String to Integer (atoi) in C++

Suppose we have to design a module, that first discards as many whitespace characters as necessary until the first non-whitespace character is reached. After that, starting from this character, it takes an optional initial plus sign or minus sign followed by as many numerical digits, and interprets them as a numerical value.

When the first sequence of non-whitespace characters in str is not a valid integral number, or when no such sequence exists because either str is empty or it contains only whitespaces, no conversion will be performed.

So if the input is like “-45”, the output will be -45.

To solve this, we will follow these steps −

• sign := 1, base := 0, i := 0, n := size of the string s
• while i < n and s[i] is whitespace, then increase i by 1
• if the first character is – then sign := -1, otherwise sign := 1
• while s[i] in range ‘0’ to ‘9’
  • read each character and convert it to an integer, then adjust the base calculation by increasing base for each character.
• return the number * sign

Example(C++)

Let us see the following implementation to get a better understanding −

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
   int myAtoi(string str) {
      int sign = 1;
      int base = 0;
      int i = 0;
      int n = str.size();
      while(i < n && str[i] == ' '){
         i++;
      }
      if(str[i] == '-' || str[i] == '+') sign = 1 - 2*(str[i++] =='-');
      while(str[i] >= '0' && str[i] <='9'){
         if(base > INT_MAX/10 || base == INT_MAX/10 && str[i]- '0' > INT_MAX %10){
            if(sign == 1)return INT_MAX;
               return INT_MIN;
            }
            base = (base * 10) + (str[i++] - '0');
         }
         return base * sign;
   }
};
main(){
   Solution ob;
   cout << ob.myAtoi("-45")<<endl;
   cout << ob.myAtoi(" 56")<<endl;
   cout << ob.myAtoi("100")<<endl;
}

Input

"-45"
" 56"
"100"

Output

-45
56
100