Consider a series of numbers composed of only digits 4 and 7. The first few numbers in the series are 4, 7, 44, 47, 74, 77, 444, .. etc. Given a number constructed by 4, 7 digits only, we need to find the position of this number in this series.

Examples:

Input : 7 Output : pos = 2 Input : 444 Output : pos = 7

It is reverse of the following article :

Find n-th element in a series with only 2 digits (4 and 7) allowed | Set 2 (log(n) method)

"" / \ 1(4) 2(7) / \ / \ 3(44) 4(47) 5(74) 6(77) / \ / \ / \ / \

The idea is based on the fact that all even positioned numbers have 7 as the last digit and all odd positioned numbers have 4 as the last digit.

If the number is 4 then it is the left node of the tree, then it corresponds to (pos*2)+1. Else right child node(7) corresponds to (pos*2)+2.

## C++

`// C++ program to find position of a number` `// in a series of numbers with 4 and 7 as the` `// only digits.` `#include <iostream>` `#include <algorithm>` `using` `namespace` `std;` `int` `findpos(string n)` `{` ` ` `int` `i = 0, pos = 0;` ` ` `while` `(n[i] != ` `'\0'` `) {` ` ` `// check all digit position` ` ` `switch` `(n[i])` ` ` `{` ` ` `// if number is left then pos*2+1` ` ` `case` `'4'` `:` ` ` `pos = pos * 2 + 1;` ` ` `break` `;` ` ` `// if number is right then pos*2+2` ` ` `case` `'7'` `:` ` ` `pos = pos * 2 + 2;` ` ` `break` `;` ` ` `}` ` ` `i++;` ` ` `}` ` ` `return` `pos;` `}` `// Driver code` `int` `main()` `{` ` ` `// given a number which is constructed` ` ` `// by 4 and 7 digit only` ` ` `string n = ` `"774"` `;` ` ` `cout << findpos(n);` ` ` `return` `0;` `}` |

## Java

`// java program to find position of a number` `// in a series of numbers with 4 and 7 as the` `// only digits.` `import` `java.util.*;` `class` `GFG {` ` ` ` ` `static` `int` `findpos(String n)` ` ` `{` ` ` ` ` `int` `k = ` `0` `, pos = ` `0` `, i = ` `0` `;` ` ` `while` `(k != n.length()) {` ` ` `// check all digit position` ` ` `switch` `(n.charAt(i)) {` ` ` `// if number is left then pos*2+1` ` ` `case` `'4'` `:` ` ` `pos = pos * ` `2` `+ ` `1` `;` ` ` `break` `;` ` ` `// if number is right then pos*2+2` ` ` `case` `'7'` `:` ` ` `pos = pos * ` `2` `+ ` `2` `;` ` ` `break` `;` ` ` `}` ` ` ` ` `i++;` ` ` `k++;` ` ` `}` ` ` ` ` `return` `pos;` ` ` `}` ` ` `// Driver code` ` ` `public` `static` `void` `main(String[] args)` ` ` `{` ` ` ` ` `// given a number which is constructed` ` ` `// by 4 and 7 digit only` ` ` `String n = ` `"774"` `;` ` ` ` ` `System.out.println(findpos(n));` ` ` `}` `}` `// This code is contributed by Sam007.` |

## Python3

`# python program to find position` `# of a number in a series of` `# numbers with 4 and 7 as the` `# only digits.` `def` `findpos(n):` ` ` `i ` `=` `0` ` ` `j ` `=` `len` `(n)` ` ` `pos ` `=` `0` ` ` `while` `(i<j):` ` ` ` ` `# check all digit position` ` ` `# if number is left then` ` ` `# pos*2+1` ` ` `if` `(n[i] ` `=` `=` `'4'` `):` ` ` `pos ` `=` `pos ` `*` `2` `+` `1` ` ` ` ` `# if number is right then` ` ` `# pos*2+2` ` ` `if` `(n[i] ` `=` `=` `'7'` `):` ` ` `pos ` `=` `pos ` `*` `2` `+` `2` ` ` ` ` `i` `=` `i` `+` `1` ` ` ` ` `return` `pos` `# Driver code` `# given a number which is constructed` `# by 4 and 7 digit only` `n ` `=` `"774"` `print` `(findpos(n))` `# This code is contributed by Sam007` |

## C#

`// C# program to find position of` `// a number in a series of numbers` `// with 4 and 7 as the only digits.` `using` `System;` `class` `GFG` `{` ` ` `static` `int` `findpos(String n)` ` ` `{` ` ` ` ` `int` `k = 0, pos = 0, i = 0;` ` ` `while` `(k != n.Length) {` ` ` `// check all digit position` ` ` `switch` `(n[i]) {` ` ` `// if number is left then pos*2+1` ` ` `case` `'4'` `:` ` ` `pos = pos * 2 + 1;` ` ` `break` `;` ` ` `// if number is right then pos*2+2` ` ` `case` `'7'` `:` ` ` `pos = pos * 2 + 2;` ` ` `break` `;` ` ` `}` ` ` ` ` `i++;` ` ` `k++;` ` ` `}` ` ` ` ` `return` `pos;` ` ` `}` ` ` `// Driver code` ` ` `static` `void` `Main()` ` ` `{` ` ` ` ` `// given a number which is constructed` ` ` `// by 4 and 7 digit only` ` ` `String n = ` `"774"` `;` ` ` ` ` `Console.Write(findpos(n));` ` ` `}` ` ` `}` `// This code is contributed by Sam007` |

## PHP

`<?php` `// PHP program to find position of a number` `// in a series of numbers with 4 and 7 as the` `// only digits.` `function` `findpos(` `$n` `)` `{` ` ` `$i` `= 0;` ` ` `$pos` `= 0;` ` ` `while` `(` `$i` `< ` `strlen` `(` `$n` `)) {` ` ` `// check all digit position` ` ` `switch` `(` `$n` `[` `$i` `])` ` ` `{` ` ` `// if number is left then pos*2+1` ` ` `case` `'4'` `:` ` ` `$pos` `= ` `$pos` `* 2 + 1;` ` ` `break` `;` ` ` `// if number is right then pos*2+2` ` ` `case` `'7'` `:` ` ` `$pos` `= ` `$pos` `* 2 + 2;` ` ` `break` `;` ` ` `}` ` ` `$i` `++;` ` ` `}` ` ` `return` `$pos` `;` `}` ` ` `// Driver code` ` ` `// given a number which` ` ` `// is constructed by 4` ` ` `// and 7 digit only` ` ` `$n` `= ` `"774"` `;` ` ` `echo` `findpos(` `$n` `);` ` ` `// This code is contributed by Sam007` `?>` |

## Javascript

`<script>` ` ` `// Javascript program to find position of a number` `// in a series of numbers with 4 and 7 as the` `// only digits.` ` ` `function` `findpos(n)` `{` ` ` `let i = 0;` ` ` `let pos = 0;` ` ` `while` `(i < n.length) {` ` ` ` ` `// check all digit position` ` ` `switch` `(n[i])` ` ` `{` ` ` ` ` `// if number is left then pos*2+1` ` ` `case` `'4'` `:` ` ` `pos = pos * 2 + 1;` ` ` `break` `;` ` ` ` ` `// if number is right then pos*2+2` ` ` `case` `'7'` `:` ` ` `pos = pos * 2 + 2;` ` ` `break` `;` ` ` `}` ` ` `i++;` ` ` `}` ` ` `return` `pos;` `}` ` ` ` ` `// Driver code` ` ` `// given a number which` ` ` `// is constructed by 4` ` ` `// and 7 digit only` ` ` `let n = ` `"774"` `;` ` ` `document.write(findpos(n));` ` ` `// This code is contributed by _saurabh_jaiswal` `</script>` |

Output:

13

This article is contributed by **Devanshu Agarwal**. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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