Как найти индекс числа в массиве чисел

Добавьте тогда пример, чтобы не был голый текст, например: def index(items, value): for i, x in enumerate(items): if x == value: return i это после цикла: return -1. После пример использования: s = [3,4,1,2,5] print(index(s, 1))

7 июн 2018 в 8:07

Given an array of N elements and an element K, find the index of an array element in Java. 
Examples: 

Input: a[] = { 5, 4, 6, 1, 3, 2, 7, 8, 9 }, K = 5
Output: 0

Input: a[] = { 5, 4, 6, 1, 3, 2, 7, 8, 9 }, K = 7
Output: 6

An element in an array of N integers can be searched using the below-mentioned methods.  

1. Linear Search: Doing a linear search in an array, the element can be found in O(N) complexity. 
   Below is the implementation of the linear-search approach:

Index position of 5 is: 0
Index position of 7 is: 6

Time Complexity: O(N)
Auxiliary Space: O(1)

2. Binary search: Binary search can also be used to find the index of the array element in an array. But the binary search can only be used if the array is sorted. Java provides us with an inbuilt function which can be found in the Arrays library of Java which will return the index if the element is present, else it returns -1. The complexity will be O(log n). 
Below is the implementation of Binary search. 

Index position of 5 is: 4
Index position of 7 is: 6

Time Complexity: O(log N)
Auxiliary Space: O(1)

3. Guava: Guava is an open source, Java-based library developed by Google. It provides utility methods for collections, caching, primitives support, concurrency, common annotations, string processing, I/O, and validations. Guava provides several-utility class pertaining to be primitive like Ints for int, Longs for long, Doubles for double etc. Each utility class has an indexOf() method that returns the index of the first appearance of the element in array.

Below is the implementation of Guava.

Index position of 5 is: 0
Index position of 7 is: 6

Time Complexity: O(N)
Auxiliary Space: O(1)

4. Stream API: Stream is a new abstract layer introduced in Java 8. Using stream, you can process data in a declarative way similar to SQL statements. The stream represents a sequence of objects from a source, which supports aggregate operations. In order to find the index of an element Stream package provides utility, IntStream. Using the length of an array we can get an IntStream of array indices from 0 to n-1, where n is the length of an array.
Below is the implementation of Stream API approach. 

Index position of 5 is: 0
Index position of 7 is: 6

Time Complexity: O(N)
Auxiliary Space: O(1)

5. Using ArrayList

In this approach, we will convert the array into ArrayList, and then we will use the indexOf method of ArrayList to get the index of the element.

Output:

Index position of 5 is: 0
Index position of 7 is: 6

Time Complexity: O(N)
Auxiliary Space: O(N)

6. Using Recursion

We will use recursion to find the first index of the given element.

Output:

Index position of 5 is: 0
Index position of 7 is: 6

Time Complexity: O(N)
Auxiliary Space: O(1)

Last Updated :
20 Feb, 2023

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Чтобы найти индекс искомого элемента в массиве можно в цикле перебрать все элементы и сравнить их с искомым. Если они равны, будет выведен индекс первого подходящего элемента.

// Число, которое будем искать
int num = 3;

// Переменная для хранения индекса,
// найденного числа
int index = -1;

int[] arr = {1, 2, 3, 4, 5};

for (int i = 0; i < arr.length; i++) {
    // Если элемент и число равны, то 
    // сохраняй индекс
    if (arr[i] == num) {
        index = i;
    }
}

System.out.println(index); // => 2

Также можно воспользоваться пакетом org.apache.commons.lang, методом indexOf() из класса ArrayUtils для нахождения индекса элемента.

import org.apache.commons.lang3.ArrayUtils;

public class Example {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};

        // индекс числа 3
        int index = ArrayUtils.indexOf(arr, 3);

        System.out.println(index); // => 2
    }
}