);
for (var j = start; j < end; j++) {
if (t_matches[j]) continue;
if (s.charAt(i) != t.charAt(j)) continue;
s_matches[i] = true;
t_matches[j] = true;
matches++;
break;
}
}
if (matches == 0) return 0;
var k = 0;
for (var i = 0; i < s_len; i++) {
if (!s_matches[i]) continue;
while (!t_matches[k]) k++;
if (s.charAt(i) != t.charAt(k)) transpositions++;
k++;
}
return ((matches / s_len) +
(matches / t_len) +
((matches - transpositions / 2.0) / matches)) / 3.0;
}
```
</section>