# Count of binary strings of given length consisting of at least one 1

Given an integer **N**, the task is to print the number of binary strings of length N which at least one ‘1’.

**Examples:**

Input:2Output:3Explanation:

“01”, “10” and “11” are the possible strings

Input:3Output:7Explanation:

“001”, “011”, “010”, “100”, “101”, “110” and “111” are the possible strings

**Approach:**

We can observe that:

Only one string of length N does not contain any 1, the one filled with only 0’s.

Since2strings are possible of length N, the required answer is^{N}2.^{N}– 1

Follow the steps below to solve the problem:

- Initialize X = 1.
- Compute upto
**2**by performing bitwise left shift on X, N-1 times.^{N} - Finally, print X – 1 as the required answer.

Below is the implementation of our approach:

## C++

`// C++ Program to implement` `// the above approach` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// Function to return` `// the count of strings` `long` `count_strings(` `long` `n)` `{` ` ` `int` `x = 1;` ` ` `// Calculate pow(2, n)` ` ` `for` `(` `int` `i = 1; i < n; i++) {` ` ` `x = (1 << x);` ` ` `}` ` ` `// Return pow(2, n) - 1` ` ` `return` `x - 1;` `}` `// Driver Code` `int` `main()` `{` ` ` `long` `n = 3;` ` ` `cout << count_strings(n);` ` ` `return` `0;` `}` |

## Java

`// Java program to implement` `// the above approach` `import` `java.util.*;` `class` `GFG{` `// Function to return` `// the count of Strings` `static` `long` `count_Strings(` `long` `n)` `{` ` ` `int` `x = ` `1` `;` ` ` `// Calculate Math.pow(2, n)` ` ` `for` `(` `int` `i = ` `1` `; i < n; i++)` ` ` `{` ` ` `x = (` `1` `<< x);` ` ` `}` ` ` `// Return Math.pow(2, n) - 1` ` ` `return` `x - ` `1` `;` `}` `// Driver Code` `public` `static` `void` `main(String[] args)` `{` ` ` `long` `n = ` `3` `;` ` ` `System.out.print(count_Strings(n));` `}` `}` `// This code is contributed by Amit Katiyar` |

## Python3

`# Python3 program to implement` `# the above approach` `# Function to return` `# the count of Strings` `def` `count_Strings(n):` ` ` ` ` `x ` `=` `1` `;` ` ` `# Calculate pow(2, n)` ` ` `for` `i ` `in` `range` `(` `1` `, n):` ` ` `x ` `=` `(` `1` `<< x);` ` ` `# Return pow(2, n) - 1` ` ` `return` `x ` `-` `1` `;` `# Driver Code` `if` `__name__ ` `=` `=` `'__main__'` `:` ` ` ` ` `n ` `=` `3` `;` ` ` `print` `(count_Strings(n));` `# This code is contributed by Princi Singh` |

## C#

`// C# program to implement` `// the above approach` `using` `System;` `class` `GFG{` `// Function to return` `// the count of Strings` `static` `long` `count_Strings(` `long` `n)` `{` ` ` `int` `x = 1;` ` ` `// Calculate Math.Pow(2, n)` ` ` `for` `(` `int` `i = 1; i < n; i++)` ` ` `{` ` ` `x = (1 << x);` ` ` `}` ` ` ` ` `// Return Math.Pow(2, n) - 1` ` ` `return` `x - 1;` `}` `// Driver Code` `public` `static` `void` `Main(String[] args)` `{` ` ` `long` `n = 3;` ` ` `Console.Write(count_Strings(n));` `}` `}` `// This code is contributed by Amit Katiyar` |

## Javascript

`<script>` `// Javascript program to implement` `// the above approach` ` ` `// Function to return` ` ` `// the count of Strings` ` ` `function` `count_Strings(n)` ` ` `{` ` ` `var` `x = 1;` ` ` `// Calculate Math.pow(2, n)` ` ` `for` `(i = 1; i < n; i++) {` ` ` `x = (1 << x);` ` ` `}` ` ` `// Return Math.pow(2, n) - 1` ` ` `return` `x - 1;` ` ` `}` ` ` `// Driver Code` ` ` ` ` `var` `n = 3;` ` ` `document.write(count_Strings(n));` `// This code is contributed by todaysgaurav` `</script>` |

**Output:**

3

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