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 give the corresponding digit for the given character.

Below is the implementation of the above approach:

C++

// C++ program for the 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;
}

C

// 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;
}

Java

// 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

Python3

# 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

C#

// 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

Output:

123

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