# Java Equivalent of C++’s lower_bound() Method

• Last Updated : 29 Apr, 2022

The lower_bound() method of C++ returns the index of the first element in the array which has a value not less than the key. This means that the function returns the index of the next smallest number just greater than or equal to that number. If there are multiple values that are equal to the number, lower_bound() returns the index of the first such value.

Examples:

Input  : 4 6 10 12 18 18 20 20 30 45
Output : lower_bound for element 18 at index 4

Input  : 4 6 10 12 16 20 28
Output : lower_bound for element 18 at index 5

Input  : 24 26 40 56
Output : lower_bound for element 18 at index 0

Input  : 4 6 10 12 16 17
Output : lower_bound for element 18 at index 6

Now let us discuss the methods in order to use lower_bound() method in order to get the index of the next smallest number just greater than or equal to that number.

Methods:

1. Naive Approach
2. Using binary search iteratively
3. Using binary search recursively
4. Using binarySearch() method of Arrays utility class

Method 1: Using linear search
We can use linear search to find lower_bound. We will iterate over the array starting from the 0th index until we find a value equal to or greater than the key.

Below is the implementation of the above approach:

## Java

 `// Java program for finding lower bound` `// using linear search`   `// Importing Arrays utility class` `import` `java.util.Arrays;`   `// Main class` `class` `GFG {`   `    ``// Method 1` `    ``// To find lower bound of given key` `    ``static` `int` `lower(``int` `array[], ``int` `key)` `    ``{` `        ``int` `lowerBound = ``0``;`   `        ``// Traversing the array using length function` `        ``while` `(lowerBound < array.length) {`   `            ``// If key is lesser than current value` `            ``if` `(key > array[lowerBound])` `                ``lowerBound++;`   `            ``// This is either the first occurrence of key` `            ``// or value just greater than key` `            ``else` `                ``return` `lowerBound;` `        ``}`   `        ``return` `lowerBound;` `    ``}`   `    ``// Method 2` `    ``// Main driver method` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``// Custom array input over which lower bound is to` `        ``// be operated by passing a key` `        ``int` `array[]` `            ``= { ``4``, ``6``, ``10``, ``12``, ``18``, ``18``, ``20``, ``20``, ``30``, ``45` `};` `        ``int` `key = ``18``;`   `        ``// Sort the array using Arrays.sort() method` `        ``Arrays.sort(array);`   `        ``// Printing the lower bound` `        ``System.out.println(lower(array, key));` `    ``}` `}`

Output

`4`

Time Complexity: O(N), where N is the number of elements in the array.
Auxiliary Space: O(1)

We can use an efficient approach of binary search to search the key in the sorted array in O(log2 n) as proposed in the below example

Method  2: Using binary search iteratively

Procedure:

1. Initialize the low as 0 and high as N.
2. Compare key with the middle element(arr[mid])
3. If the middle element is greater than or equal to the key then update the high as a middle index(mid).
4. Else update low as mid + 1.
5. Repeat step 2 to step 4 until low is less than high.
6. After all the above steps the low is the lower_bound of a key in the given array.

Below is the implementation of the above approach:

## Java

 `// Java program to Find lower bound` `// Using Binary Search Iteratively`   `// Importing Arrays utility class` `import` `java.util.Arrays;`   `// Main class` `public` `class` `GFG {`   `    ``// Method 1` `    ``// Iterative approach to find lower bound` `    ``// using binary search technique` `    ``static` `int` `lower_bound(``int` `array[], ``int` `key)` `    ``{` `        ``// Initialize starting index and` `        ``// ending index` `        ``int` `low = ``0``, high = array.length;` `        ``int` `mid;`   `        ``// Till high does not crosses low` `        ``while` `(low < high) {`   `            ``// Find the index of the middle element` `            ``mid = low + (high - low) / ``2``;`   `            ``// If key is less than or equal` `            ``// to array[mid], then find in` `            ``// left subarray` `            ``if` `(key <= array[mid]) {` `                ``high = mid;` `            ``}`   `            ``// If key is greater than array[mid],` `            ``// then find in right subarray` `            ``else` `{`   `                ``low = mid + ``1``;` `            ``}` `        ``}`   `        ``// If key is greater than last element which is` `        ``// array[n-1] then lower bound` `        ``// does not exists in the array` `        ``if` `(low < array.length && array[low] < key) {` `            ``low++;` `        ``}`   `        ``// Returning the lower_bound index` `        ``return` `low;` `    ``}`   `    ``// Method 2` `    ``// Driver main method` `    ``public` `static` `void` `main(String[] args)` `    ``{`   `        ``// Custom array and key input over which lower bound` `        ``// is computed` `        ``int` `array[]` `            ``= { ``4``, ``6``, ``10``, ``12``, ``18``, ``18``, ``20``, ``20``, ``30``, ``45` `};` `        ``int` `key = ``18``;`   `        ``// Sort the array  using Arrays.sort() method` `        ``Arrays.sort(array);`   `        ``// Printing the lower bound` `        ``System.out.println(lower_bound(array, key));` `    ``}` `}`

Output

`4`

Now as usual optimizing further away by providing a recursive approach following the same procedure as discussed above.

Method 3: Using binary search recursively

## Java

 `// Java program to Find Lower Bound` `// Using Binary Search Recursively`   `// Importing Arrays utility class` `import` `java.util.Arrays;`   `// Main class` `public` `class` `GFG {`   `    ``// Method 1` `    ``// To find lower bound using binary search technique` `    ``static` `int` `recursive_lower_bound(``int` `array[], ``int` `low,` `                                     ``int` `high, ``int` `key)` `    ``{` `        ``// Base Case` `        ``if` `(low > high) {` `            ``return` `low;` `        ``}`   `        ``// Find the middle index` `        ``int` `mid = low + (high - low) / ``2``;`   `        ``// If key is lesser than or equal to` `        ``// array[mid] , then search` `        ``// in left subarray` `        ``if` `(key <= array[mid]) {`   `            ``return` `recursive_lower_bound(array, low,` `                                         ``mid - ``1``, key);` `        ``}`   `        ``// If key is greater than array[mid],` `        ``// then find in right subarray` `        ``return` `recursive_lower_bound(array, mid + ``1``, high,` `                                     ``key);` `    ``}`   `    ``// Method 2` `    ``// To compute the lower bound` `    ``static` `int` `lower_bound(``int` `array[], ``int` `key)` `    ``{` `        ``// Initialize starting index and` `        ``// ending index` `        ``int` `low = ``0``, high = array.length;`   `        ``// Call recursive lower bound method` `        ``return` `recursive_lower_bound(array, low, high, key);` `    ``}`   `    ``// Method 3` `    ``// Main driver method` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``// Custom array and key over which lower bound is to` `        ``// be computed` `        ``int` `array[]` `            ``= { ``4``, ``6``, ``10``, ``12``, ``18``, ``18``, ``20``, ``20``, ``30``, ``45` `};` `        ``int` `key = ``18``;`   `        ``// Sorting the array using Arrays.sort() method` `        ``Arrays.sort(array);`   `        ``// Printing the lower bound` `        ``System.out.println(lower_bound(array, key));` `    ``}` `}`

Output

`4`

We can also use the in-built binary search implementation of the Arrays utility class (or Collections utility class). The function returns an index of the search key, if it is contained in the array; otherwise, (-(insertion point) – 1). The insertion point is defined as the point at which the key would be inserted into the array.

Approach:

1. Sort the array before applying binary search
2. Search the index of the key in the sorted array using Arrays.binarysearch()
• Check if it key is present in the array, if true then return the index of the key as a positive value.
• Otherwise, a negative value which specifies the position at which the key should be added to the sorted array.
3. If the key is present in the array we move leftwards to find its first occurrence
4. else, we would have got a negative value of an index, using that to calculate the value of the “insertion point” (i.e, the index of the first element greater than the key)
5. Print it.

Below is the implementation of the above approach:

## Java

 `// Java program to find lower bound` `// using binarySearch() method of Arrays class`   `// Importing Arrays utility class` `import` `java.util.Arrays;`   `// Main class` `public` `class` `GFG {`   `    ``// Method 1` `    ``// To find lower bound using binary search` `    ``// implementation of Arrays utility class` `    ``static` `int` `lower_bound(``int` `array[], ``int` `key)` `    ``{`   `        ``int` `index = Arrays.binarySearch(array, key);`   `        ``// If key is not present in the array` `        ``if` `(index < ``0``) {`   `            ``// Index specify the position of the key` `            ``// when inserted in the sorted array` `            ``// so the element currently present at` `            ``// this position will be the lower bound` `            ``return` `Math.abs(index) - ``1``;` `        ``}`   `        ``// If key is present in the array` `        ``// we move leftwards to find its first occurrence` `        ``else` `{`   `            ``// Decrement the index to find the first` `            ``// occurrence of the key`   `            ``while` `(index > ``0``) {`   `                ``// If previous value is same` `                ``if` `(array[index - ``1``] == key)` `                    ``index--;`   `                ``// Previous value is different which means` `                ``// current index is the first occurrence of` `                ``// the key` `                ``else` `                    ``return` `index;` `            ``}`   `            ``return` `index;` `        ``}` `    ``}`   `    ``// Method 2` `    ``// Main driver method` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``//` `        ``int` `array[]` `            ``= { ``4``, ``6``, ``10``, ``12``, ``18``, ``18``, ``20``, ``20``, ``30``, ``45` `};` `        ``int` `key = ``18``;`   `        ``// Sort the array before applying binary search` `        ``Arrays.sort(array);` `      `  `        ``// Printing the lower bound ` `        ``System.out.println(lower_bound(array, key));` `    ``}` `}`

Output

`4`

Note: We can also find mid-value via any one of them

int mid = (high + low)/ 2;
int mid = (low + high) >>> 1;

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