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How to Check if an Array Contains a Value in Java Efficiently?---reference

2014-04-09 08:39 381 查看

How to check if an array (unsorted) contains a certain value? This is a very useful and frequently used operation in Java. It is also a top voted question on Stack Overflow. As shown in top voted answers, checking if an array contains a certain value can be done in several different ways, but the time complexity could be very different. In the following I will show the time each method takes.

1. 4 Different Ways to Check If an Array Contains a Value

1) Using List:

public static boolean useList(String[] arr, String targetValue) {
return Arrays.asList(arr).contains(targetValue);
}

2) Using Set:

public static boolean useSet(String[] arr, String targetValue) {
Set<String> set = new HashSet<String>(Arrays.asList(arr));
return set.contains(targetValue);
}

3) Using a simple loop:

public static boolean useLoop(String[] arr, String targetValue) {
for(String s: arr){
if(s.equals(targetValue))
return true;
}
return false;
}

4) Using Arrays.binarySearech():

public static boolean useArraysBinarySearch(String[] arr, String targetValue) {
int a =  Arrays.binarySearch(arr, targetValue);
if(a > 0)
return true;
else
return false;
}

2. Time Complexity

The approximate time complexity can be compared by using the following code. It is not precise, just search an array of size 5, 1k, 10k, but the idea is clear.

public static void main(String[] args) {
String[] arr = new String[] {  "CD",  "BC", "EF", "DE", "AB"};

//use list
long startTime = System.nanoTime();
for (int i = 0; i < 100000; i++) {
useList(arr, "A");
}
long endTime = System.nanoTime();
long duration = endTime - startTime;
System.out.println("useList:  " + duration / 1000000);

//use set
startTime = System.nanoTime();
for (int i = 0; i < 100000; i++) {
useSet(arr, "A");
}
endTime = System.nanoTime();
duration = endTime - startTime;
System.out.println("useSet:  " + duration / 1000000);

//use loop
startTime = System.nanoTime();
for (int i = 0; i < 100000; i++) {
useLoop(arr, "A");
}
endTime = System.nanoTime();
duration = endTime - startTime;
System.out.println("useLoop:  " + duration / 1000000);

//use Arrays.binarySearch()
startTime = System.nanoTime();
for (int i = 0; i < 100000; i++) {
useArraysBinarySearch(arr, "A");
}
endTime = System.nanoTime();
duration = endTime - startTime;
System.out.println("useArrayBinary:  " + duration / 1000000);
}

Result:

useList:  13
useSet:  72
useLoop:  5
useArraysBinarySearch:  9

Use a larger array (1k):

String[] arr = new String[1000];

Random s = new Random();
for(int i=0; i< 1000; i++){
arr[i] = String.valueOf(s.nextInt());
}

Result:

useList:  112
useSet:  2055
useLoop:  99
useArrayBinary:  12

Use a larger array (10k):

String[] arr = new String[10000];

Random s = new Random();
for(int i=0; i< 10000; i++){
arr[i] = String.valueOf(s.nextInt());
}

Result:

useList:  1590
useSet:  23819
useLoop:  1526
useArrayBinary:  12

Clearly, using a simple loop method is more efficient than using any collection. A lot of developers use the first method, but it is inefficient. Pushing the array to another Collection type will require spin through all elements to read them in before doing anything with the collection type.

The array must be sorted, if Arrays.binarySearch() method is used. In this case, the array is not sorted, therefore, it should not be used.

Actually, if you really need to check if a value is contained in some array/collection efficiently, a sorted list or tree can do it in O(log(n)) or hashset can do it in O(1).

reference from:http://www.programcreek.com/2014/04/check-if-array-contains-a-value-java/

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