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Lists

Output

From a List Produce a List of Duplicate Entries

Taking a list:
["andrew", "bob", "chris", "bob"]

Write the code to produce a list of duplicates in the list:
["bob"]

php

$arr = array('andrew', 'bob', 'chris', 'bob', 'chris', 'john', 'mary', 'lucy');

function match($a, $b)
{
// This is a separate function so you could include checking
// with loose comparisons or lowercase both strings, etc.
return $a === $b;
}

$results = array();

sort($arr);

for($i = 0; $i < count($arr) - 1; $i++)
{
if (match($arr[$i], $arr[$i+1]))
{
$results[] = $arr[$i];
$i++;
}
}
print_r($results);

php

$arr = array('lucy', 'andrew', 'lucy', 'bob', 'chris', 'bob', 'chris', 'john', 'mary');

function match($a, $b)
{
// This is a separate function so you could include checking
// with loose comparisons or lowercase both strings, etc.
return $a === $b;
}

$results = array();

for($i = 0; $i < count($arr); $i++)
{
if (isset($results[$arr[$i]]))
{
$results[$arr[$i]]++;
}
else
{
$results[$arr[$i]] = 0;
}
}

$out = array();

foreach($results as $name => $count)
{
echo $name . ':' . $count . '<br />';
if ($count > 0)
{
$out[] = $name;
}
}

print_r($out);

php 5.3

<?php
$array = array("andrew", "bob", "chris", "bob");
$counts = array_count_values($array);

$duplicates = array_filter(
array_unique($array),
function($key) use ($counts) {
return $counts[$key] > 1;
}
);
?>

clojure

(->> '("andrew" "bob" "chris" "bob")
(group-by identity)
(filter #(> (count (second %)) 1))
(map first))

cpp C++0x/C++11 (gcc >=4.6)

vector<string> lst = { "andrew", "bob", "chris", "bob" };
vector<string> lst_no_dups;
vector<string> tmp;
vector<string> dups;

sort(lst.begin(), lst.end());
unique_copy(lst.begin(), lst.end(), back_inserter(lst_no_dups));
set_difference(lst.begin(), lst.end(),
lst_no_dups.begin(), lst_no_dups.end(),
back_inserter(tmp));
unique_copy(tmp.begin(), tmp.end(), back_inserter(dups));

cout << dups << endl;

cpp C++0x/C++11 (gcc >=4.6)

list<string> lst = { "andrew", "bob", "chris", "bob" };
map<string,int> num_identical;
list<string> dups;

for (auto &s: lst)
num_identical[s]++;
for (auto &n: num_identical)
if (n.second > 1)
dups.push_back(n.first);
cout << dups << endl;

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