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Convert String to Integer

Write a program to implement atoi function.

 1) discard all the leading spaces
2) the string may optionally contain leading sign character
-12
+998
are valid
3) After these, if the next character is not a digit, then the return value must be 0
4) If there are alphabets or other characters after digits, the conversion should stop.

e.g.
-12ab89 = -12
ab129 = 0
890jkl = 890

How do we write code for this?

  1. For spaces, we must use trim function.
  2. For sign we should check if the 0th character is sign and if it is -, set the negative flag.
  3. While the character is digit, 
    1. scan the character d
    2. Multiply previous sum s by 10
    3. Add the d -> s = s*10 +d

Not too complicated right. If we assign s to 0 initially, if there are no leading digits, we stop conversion and return 0. If there are digits followed by alpha, we stop at first alpha then return the value.

import java.util.Scanner;

public class Atoi {
    int stringToInteger(String str){
        str = str.trim();
        char arr[] = str.toCharArray();
        int len = arr.length;
        int sum = 0;
        boolean isNegative = false;
        int i=0;
         if(arr[0]=='-' ){
            isNegative = true;i++;
        }else if(arr[0]=='+'){
            i++;
        }
        for(;i<len;i++){
            if(!Character.isDigit(arr[i])){
                break;/* we have a non-digit. stop conversion*/
            }
            sum = sum*10+(arr[i]-48);/*unicode value of 0 is 48*/
        }
        if(isNegative)
            return -1*sum;
        return sum;
    }
    public static void main(String args[]){
        String str;
        System.out.println("Enter a string");
        Scanner scanner = new Scanner(System.in);
        str = scanner.nextLine();
        Atoi obj = new Atoi();
        System.out.println("THe number is "+obj.stringToInteger(str));
    }
}
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