I'm sorting strings that are comprised of text and numbers.
I want the sort to sort the number parts as numbers, not alphanumeric.
For example I want: abc1def, ..., abc9def, abc10def
instead of: abc10def, abc1def, ..., abc9def
Does anyone know an algorithm for this (in particular in c++)
Thanks
I asked this exact question (although in Java) and got pointed to http://www.davekoelle.com/alphanum.html which has an algorithm and implementations of it in many languages.
Update 14 years later: Dave Koelle’s blog has gone off line and I can’t find his actual algorithm, but here’s an implementation.
https://github.com/cblanc/koelle-sort
Several natural sort implementations for C++ are available. A brief review:
natural_sort<> - based on Boost.Regex.
In my tests, it's roughly 20 times slower than other options.
Dirk Jagdmann's alnum.hpp, based on Dave Koelle's alphanum algorithm
Potential integer overlow issues for values over MAXINT
Martin Pool's natsort - written in C, but trivially usable from C++.
The only C/C++ implementation I've seen to offer a case insensitive version, which would seem to be a high priority for a "natural" sort.
Like the other implementations, it doesn't actually parse decimal points, but it does special case leading zeroes (anything with a leading 0 is assumed to be a fraction), which is a little weird but potentially useful.
PHP uses this algorithm.
This is known as natural sorting. There's an algorithm here that looks promising.
Be careful of problems with non-ASCII characters (see Jeff's blog entry on the subject).
Partially reposting my another answer:
bool compareNat(const std::string& a, const std::string& b){
if (a.empty())
return true;
if (b.empty())
return false;
if (std::isdigit(a[0]) && !std::isdigit(b[0]))
return true;
if (!std::isdigit(a[0]) && std::isdigit(b[0]))
return false;
if (!std::isdigit(a[0]) && !std::isdigit(b[0]))
{
if (a[0] == b[0])
return compareNat(a.substr(1), b.substr(1));
return (toUpper(a) < toUpper(b));
//toUpper() is a function to convert a std::string to uppercase.
}
// Both strings begin with digit --> parse both numbers
std::istringstream issa(a);
std::istringstream issb(b);
int ia, ib;
issa >> ia;
issb >> ib;
if (ia != ib)
return ia < ib;
// Numbers are the same --> remove numbers and recurse
std::string anew, bnew;
std::getline(issa, anew);
std::getline(issb, bnew);
return (compareNat(anew, bnew));
}
toUpper() function:
std::string toUpper(std::string s){
for(int i=0;i<(int)s.length();i++){s[i]=toupper(s[i]);}
return s;
}
Usage:
std::vector<std::string> str;
str.push_back("abc1def");
str.push_back("abc10def");
...
std::sort(str.begin(), str.end(), compareNat);
To solve what is essentially a parsing problem a state machine (aka finite state automaton) is the way to go. Dissatisfied with the above solutions i wrote a simple one-pass early bail-out algorithm that beats C/C++ variants suggested above in terms of performance, does not suffer from numerical datatype overflow errors, and is easy to modify to add case insensitivity if required.
sources can be found here
For those that arrive here and are already using Qt in their project, you can use the QCollator class. See this question for details.
Avalanchesort is a recursive variation of naturall sort, whiche merge runs, while exploring the stack of sorting-datas. The algorithim will sort stable, even if you add datas to your sorting-heap, while the algorithm is running/sorting.
The search-principle is simple. Only merge runs with the same rank.
After finding the first two naturell runs (rank 0), avalanchesort merge them to a run with rank 1. Then it call avalanchesort, to generate a second run with rank 1 and merge the two runs to a run with rank 2. Then it call the avalancheSort to generate a run with rank 2 on the unsorted datas....
My Implementation porthd/avalanchesort divide the sorting from the handling of the data using interface injection. You can use the algorithmn for datastructures like array, associative arrays or lists.
/**
* #param DataListAvalancheSortInterface $dataList
* #param DataRangeInterface $beginRange
* #param int $avalancheIndex
* #return bool
*/
public function startAvalancheSort(DataListAvalancheSortInterface $dataList)
{
$avalancheIndex = 0;
$rangeResult = $this->avalancheSort($dataList, $dataList->getFirstIdent(), $avalancheIndex);
if (!$dataList->isLastIdent($rangeResult->getStop())) {
do {
$avalancheIndex++;
$lastIdent = $rangeResult->getStop();
if ($dataList->isLastIdent($lastIdent)) {
$rangeResult = new $this->rangeClass();
$rangeResult->setStart($dataList->getFirstIdent());
$rangeResult->setStop($dataList->getLastIdent());
break;
}
$nextIdent = $dataList->getNextIdent($lastIdent);
$rangeFollow = $this->avalancheSort($dataList, $nextIdent, $avalancheIndex);
$rangeResult = $this->mergeAvalanche($dataList, $rangeResult, $rangeFollow);
} while (true);
}
return $rangeResult;
}
/**
* #param DataListAvalancheSortInterface $dataList
* #param DataRangeInterface $range
* #return DataRangeInterface
*/
protected function findRun(DataListAvalancheSortInterface $dataList,
$startIdent)
{
$result = new $this->rangeClass();
$result->setStart($startIdent);
$result->setStop($startIdent);
do {
if ($dataList->isLastIdent($result->getStop())) {
break;
}
$nextIdent = $dataList->getNextIdent($result->getStop());
if ($dataList->oddLowerEqualThanEven(
$dataList->getDataItem($result->getStop()),
$dataList->getDataItem($nextIdent)
)) {
$result->setStop($nextIdent);
} else {
break;
}
} while (true);
return $result;
}
/**
* #param DataListAvalancheSortInterface $dataList
* #param $beginIdent
* #param int $avalancheIndex
* #return DataRangeInterface|mixed
*/
protected function avalancheSort(DataListAvalancheSortInterface $dataList,
$beginIdent,
int $avalancheIndex = 0)
{
if ($avalancheIndex === 0) {
$rangeFirst = $this->findRun($dataList, $beginIdent);
if ($dataList->isLastIdent($rangeFirst->getStop())) {
// it is the last run
$rangeResult = $rangeFirst;
} else {
$nextIdent = $dataList->getNextIdent($rangeFirst->getStop());
$rangeSecond = $this->findRun($dataList, $nextIdent);
$rangeResult = $this->mergeAvalanche($dataList, $rangeFirst, $rangeSecond);
}
} else {
$rangeFirst = $this->avalancheSort($dataList,
$beginIdent,
($avalancheIndex - 1)
);
if ($dataList->isLastIdent($rangeFirst->getStop())) {
$rangeResult = $rangeFirst;
} else {
$nextIdent = $dataList->getNextIdent($rangeFirst->getStop());
$rangeSecond = $this->avalancheSort($dataList,
$nextIdent,
($avalancheIndex - 1)
);
$rangeResult = $this->mergeAvalanche($dataList, $rangeFirst, $rangeSecond);
}
}
return $rangeResult;
}
protected function mergeAvalanche(DataListAvalancheSortInterface $dataList, $oddListRange, $evenListRange)
{
$resultRange = new $this->rangeClass();
$oddNextIdent = $oddListRange->getStart();
$oddStopIdent = $oddListRange->getStop();
$evenNextIdent = $evenListRange->getStart();
$evenStopIdent = $evenListRange->getStop();
$dataList->initNewListPart($oddListRange, $evenListRange);
do {
if ($dataList->oddLowerEqualThanEven(
$dataList->getDataItem($oddNextIdent),
$dataList->getDataItem($evenNextIdent)
)) {
$dataList->addListPart($oddNextIdent);
if ($oddNextIdent === $oddStopIdent) {
$restTail = $evenNextIdent;
$stopTail = $evenStopIdent;
break;
}
$oddNextIdent = $dataList->getNextIdent($oddNextIdent);
} else {
$dataList->addListPart($evenNextIdent);
if ($evenNextIdent === $evenStopIdent) {
$restTail = $oddNextIdent;
$stopTail = $oddStopIdent;
break;
}
$evenNextIdent = $dataList->getNextIdent($evenNextIdent);
}
} while (true);
while ($stopTail !== $restTail) {
$dataList->addListPart($restTail);
$restTail = $dataList->getNextIdent($restTail);
}
$dataList->addListPart($restTail);
$dataList->cascadeDataListChange($resultRange);
return $resultRange;
}
}
My algorithm with test code of java version. If you want to use it in your project you can define a comparator yourself.
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.function.Consumer;
public class FileNameSortTest {
private static List<String> names = Arrays.asList(
"A__01__02",
"A__2__02",
"A__1__23",
"A__11__23",
"A__3++++",
"B__1__02",
"B__22_13",
"1_22_2222",
"12_222_222",
"2222222222",
"1.sadasdsadsa",
"11.asdasdasdasdasd",
"2.sadsadasdsad",
"22.sadasdasdsadsa",
"3.asdasdsadsadsa",
"adsadsadsasd1",
"adsadsadsasd10",
"adsadsadsasd3",
"adsadsadsasd02"
);
public static void main(String...args) {
List<File> files = new ArrayList<>();
names.forEach(s -> {
File f = new File(s);
try {
if (!f.exists()) {
f.createNewFile();
}
files.add(f);
} catch (IOException e) {
e.printStackTrace();
}
});
files.sort(Comparator.comparing(File::getName));
files.forEach(f -> System.out.print(f.getName() + " "));
System.out.println();
files.sort(new Comparator<File>() {
boolean caseSensitive = false;
int SPAN_OF_CASES = 'a' - 'A';
#Override
public int compare(File left, File right) {
char[] csLeft = left.getName().toCharArray(), csRight = right.getName().toCharArray();
boolean isNumberRegion = false;
int diff=0, i=0, j=0, lenLeft=csLeft.length, lenRight=csRight.length;
char cLeft = 0, cRight = 0;
for (; i<lenLeft && j<lenRight; i++, j++) {
cLeft = getCharByCaseSensitive(csLeft[i]);
cRight = getCharByCaseSensitive(csRight[j]);
boolean isNumericLeft = isNumeric(cLeft), isNumericRight = isNumeric(cRight);
if (isNumericLeft && isNumericRight) {
// Number start!
if (!isNumberRegion) {
isNumberRegion = true;
// Remove prefix '0'
while (i < lenLeft && cLeft == '0') i++;
while (j < lenRight && cRight == '0') j++;
if (i == lenLeft || j == lenRight) break;
}
// Diff start: calculate the diff value.
if (cLeft != cRight && diff == 0)
diff = cLeft - cRight;
} else {
if (isNumericLeft != isNumericRight) {
// One numeric and one char.
if (isNumberRegion)
return isNumericLeft ? 1 : -1;
return cLeft - cRight;
} else {
// Two chars: if (number) diff don't equal 0 return it.
if (diff != 0)
return diff;
// Calculate chars diff.
diff = cLeft - cRight;
if (diff != 0)
return diff;
// Reset!
isNumberRegion = false;
diff = 0;
}
}
}
// The longer one will be put backwards.
return (i == lenLeft && j == lenRight) ? cLeft - cRight : (i == lenLeft ? -1 : 1) ;
}
private boolean isNumeric(char c) {
return c >= '0' && c <= '9';
}
private char getCharByCaseSensitive(char c) {
return caseSensitive ? c : (c >= 'A' && c <= 'Z' ? (char) (c + SPAN_OF_CASES) : c);
}
});
files.forEach(f -> System.out.print(f.getName() + " "));
}
}
The output is,
1.sadasdsadsa 11.asdasdasdasdasd 12_222_222 1_22_2222 2.sadsadasdsad 22.sadasdasdsadsa 2222222222 3.asdasdsadsadsa A__01__02 A__11__23 A__1__23 A__2__02 A__3++++ B__1__02 B__22_13 adsadsadsasd02 adsadsadsasd1 adsadsadsasd10 adsadsadsasd3
1.sadasdsadsa 1_22_2222 2.sadsadasdsad 3.asdasdsadsadsa 11.asdasdasdasdasd 12_222_222 22.sadasdasdsadsa 2222222222 A__01__02 A__1__23 A__2__02 A__3++++ A__11__23 adsadsadsasd02 adsadsadsasd1 adsadsadsasd3 adsadsadsasd10 B__1__02 B__22_13
Process finished with exit code 0
// -1: s0 < s1; 0: s0 == s1; 1: s0 > s1
static int numericCompare(const string &s0, const string &s1) {
size_t i = 0, j = 0;
for (; i < s0.size() && j < s1.size();) {
string t0(1, s0[i++]);
while (i < s0.size() && !(isdigit(t0[0]) ^ isdigit(s0[i]))) {
t0.push_back(s0[i++]);
}
string t1(1, s1[j++]);
while (j < s1.size() && !(isdigit(t1[0]) ^ isdigit(s1[j]))) {
t1.push_back(s1[j++]);
}
if (isdigit(t0[0]) && isdigit(t1[0])) {
size_t p0 = t0.find_first_not_of('0');
size_t p1 = t1.find_first_not_of('0');
t0 = p0 == string::npos ? "" : t0.substr(p0);
t1 = p1 == string::npos ? "" : t1.substr(p1);
if (t0.size() != t1.size()) {
return t0.size() < t1.size() ? -1 : 1;
}
}
if (t0 != t1) {
return t0 < t1 ? -1 : 1;
}
}
return i == s0.size() && j == s1.size() ? 0 : i != s0.size() ? 1 : -1;
}
I am not very sure if it is you want, anyway, you can have a try:-)
Anyone know if this is a Dart bug or is it my misunderstanding of how Dart coding works?
I am learning Dart to investigate feasibility of eventually using Flutter; however, while exploring the language, I found a weird behavior (maybe a bug). I tried repro'ing it by writing a similar pattern of code, but have yet to figure out what causes it. In the attached code, I wrote a quicksort class. In that class, it counts the number of times the "sort" method is recursed and saves the count in a class member called "recurseCount".
From the main() class, if I use the QuickSort class directly, I have no issue getting back the recurseCount member; however, if I call it from a different class (called "Tester"), I do not get the correct value for "recurseCount". Why would calling a class from a separate class cause members to not provide the correct values?
import 'package:test/test.dart';
import 'dart:math' as _math;
// ***********************
enum SortOrder { ascending, descending, unsorted }
class QuickSort {
List list = [];
SortOrder sortOrder = SortOrder.unsorted;
int recurseCount = 0;
QuickSort({this.list}) {
if (list != null && list.length > 1) {
list = sort(useRandomPivot: true);
}
}
List sort(
{List iList,
int leftIndex = 0,
int rightIndex,
bool useRandomPivot = true}) {
if (iList != null) list = iList;
if (list.isEmpty) return [];
rightIndex ??= list.length - 1;
if (rightIndex > list.length - 1) rightIndex = list.length - 1;
if (leftIndex < rightIndex) {
recurseCount++;
var partitionIndex =
_partition(leftIndex, rightIndex, useRandomPivot: useRandomPivot);
if (partitionIndex == -1) {
//already sorted List
if (sortOrder == SortOrder.ascending) {
return list;
} else {
//SortOrder.descending
list = list.reversed.toList(); // Time Complexity of O(n)
sortOrder = SortOrder.ascending;
return list;
}
} else {
sort(leftIndex: leftIndex, rightIndex: partitionIndex - 1);
sort(leftIndex: partitionIndex + 1, rightIndex: rightIndex);
}
} else {
sortOrder = SortOrder.ascending;
}
return list;
}
int _partition(int leftIndex, int rightIndex, {bool useRandomPivot = true}) {
// in case the array is already sorted from the start; only run through the partition'ing one time
// note: regardless of ascending or descending order
if (leftIndex == 0 && rightIndex >= list.length - 1) {
sortOrder = checkSorting(list);
if (sortOrder != SortOrder.unsorted) {
return -1;
}
if (useRandomPivot) {
var random = _math.Random();
var randomIndex = random.nextInt(rightIndex - leftIndex);
_swapElements(randomIndex, rightIndex);
}
}
int pivotVal = list[rightIndex]; //select the last item as the pivot
var headIndex = leftIndex - 1;
for (var scanIndex = leftIndex; scanIndex < rightIndex; scanIndex++) {
if (list[scanIndex] <= pivotVal) {
headIndex++;
_swapElements(headIndex, scanIndex);
}
}
var partitionIndex = headIndex + 1;
_swapElements(partitionIndex, rightIndex);
return partitionIndex;
}
void _swapElements(position1, position2) {
int tempVal = list[position1];
list[position1] = list[position2];
list[position2] = tempVal;
}
SortOrder checkSorting(List arr) {
var isAsc = true;
var isDesc = true;
for (var i = 0; i < arr.length - 1; i++) {
if (arr[i] < arr[i + 1]) isDesc = false;
if (arr[i] > arr[i + 1]) isAsc = false;
if (!isDesc && !isAsc) break; // not sorted Asc or Desc
}
if (isAsc) {
return SortOrder.ascending;
} else if (isDesc) return SortOrder.descending;
return SortOrder.unsorted;
}
}
// ***********************TESTS*********************************
num log2(num n) => _math.log(n) / _math.ln2;
List getRandomIntList({int min = 0, int max = 10000, int len}) {
var random = _math.Random();
var tempList = List(len);
for (var i = 0; i < len; i++) {
tempList[i] = random.nextInt(max - min);
}
return tempList;
}
class Tester {
static num _runControlTest(List list) {
var stopwatch = Stopwatch();
stopwatch.start();
list.sort();
stopwatch.stop();
return stopwatch.elapsedMicroseconds;
}
static void runTests(List list, String groupName,
{bool useRandomPivot = false, bool sortFromConstructor = false}) {
// CONTROL
var controlElapsedTimeMicroSec = 0;
controlElapsedTimeMicroSec = _runControlTest(list);
// END CONTROL
var expectedRecursionCountLogN = log2(list.length).ceil();
var expectedRecursionCountNLogN = list.length * expectedRecursionCountLogN;
var qs = QuickSort();
var stopwatch = Stopwatch();
stopwatch.start();
list =
qs.sort(iList: list, useRandomPivot: useRandomPivot); // METHOD TO TEST
stopwatch.stop();
var elapsedTimeMicroSec = stopwatch.elapsedMicroseconds;
var recursionCount = qs
.recurseCount; //NOTE (BUG in Dart?): unable to get the recurseCount correctly from within this class/method
group(groupName, () {
var testSubject =
'Time Taken: ${controlElapsedTimeMicroSec} microseconds';
var reason =
'The built in sort took ${controlElapsedTimeMicroSec} microseconds, while the test took ${elapsedTimeMicroSec}.';
test(testSubject, () {
expect(
elapsedTimeMicroSec, lessThanOrEqualTo(controlElapsedTimeMicroSec),
reason: reason);
});
testSubject =
'Time Complexity: ${recursionCount} vs ${expectedRecursionCountNLogN}';
reason =
'Time Complexity of ${recursionCount} is greater than either range (LogN) ${expectedRecursionCountLogN} or (N*LogN) ${expectedRecursionCountNLogN}';
test(testSubject, () {
expect(recursionCount, lessThanOrEqualTo(expectedRecursionCountNLogN),
reason: reason);
});
});
}
}
void main() {
var min = 0;
var len = 1000000;
var max = len;
var originalList = getRandomIntList(min: min, max: max, len: len);
var list = List.from(originalList);
// BUG? When called within this Tester.runTests the sort method does NOT return the correct recurseCount
Tester.runTests(list, 'UNSORTED_RIGHT_PIVOT', useRandomPivot: false);
list = List.from(originalList);
var qs = QuickSort();
// When called directly from main() the sort method DOES return the correct recurseCount
var stopwatch = Stopwatch()..start();
qs.sort(iList: list, useRandomPivot: true); //METHOD TO TEST
stopwatch.stop();
group('UNSORTED_RANDOM_PIVOT', () {
test('Time Taken: ${stopwatch.elapsedMicroseconds} microseconds', () {
expect(stopwatch.elapsedMicroseconds,
lessThanOrEqualTo(Tester._runControlTest(originalList)));
});
var nLogN = (list.length * (log2(list.length).ceil()));
test('Time Complexity: ${qs.recurseCount} vs $nLogN', () {
expect(qs.recurseCount, lessThanOrEqualTo(nLogN));
});
});
}
I was trying to do this function in C but for some reason is giving me "Illegal instruction: 4"... From what I looked up it may be because I'm using iOs, but still I'm using VSCode and not a project so I have no idea how to correct it.
The function is the following:
void divideSocios(void){
int i;
for (i = 0; i < MAXFILA || socios[i].next != -1; i++){
if (socios[i].sc.pago >= 50){
if (semDiv[0].next == 0){
semDiv[0].next = -1;
semDiv[0].sc.id = socios[i].sc.id;
semDiv[0].sc.pago = socios[i].sc.pago;
strcpy(semDiv[0].sc.nome, socios[i].sc.nome);
}
else{
for (; i < MAXFILA && semDiv[i].next != -1; i++);
semDiv[i-1].next = i;
semDiv[i].next = -1;
semDiv[i].sc.pago = socios[i].sc.pago;
semDiv[i].sc.id = socios[i].sc.id;
strcpy(semDiv[i].sc.nome, socios[i].sc.nome);
}
} else {
if (comDiv[0].next == 0){
comDiv[0].next = -1;
comDiv[0].sc.id = socios[i].sc.id;
comDiv[0].sc.pago = socios[i].sc.pago;
strcpy(comDiv[0].sc.nome, socios[i].sc.nome);
}
else{
for (; i < MAXFILA && comDiv[i].next != -1; i++);
comDiv[i-1].next = i;
comDiv[i].next = -1;
comDiv[i].sc.pago = socios[i].sc.pago;
comDiv[i].sc.id = socios[i].sc.id;
strcpy(comDiv[i].sc.nome, socios[i].sc.nome);
}
}
}
}
I basically have 3 different linked lists by matrix, each node with a struct and the index of the current node, and depending on the "pago" atribute from the struct of the node I want to separate them from the original list between the other 2.
I am using Seive method to calculate prime numbers up to a limit (say 100).
I constructed a List<bool> to hold if an index (which will eventually be the number) is prime or not. This part is fine.
Finally, I need return the numbers (index). Now I am using:
for(var i=2;i<=max;i++) {
if(!is_composite[i]){
primes.add(i);
}
}
return primes;
This will give the correct list containing only the prime numbers.
I am wondering if I can do a filter on is_composite and returns the "index" associated?
As requested, the complete code:
List<int> findPrimes(int max) {
var is_composite=List<bool>(max+1);
var primes=List<int>();
is_composite.fillRange(0, max, false);
for(var i=4;i<=max;i+=2) {
is_composite[i]=true;
}
var next_prime=3;
var stop=sqrt(max);
while(next_prime<=stop) {
for(var i=next_prime*2;i<=max;i+=next_prime) {
is_composite[i]=true;
}
next_prime+=2;
while(next_prime<=max && is_composite[next_prime]) {
next_prime+=2;
}
}
// TODO: is there a beter way instead of iterating the list?
for(var i=2;i<=max;i++) {
if(!is_composite[i]){
primes.add(i);
}
}
return primes;
}
For the given algorithm you could properly reorder some stuff to make it easier to get the result by changing your is_composite list of boolean into a is_not_composite list of integers which are closer to what your want to return as the result:
List<int> findPrimes(int max) {
final is_not_composite = List.generate(max + 1, (i) => i);
for (var i = 4; i <= max; i += 2) {
is_not_composite.remove(i);
}
var next_prime = 3;
final stop = sqrt(max);
while (next_prime <= stop) {
for (var i = next_prime * 2; i <= max; i += next_prime) {
is_not_composite.remove(i);
}
next_prime += 2;
while (next_prime <= max && !is_not_composite.contains(next_prime)) {
next_prime += 2;
}
}
return is_not_composite.sublist(2);
}
i'm trying to make an app that can create a list of available wifi access point. here's part of the code i used:
x = new BroadcastReceiver()
{
#Override
public void onReceive(Context c, Intent intent)
{
results = wifi.getScanResults();
size = results.size();
if (results != null) {
for (int i=0; i<size; i++){
ScanResult scanresult = wifi.getScanResults().get(i);
String ssid = scanresult.SSID;
int rssi = scanresult.level;
String rssiString = String.valueOf(rssi);
textStatus.append(ssid + "," + rssiString);
textStatus.append("\n");
}
unregisterReceiver(x); //stops the continuous scan
textState.setText("Scanning complete!");
} else {
unregisterReceiver(x);
textState.setText("Nothing is found. Please make sure you are under any wifi coverage");
}
}
};
both textStatus and textState is a TextView.
i can get this to work but sometimes the result shows duplicate SSID but with different signal level, in a single scan. there might be 3-4 same SSIDs but with different signal level.
is it really different SSIDs and what differs them? can anyone explain?
Are you having several router modems for the same network? For example: A company has a big wireless network with multiple router modems installed in several places so every room has Wifi. If you do that scan you will get a lot of results with the same SSIDs but with different acces points, and thus different signal level.
EDIT:
According to Walt's comment you can also have multiple results despite having only one access point if your modem is dual-band.
use below code to to remove duplicate ssids with highest signal strength
public void onReceive(Context c, Intent intent) {
ArrayList<ScanResult> mItems = new ArrayList<>();
List<ScanResult> results = wifiManager.getScanResults();
wifiListAdapter = new WifiListAdapter(ConnectToInternetActivity.this, mItems);
lv.setAdapter(wifiListAdapter);
int size = results.size();
HashMap<String, Integer> signalStrength = new HashMap<String, Integer>();
try {
for (int i = 0; i < size; i++) {
ScanResult result = results.get(i);
if (!result.SSID.isEmpty()) {
String key = result.SSID + " "
+ result.capabilities;
if (!signalStrength.containsKey(key)) {
signalStrength.put(key, i);
mItems.add(result);
wifiListAdapter.notifyDataSetChanged();
} else {
int position = signalStrength.get(key);
ScanResult updateItem = mItems.get(position);
if (calculateSignalStength(wifiManager, updateItem.level) >
calculateSignalStength(wifiManager, result.level)) {
mItems.set(position, updateItem);
wifiListAdapter.notifyDataSetChanged();
}
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
This is my simple Solution please and it is work for me
private void scanWifiListNew() {
wifiManager.startScan();
List<ScanResult> wifiList = wifiManager.getScanResults();
mWiFiList = new ArrayList<>();
for(ScanResult result: wifiList){
checkItemExists(mWiFiList, result);
}
setAdapter(mWiFiList);
}
private void printList(List<ScanResult> list){
for(ScanResult result: list){
int level = WifiManager.calculateSignalLevel(result.level, 100);
System.out.println(result.SSID + " Level is " + level + " out of 100");
}
}
private void checkItemExists(List<ScanResult> newWiFiList, ScanResult resultNew){
int indexToRemove = -1;
if(newWiFiList.size() > 0) {
for (int i = 0; i < newWiFiList.size(); i++) {
ScanResult resultCurrent = newWiFiList.get(i);
if (resultCurrent.SSID.equals(resultNew.SSID)) {
int levelCurrent = WifiManager.calculateSignalLevel(resultCurrent.level, 100);
int levelNew = WifiManager.calculateSignalLevel(resultNew.level, 100);
if (levelNew > levelCurrent) {
indexToRemove = i;
break;
}else indexToRemove = -2;
}
}
if(indexToRemove > -1){
newWiFiList.remove(indexToRemove);
newWiFiList.add(indexToRemove,resultNew);
}else if(indexToRemove == -1)newWiFiList.add(resultNew);
} else newWiFiList.add(resultNew);
}
private void setAdapter(List<ScanResult> list) {
listAdapter = new WifiListAdapter(getActivity().getApplicationContext(), list);
wifiListView.setAdapter(listAdapter);
}