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Write your own String to Integer (atoi) implementation in Java

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Write your own String to Integer (atoi) converter.

Implement your own ASCII to Integer (atoi) method which converts a string to an integer.

Input 1: "123"
Output: 123 (as int)

Input 2: "0"
Output: 0 (as int)

Input 3: "8"
Output: 8 (as int)

Algorithm

Before we look into the algorithm, lets understand the ASCII ranges for 0-9.

ASCII  Char 
--------------- 
 48   0    
 49   1    
 50   2    
 51   3    
 52   4    
 53   5    
 54   6    
 55   7    
 56   8    
 57   9 

so if we have char '4' and we want to get the Integer representation of '4', we can get by

int number = '4' - '0' which would be 52 - 48 = 4 (an integer 4), 

similarly, if we want to get the integer representation of char '7', just subtract it by '0'.
int number = '7' - '0' which would be 55 - 48 = 7 (an integer 7), similarly

Lets jump to original problem, if given the String "123"

Iterate the String, get the first character '1', convert it to Integer as shown above.
Similarly for all the characters of the String.

For getting the integer number, we will use the technique, 

sum = 0 initially.
sum = (sum * 10) + (str.charAt(i) - '0') 

Java Program to convert Ascii To Integer.

    

package javabypatel;

public class ASCIIToInteger {
    public static void main(String[] args) {
        System.out.println(asciiToInt("10"));
    }

    private static int asciiToInt(String num) {
        int result = 0;
        for (int i = 0; i < num.length(); i++) {
            char ch = num.charAt(i);
            result = (result * 10) + (ch - '0');
        }
        return result;
    }
}
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