# Queries on count of points lie inside a circle

Given n coordinate (x, y) of points on 2D plane and Q queries. Each query contains an integer r, the task is to count the number of points lying inside or on the circumference of the circle having radius r and centered at the origin.

Examples :

```Input : n = 5
Coordinates:
1 1
2 2
3 3
-1 -1
4 4

Query 1: 3
Query 2: 32

Output :
3
5
For first query radius = 3, number of points lie
inside or on the circumference are (1, 1), (-1, -1),
(2, 2). There are only 3 points lie inside or on
the circumference of the circle.
For second query radius = 32, all five points are
inside the circle.
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

The equation for the circle centered at origin (0, 0) with radius r, x2 + y2 = r2. And condition for a point at (x1, y1) to lie inside or on the circumference, x12 + y12 <= r2.

A Naive approach can be for each query, traverse through all points and check the condition. This take O(n*Q) time complexity.

An Efficient approach is to precompute x2 + y2 for each point coordinate and store them in an array p[]. Now, sort the array p[]. Then apply binary search on the array to find last index with condition p[i] <= r2 for each query.

Below is the implementation of this approach:

## C++

 `// C++ program to find number of points lie inside or ` `// on the cirumference of circle for Q queries. ` `#include ` `using` `namespace` `std; ` ` `  `// Computing the x^2 + y^2 for each given points ` `// and sorting them. ` `void` `preprocess(``int` `p[], ``int` `x[], ``int` `y[], ``int` `n) ` `{ ` `    ``for` `(``int` `i = 0; i < n; i++) ` `        ``p[i] = x[i] * x[i] + y[i] * y[i]; ` ` `  `    ``sort(p, p + n); ` `} ` ` `  `// Return count of points lie inside or on circumference ` `// of circle using binary search on p[0..n-1] ` `int` `query(``int` `p[], ``int` `n, ``int` `rad) ` `{ ` `    ``int` `start = 0, end = n - 1; ` `    ``while` `((end - start) > 1) { ` `        ``int` `mid = (start + end) / 2; ` `        ``double` `tp = ``sqrt``(p[mid]); ` ` `  `        ``if` `(tp > (rad * 1.0)) ` `            ``end = mid - 1; ` `        ``else` `            ``start = mid; ` `    ``} ` ` `  `    ``double` `tp1 = ``sqrt``(p[start]), tp2 = ``sqrt``(p[end]); ` ` `  `    ``if` `(tp1 > (rad * 1.0)) ` `        ``return` `0; ` `    ``else` `if` `(tp2 <= (rad * 1.0)) ` `        ``return` `end + 1; ` `    ``else` `        ``return` `start + 1; ` `} ` ` `  `// Driven Program ` `int` `main() ` `{ ` `    ``int` `x[] = { 1, 2, 3, -1, 4 }; ` `    ``int` `y[] = { 1, 2, 3, -1, 4 }; ` `    ``int` `n = ``sizeof``(x) / ``sizeof``(x); ` ` `  `    ``// Compute distances of all points and keep ` `    ``// the distances sorted so that query can ` `    ``// work in O(logn) using Binary Search. ` `    ``int` `p[n]; ` `    ``preprocess(p, x, y, n); ` ` `  `    ``// Print number of points in a circle of radius 3. ` `    ``cout << query(p, n, 3) << endl; ` ` `  `    ``// Print number of points in a circle of radius 32. ` `    ``cout << query(p, n, 32) << endl; ` `    ``return` `0; ` `} `

## Java

 `// JAVA Code for Queries on count of ` `// points lie inside a circle ` `import` `java.util.*; ` ` `  `class` `GFG { ` ` `  `    ``// Computing the x^2 + y^2 for each given points ` `    ``// and sorting them. ` `    ``public` `static` `void` `preprocess(``int` `p[], ``int` `x[], ` `                                  ``int` `y[], ``int` `n) ` `    ``{ ` `        ``for` `(``int` `i = ``0``; i < n; i++) ` `            ``p[i] = x[i] * x[i] + y[i] * y[i]; ` ` `  `        ``Arrays.sort(p); ` `    ``} ` ` `  `    ``// Return count of points lie inside or on ` `    ``// circumference of circle using binary ` `    ``// search on p[0..n-1] ` `    ``public` `static` `int` `query(``int` `p[], ``int` `n, ``int` `rad) ` `    ``{ ` `        ``int` `start = ``0``, end = n - ``1``; ` `        ``while` `((end - start) > ``1``) { ` `            ``int` `mid = (start + end) / ``2``; ` `            ``double` `tp = Math.sqrt(p[mid]); ` ` `  `            ``if` `(tp > (rad * ``1.0``)) ` `                ``end = mid - ``1``; ` `            ``else` `                ``start = mid; ` `        ``} ` ` `  `        ``double` `tp1 = Math.sqrt(p[start]); ` `        ``double` `tp2 = Math.sqrt(p[end]); ` ` `  `        ``if` `(tp1 > (rad * ``1.0``)) ` `            ``return` `0``; ` `        ``else` `if` `(tp2 <= (rad * ``1.0``)) ` `            ``return` `end + ``1``; ` `        ``else` `            ``return` `start + ``1``; ` `    ``} ` ` `  `    ``/* Driver program to test above function */` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``int` `x[] = { ``1``, ``2``, ``3``, -``1``, ``4` `}; ` `        ``int` `y[] = { ``1``, ``2``, ``3``, -``1``, ``4` `}; ` `        ``int` `n = x.length; ` ` `  `        ``// Compute distances of all points and keep ` `        ``// the distances sorted so that query can ` `        ``// work in O(logn) using Binary Search. ` `        ``int` `p[] = ``new` `int``[n]; ` `        ``preprocess(p, x, y, n); ` ` `  `        ``// Print number of points in a circle of ` `        ``// radius 3. ` `        ``System.out.println(query(p, n, ``3``)); ` ` `  `        ``// Print number of points in a circle of ` `        ``// radius 32. ` `        ``System.out.println(query(p, n, ``32``)); ` `    ``} ` `} ` `// This code is contributed by Arnav Kr. Mandal. `

## Python 3

 `# Python 3 program to find number of  ` `# points lie inside or on the cirumference  ` `# of circle for Q queries. ` `import` `math ` ` `  `# Computing the x^2 + y^2 for each  ` `# given points and sorting them. ` `def` `preprocess(p, x, y, n): ` `    ``for` `i ``in` `range``(n): ` `        ``p[i] ``=` `x[i] ``*` `x[i] ``+` `y[i] ``*` `y[i] ` ` `  `    ``p.sort() ` ` `  `# Return count of points lie inside  ` `# or on circumference of circle using ` `# binary search on p[0..n-1] ` `def` `query(p, n, rad): ` ` `  `    ``start ``=` `0` `    ``end ``=` `n ``-` `1` `    ``while` `((end ``-` `start) > ``1``): ` `        ``mid ``=` `(start ``+` `end) ``/``/` `2` `        ``tp ``=` `math.sqrt(p[mid]) ` ` `  `        ``if` `(tp > (rad ``*` `1.0``)): ` `            ``end ``=` `mid ``-` `1` `        ``else``: ` `            ``start ``=` `mid ` ` `  `    ``tp1 ``=` `math.sqrt(p[start]) ` `    ``tp2 ``=` `math.sqrt(p[end]) ` ` `  `    ``if` `(tp1 > (rad ``*` `1.0``)): ` `        ``return` `0` `    ``elif` `(tp2 <``=` `(rad ``*` `1.0``)): ` `        ``return` `end ``+` `1` `    ``else``: ` `        ``return` `start ``+` `1` ` `  `# Driver Code ` `if` `__name__ ``=``=` `"__main__"``: ` `     `  `    ``x ``=` `[ ``1``, ``2``, ``3``, ``-``1``, ``4` `] ` `    ``y ``=` `[ ``1``, ``2``, ``3``, ``-``1``, ``4` `] ` `    ``n ``=` `len``(x) ` ` `  `    ``# Compute distances of all points and keep ` `    ``# the distances sorted so that query can ` `    ``# work in O(logn) using Binary Search. ` `    ``p ``=` `[``0``] ``*` `n ` `    ``preprocess(p, x, y, n) ` ` `  `    ``# Print number of points in a  ` `    ``# circle of radius 3. ` `    ``print``(query(p, n, ``3``)) ` ` `  `    ``# Print number of points in a  ` `    ``# circle of radius 32. ` `    ``print``(query(p, n, ``32``)) ` ` `  `# This code is contributed by ita_c `

## C#

 `// C# Code for Queries on count of ` `// points lie inside a circle ` `using` `System; ` ` `  `class` `GFG { ` ` `  `    ``// Computing the x^2 + y^2 for each  ` `    ``// given points and sorting them. ` `    ``public` `static` `void` `preprocess(``int``[] p, ``int``[] x, ` `                                    ``int``[] y, ``int` `n) ` `    ``{ ` `        ``for` `(``int` `i = 0; i < n; i++) ` `            ``p[i] = x[i] * x[i] + y[i] * y[i]; ` ` `  `        ``Array.Sort(p); ` `    ``} ` ` `  `    ``// Return count of points lie inside or on ` `    ``// circumference of circle using binary ` `    ``// search on p[0..n-1] ` `    ``public` `static` `int` `query(``int``[] p, ``int` `n, ``int` `rad) ` `    ``{ ` `        ``int` `start = 0, end = n - 1; ` `        ``while` `((end - start) > 1) { ` `            ``int` `mid = (start + end) / 2; ` `            ``double` `tp = Math.Sqrt(p[mid]); ` ` `  `            ``if` `(tp > (rad * 1.0)) ` `                ``end = mid - 1; ` `            ``else` `                ``start = mid; ` `        ``} ` ` `  `        ``double` `tp1 = Math.Sqrt(p[start]); ` `        ``double` `tp2 = Math.Sqrt(p[end]); ` ` `  `        ``if` `(tp1 > (rad * 1.0)) ` `            ``return` `0; ` `        ``else` `if` `(tp2 <= (rad * 1.0)) ` `            ``return` `end + 1; ` `        ``else` `            ``return` `start + 1; ` `    ``} ` ` `  `    ``/* Driver program to test above function */` `    ``public` `static` `void` `Main() ` `    ``{ ` `        ``int``[] x = { 1, 2, 3, -1, 4 }; ` `        ``int``[] y = { 1, 2, 3, -1, 4 }; ` `        ``int` `n = x.Length; ` ` `  `        ``// Compute distances of all points and keep ` `        ``// the distances sorted so that query can ` `        ``// work in O(logn) using Binary Search. ` `        ``int``[] p = ``new` `int``[n]; ` `        ``preprocess(p, x, y, n); ` ` `  `        ``// Print number of points in a circle of ` `        ``// radius 3. ` `        ``Console.WriteLine(query(p, n, 3)); ` ` `  `        ``// Print number of points in a circle of ` `        ``// radius 32. ` `        ``Console.WriteLine(query(p, n, 32)); ` `    ``} ` `} ` ` `  `// This code is contributed by vt_m. `

Output:

```3
5
```

Time Complexity: O(n log n) for preprocessing and O(Q Log n) for Q queries.

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Improved By : vt_m, chitranayal

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