Convert string to integer without using any in-built functions

Given a string str, the task is to convert the given string into the number without using any inbuilt function.

Examples: 

Input: str = “985632”
Output: 985632
Explanation:
Given input is in string form and returned output is in integer form.

Input: str = “123”
Output: 123
Explanation:
Given input is in string form and returned output is in integer form.

Approach: The idea is to use the ASCII value of the digits from 0 to 9 start from 48 – 57.
Therefore, to change the numeric character to an integer subtract 48 from the ASCII value of the character will given the corresponding digit for the given character.



Below is the implementation of the above approach:

C++

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// C++ program for tha above approach 
#include <iostream> 
using namespace std; 
  
// Function to convert string to 
// integer without using functions 
void convert(string s) 
    // Initialize a variable 
    int num = 0; 
    int n = s.length(); 
  
    // Iterate till length of the string 
    for (int i = 0; i < n; i++) 
  
        // Subtract 48 from the current digit 
        num = num * 10 + (int(s[i]) - 48); 
  
    // Print the answer 
    cout << num; 
  
// Driver Code 
int main() 
    // Given string of number 
    char s[] = "123"
  
    // Function Call 
    convert(s); 
    return 0; 

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C

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// C program for the above approach 
#include <stdio.h> 
#include <string.h> 
  
// Function to convert string to 
// integer without using functions 
void convert(char s[]) 
    // Initialize a variable 
    int num = 0; 
    int n = strlen(s); 
  
    // Iterate till length of the string 
    for (int i = 0; i < n; i++) 
  
        // Subtract 48 from the current digit 
        num = num * 10 + (s[i] - 48); 
  
    // Print the answer 
    printf("%d", num); 
  
// Driver Code 
int main() 
    // Given string of number 
    char s[] = "123"
  
    // Function Call 
    convert(s); 
    return 0; 

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Java

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// Java program for the above approach 
class GFG{ 
  
// Function to convert string to 
// integer without using functions 
public static void convert(String s) 
      
    // Initialize a variable 
    int num = 0
    int n = s.length(); 
  
    // Iterate till length of the string 
    for(int i = 0; i < n; i++) 
  
        // Subtract 48 from the current digit 
        num = num * 10 + ((int)s.charAt(i) - 48); 
  
    // Print the answer 
    System.out.print(num); 
  
// Driver code 
public static void main(String[] args) 
    // Given string of number 
    String s = "123"
  
    // Function Call 
    convert(s); 
  
// This code is contributed by divyeshrabadiya07 

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Python3

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# Python3 program for the above approach 
  
# Function to convert string to 
# integer without using functions 
def convert(s): 
  
    # Initialize a variable 
    num = 0
    n = len(s) 
  
    # Iterate till length of the string 
    for i in s: 
  
        # Subtract 48 from the current digit 
        num = num * 10 + (ord(i) - 48
  
    # Print the answer 
    print(num) 
  
# Driver code 
if __name__ == '__main__'
  
    # Given string of number 
    s = "123"
  
    # Function Call 
    convert(s) 
  
# This code is contributed by Shivam Singh 

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C#

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// C# program for the above approach 
using System;
  
class GFG{
  
// Function to convert string to 
// integer without using functions 
public static void convert(string s) 
      
    // Initialize a variable 
    int num = 0; 
    int n = s.Length; 
  
    // Iterate till length of the string 
    for(int i = 0; i < n; i++) 
  
        // Subtract 48 from the current digit 
        num = num * 10 + ((int)s[i] - 48); 
  
    // Print the answer 
    Console.Write(num); 
  
// Driver code
public static void Main(string[] args)
{
      
    // Given string of number 
    string s = "123"
  
    // Function call 
    convert(s); 
}
}
  
// This code is contributed by rock_cool

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Output:

123

Time Complexity: O(N), where N is the length of the given string.
Auxiliary Space: O(1)

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