# Find i’th index character in a binary string obtained after n iterations | Set 2

Given a decimal number m, convert it into a binary string and apply n iterations, in each iteration 0 becomes “01” and 1 becomes “10”. Find ith(based indexing) index character in the string after nth iteration.

**Examples**:

Input: m = 5 i = 5 n = 3 Output: 1ExplanationIn the first case m = 5, i = 5, n = 3. Initially, the string is 101 ( binary equivalent of 5 ) After 1st iteration - 100110 After 2nd iteration - 100101101001 After 3rd iteration - 100101100110100110010110 The character at index 5 is 1, so 1 is the answer Input: m = 11 i = 6 n = 4 Output: 1

A **naive approach** to this problem has been discussed in the previous post.

**Efficient algorithm**: The first step will be to find which block the i-th character will be after N iterations are performed. In the n’th iteration distance between any two consecutive characters initially will always be equal to 2^n. For a general number m, the number of blocks will be ceil(log m). If M was 3, the string gets divided into 3 blocks. Find the block number in which kth character will lie by k / (2^n), where n is the number of iterations. Consider m=5, then the binary representation is 101. Then the distance between any 2 consecutive marked characters in any i’th iteration will be as follows

0th iteration: 101, distance = 0

1st iteration: **1**0 **0**1 **1** 0, distance = 2

2nd iteration: 1001 **0**110 **1**001, distance = 4

3rd iteration: **1**0010110 **0**1101001 **1**0010110, distance = 8

In the example k = 5 and n = 3, so Block_number, when k is 5, will be 0, as 5 / (2^3) = 0

Initially, block numbers will be

Original String : 1 0 1 Block_number : 0 1 2

There is no need to generate the entire string, only computing in the block in which the i-th character is present will give the answer. Let this character be root **root = s[Block_number]**, where s is the binary representation of “m”. Now in the final string, find the distance of the kth character from the block number, call this distance as remaining. So **remaining = k % (2^n)** will be the index of i-th character in the block. If remaining is 0, the root will be the answer. Now, in order to check whether the root is the actual answer use a boolean variable *flip* which whether we need to flip our answer or not. Following the below algorithm will give the character at the i-th index.

bool flip = true; while(remaining > 1){ if( remaining is odd ) flip = !flip remaining = remaining/2; }

Below is the implementation of the above approach:

## C++

`// C++ program to find i’th Index character ` `// in a binary string obtained after n iterations ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `// Function to find the i-th character ` `void` `KthCharacter(` `int` `m, ` `int` `n, ` `int` `k) ` `{ ` ` ` `// distance between two consecutive ` ` ` `// elements after N iterations ` ` ` `int` `distance = ` `pow` `(2, n); ` ` ` `int` `Block_number = k / distance; ` ` ` `int` `remaining = k % distance; ` ` ` ` ` `int` `s[32], x = 0; ` ` ` ` ` `// binary representation of M ` ` ` `for` `(; m > 0; x++) { ` ` ` `s[x] = m % 2; ` ` ` `m = m / 2; ` ` ` `} ` ` ` ` ` `// kth digit will be derived from root for sure ` ` ` `int` `root = s[x - 1 - Block_number]; ` ` ` ` ` `if` `(remaining == 0) { ` ` ` `cout << root << endl; ` ` ` `return` `; ` ` ` `} ` ` ` ` ` `// Check whether there is need to ` ` ` `// flip root or not ` ` ` `bool` `flip = ` `true` `; ` ` ` `while` `(remaining > 1) { ` ` ` `if` `(remaining & 1) { ` ` ` `flip = !flip; ` ` ` `} ` ` ` `remaining = remaining >> 1; ` ` ` `} ` ` ` ` ` `if` `(flip) { ` ` ` `cout << !root << endl; ` ` ` `} ` ` ` `else` `{ ` ` ` `cout << root << endl; ` ` ` `} ` `} ` ` ` `// Driver Code ` `int` `main() ` `{ ` ` ` `int` `m = 5, k = 5, n = 3; ` ` ` `KthCharacter(m, n, k); ` ` ` `return` `0; ` `} ` |

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## Java

`// Java program to find ith ` `// Index character in a binary ` `// string obtained after n iterations ` `import` `java.io.*; ` ` ` `class` `GFG ` `{ ` `// Function to find ` `// the i-th character ` `static` `void` `KthCharacter(` `int` `m, ` ` ` `int` `n, ` `int` `k) ` `{ ` ` ` `// distance between two ` ` ` `// consecutive elements ` ` ` `// after N iterations ` ` ` `int` `distance = (` `int` `)Math.pow(` `2` `, n); ` ` ` `int` `Block_number = k / distance; ` ` ` `int` `remaining = k % distance; ` ` ` ` ` `int` `s[] = ` `new` `int` `[` `32` `]; ` ` ` `int` `x = ` `0` `; ` ` ` ` ` `// binary representation of M ` ` ` `for` `(; m > ` `0` `; x++) ` ` ` `{ ` ` ` `s[x] = m % ` `2` `; ` ` ` `m = m / ` `2` `; ` ` ` `} ` ` ` ` ` `// kth digit will be ` ` ` `// derived from root ` ` ` `// for sure ` ` ` `int` `root = s[x - ` `1` `- ` ` ` `Block_number]; ` ` ` ` ` `if` `(remaining == ` `0` `) ` ` ` `{ ` ` ` `System.out.println(root); ` ` ` `return` `; ` ` ` `} ` ` ` ` ` `// Check whether there is ` ` ` `// need to flip root or not ` ` ` `Boolean flip = ` `true` `; ` ` ` `while` `(remaining > ` `1` `) ` ` ` `{ ` ` ` `if` `((remaining & ` `1` `) > ` `0` `) ` ` ` `{ ` ` ` `flip = !flip; ` ` ` `} ` ` ` `remaining = remaining >> ` `1` `; ` ` ` `} ` ` ` ` ` `if` `(flip) ` ` ` `{ ` ` ` `System.out.println((root > ` `0` `)?` `0` `:` `1` `); ` ` ` `} ` ` ` `else` ` ` `{ ` ` ` `System.out.println(root); ` ` ` `} ` `} ` ` ` `// Driver Code ` `public` `static` `void` `main (String[] args) ` `{ ` ` ` `int` `m = ` `5` `, k = ` `5` `, n = ` `3` `; ` ` ` `KthCharacter(m, n, k); ` `} ` `} ` ` ` `// This code is contributed ` `// by anuj_67. ` |

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## Python3

`# Python3 program to find ` `# i’th Index character in ` `# a binary string obtained ` `# after n iterations ` ` ` `# Function to find ` `# the i-th character ` `def` `KthCharacter(m, n, k): ` ` ` ` ` `# distance between two ` ` ` `# consecutive elements ` ` ` `# after N iterations ` ` ` `distance ` `=` `pow` `(` `2` `, n) ` ` ` `Block_number ` `=` `int` `(k ` `/` `distance) ` ` ` `remaining ` `=` `k ` `%` `distance ` ` ` ` ` `s ` `=` `[` `0` `] ` `*` `32` ` ` `x ` `=` `0` ` ` ` ` `# binary representation of M ` ` ` `while` `(m > ` `0` `) : ` ` ` `s[x] ` `=` `m ` `%` `2` ` ` `m ` `=` `int` `(m ` `/` `2` `) ` ` ` `x ` `+` `=` `1` ` ` ` ` `# kth digit will be derived ` ` ` `# from root for sure ` ` ` `root ` `=` `s[x ` `-` `1` `-` `Block_number] ` ` ` ` ` `if` `(remaining ` `=` `=` `0` `): ` ` ` `print` `(root) ` ` ` `return` ` ` ` ` `# Check whether there ` ` ` `# is need to flip root ` ` ` `# or not ` ` ` `flip ` `=` `True` ` ` `while` `(remaining > ` `1` `): ` ` ` `if` `(remaining & ` `1` `): ` ` ` `flip ` `=` `not` `(flip) ` ` ` ` ` `remaining ` `=` `remaining >> ` `1` ` ` ` ` `if` `(flip) : ` ` ` `print` `(` `not` `(root)) ` ` ` ` ` `else` `: ` ` ` `print` `(root) ` ` ` `# Driver Code ` `m ` `=` `5` `k ` `=` `5` `n ` `=` `3` `KthCharacter(m, n, k) ` ` ` `# This code is contributed ` `# by smita ` |

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## C#

`// C# program to find ith ` `// Index character in a ` `// binary string obtained ` `// after n iterations ` `using` `System; ` ` ` `class` `GFG ` `{ ` `// Function to find ` `// the i-th character ` `static` `void` `KthCharacter(` `int` `m, ` ` ` `int` `n, ` ` ` `int` `k) ` `{ ` ` ` `// distance between two ` ` ` `// consecutive elements ` ` ` `// after N iterations ` ` ` `int` `distance = (` `int` `)Math.Pow(2, n); ` ` ` `int` `Block_number = k / distance; ` ` ` `int` `remaining = k % distance; ` ` ` ` ` `int` `[]s = ` `new` `int` `[32]; ` ` ` `int` `x = 0; ` ` ` ` ` `// binary representation of M ` ` ` `for` `(; m > 0; x++) ` ` ` `{ ` ` ` `s[x] = m % 2; ` ` ` `m = m / 2; ` ` ` `} ` ` ` ` ` `// kth digit will be ` ` ` `// derived from root ` ` ` `// for sure ` ` ` `int` `root = s[x - 1 - ` ` ` `Block_number]; ` ` ` ` ` `if` `(remaining == 0) ` ` ` `{ ` ` ` `Console.WriteLine(root); ` ` ` `return` `; ` ` ` `} ` ` ` ` ` `// Check whether there is ` ` ` `// need to flip root or not ` ` ` `Boolean flip = ` `true` `; ` ` ` `while` `(remaining > 1) ` ` ` `{ ` ` ` `if` `((remaining & 1) > 0) ` ` ` `{ ` ` ` `flip = !flip; ` ` ` `} ` ` ` ` ` `remaining = remaining >> 1; ` ` ` `} ` ` ` ` ` `if` `(flip) ` ` ` `{ ` ` ` `Console.WriteLine(!(root > 0)); ` ` ` `} ` ` ` `else` ` ` `{ ` ` ` `Console.WriteLine(root); ` ` ` `} ` `} ` ` ` `// Driver Code ` `public` `static` `void` `Main () ` `{ ` ` ` `int` `m = 5, k = 5, n = 3; ` ` ` `KthCharacter(m, n, k); ` `} ` `} ` ` ` `// This code is contributed ` `// by anuj_67. ` |

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## PHP

`<?php ` `// PHP program to find i’th Index character ` `// in a binary string obtained after n iterations ` ` ` `// Function to find the i-th character ` `function` `KthCharacter(` `$m` `, ` `$n` `, ` `$k` `) ` `{ ` ` ` `// distance between two consecutive ` ` ` `// elements after N iterations ` ` ` `$distance` `= pow(2, ` `$n` `); ` ` ` `$Block_number` `= ` `intval` `(` `$k` `/ ` `$distance` `); ` ` ` `$remaining` `= ` `$k` `% ` `$distance` `; ` ` ` ` ` `$s` `= ` `array` `(32); ` ` ` `$x` `= 0; ` ` ` ` ` `// binary representation of M ` ` ` `for` `(; ` `$m` `> 0; ` `$x` `++) ` ` ` `{ ` ` ` `$s` `[` `$x` `] = ` `$m` `% 2; ` ` ` `$m` `= ` `intval` `(` `$m` `/ 2); ` ` ` `} ` ` ` ` ` `// kth digit will be derived from ` ` ` `// root for sure ` ` ` `$root` `= ` `$s` `[` `$x` `- 1 - ` `$Block_number` `]; ` ` ` ` ` `if` `(` `$remaining` `== 0) ` ` ` `{ ` ` ` `echo` `$root` `. ` `"\n"` `; ` ` ` `return` `; ` ` ` `} ` ` ` ` ` `// Check whether there is need to ` ` ` `// flip root or not ` ` ` `$flip` `= true; ` ` ` `while` `(` `$remaining` `> 1) ` ` ` `{ ` ` ` `if` `(` `$remaining` `& 1) ` ` ` `{ ` ` ` `$flip` `= !` `$flip` `; ` ` ` `} ` ` ` `$remaining` `= ` `$remaining` `>> 1; ` ` ` `} ` ` ` ` ` `if` `(` `$flip` `) ` ` ` `{ ` ` ` `echo` `!` `$root` `. ` `"\n"` `; ` ` ` `} ` ` ` `else` ` ` `{ ` ` ` `echo` `$root` `. ` `"\n"` `; ` ` ` `} ` `} ` ` ` `// Driver Code ` `$m` `= 5; ` `$k` `= 5; ` `$n` `= 3; ` `KthCharacter(` `$m` `, ` `$n` `, ` `$k` `); ` ` ` `// This code is contributed by ita_c ` `?> ` |

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**Output:**

1

**Time Complexity:** O(log Z), where Z is the distance between initially consecutive bits after N iterations

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