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Convert a String to an Integer using Recursion
• Difficulty Level : Basic
• Last Updated : 02 Jan, 2020

Given a string str representing a string, the task is to convert the given string into an integer.

Examples:

Input: str = “1234”
Output: 1234

Input: str = “0145”
Output: 145

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

Approach: Write a recursive function that will take the first digit of the string and multiply it with the appropriate power of 10 and then add the recursive result for the substring starting at the second index. The termination condition will be when the passed string consists of a single digit. In that case, return the digit represented by the string.

Below is the implementation of the above approach:

C++

 `// C++ implementation of the approach ` `#include ` `using` `namespace` `std; ` ` `  `// Recursive function to convert ` `// string to integer ` `int` `stringToInt(string str) ` `{ ` ` `  `    ``// If the number represented as a string ` `    ``// contains only a single digit ` `    ``// then returns its value ` `    ``if` `(str.length() == 1) ` `        ``return` `(str[0] - ``'0'``); ` ` `  `    ``// Recursive call for the sub-string ` `    ``// starting at the second character ` `    ``double` `y = stringToInt(str.substr(1)); ` ` `  `    ``// First digit of the number ` `    ``double` `x = str[0] - ``'0'``; ` ` `  `    ``// First digit multiplied by the ` `    ``// appropriate power of 10 and then ` `    ``// add the recursive result ` `    ``// For example, xy = ((x * 10) + y) ` `    ``x = x * ``pow``(10, str.length() - 1) + y; ` `    ``return` `int``(x); ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``string str = ``"1235"``; ` `    ``cout << (stringToInt(str)) << endl; ` `} ` ` `  `// This code is contributed by ` `// Surendra_Gangwar `

Java

 `// Java implementation of the approach ` `public` `class` `GFG { ` ` `  `    ``// Recursive function to convert string to integer ` `    ``static` `int` `stringToInt(String str) ` `    ``{ ` ` `  `        ``// If the number represented as a string ` `        ``// contains only a single digit ` `        ``// then returns its value ` `        ``if` `(str.length() == ``1``) ` `            ``return` `(str.charAt(``0``) - ``'0'``); ` ` `  `        ``// Recursive call for the sub-string ` `        ``// starting at the second character ` `        ``double` `y = stringToInt(str.substring(``1``)); ` ` `  `        ``// First digit of the number ` `        ``double` `x = str.charAt(``0``) - ``'0'``; ` ` `  `        ``// First digit multiplied by the ` `        ``// appropriate power of 10 and then ` `        ``// add the recursive result ` `        ``// For example, xy = ((x * 10) + y) ` `        ``x = x * Math.pow(``10``, str.length() - ``1``) + y; ` `        ``return` `(``int``)(x); ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``String str = ``"1235"``; ` `        ``System.out.print(stringToInt(str)); ` `    ``} ` `} `

Python3

 `     `  `# Python3 implementation of the approach ` ` `  `# Recursive function to convert ` `# string to integer ` `def` `stringToInt(``str``): ` ` `  `    ``# If the number represented as a string ` `    ``# contains only a single digit ` `    ``# then returns its value ` `    ``if` `(``len``(``str``) ``=``=` `1``): ` `        ``return` `ord``(``str``[``0``]) ``-` `ord``(``'0'``); ` ` `  `    ``# Recursive call for the sub-string ` `    ``# starting at the second character ` `    ``y ``=` `stringToInt(``str``[``1``:]); ` ` `  `    ``# First digit of the number ` `    ``x ``=` `ord``(``str``[``0``]) ``-` `ord``(``'0'``); ` `     `  `    ``# First digit multiplied by the ` `    ``# appropriate power of 10 and then ` `    ``# add the recursive result ` `    ``# For example, xy = ((x * 10) + y) ` `    ``x ``=` `x ``*` `(``10``*``*``(``len``(``str``) ``-` `1``)) ``+` `y; ` `    ``return` `int``(x); ` ` `  `# Driver code ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``str` `=` `"1235"``; ` `    ``print``(stringToInt(``str``)); ` ` `  `# This code is contributed by PrinciRaj1992  `

C#

 `// C# implementation of the approach ` `using` `System; ` ` `  `class` `GFG ` `{ ` ` `  `    ``// Recursive function to convert string to integer ` `    ``static` `int` `stringToInt(String str) ` `    ``{ ` ` `  `        ``// If the number represented as a string ` `        ``// contains only a single digit ` `        ``// then returns its value ` `        ``if` `(str.Length == 1) ` `            ``return` `(str[0] - ``'0'``); ` ` `  `        ``// Recursive call for the sub-string ` `        ``// starting at the second character ` `        ``double` `y = stringToInt(str.Substring(1)); ` ` `  `        ``// First digit of the number ` `        ``double` `x = str[0] - ``'0'``; ` ` `  `        ``// First digit multiplied by the ` `        ``// appropriate power of 10 and then ` `        ``// add the recursive result ` `        ``// For example, xy = ((x * 10) + y) ` `        ``x = x * Math.Pow(10, str.Length - 1) + y; ` `        ``return` `(``int``)(x); ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `Main(String[] args) ` `    ``{ ` `        ``String str = ``"1235"``; ` `        ``Console.Write(stringToInt(str)); ` `    ``} ` `} ` ` `  `// This code is contributed by Princi Singh `

Output:

```1235
```

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