Complexity of Converting String to Int in C++

Question

I am having trouble with converting string datatype to int in c++, I am currently using DevC++ GNU version 5.8.3 which gives an error when i use stoi() funtion. My concern is not about conversion but about the complexity and how it is done. Code Below :

#include<bits/stdc++.h>
int main(){
    string s="abc";
    int i=stoi(s,NULL,16);
}

Error : 'stoi' was not declared in this scope

So, i wrote my own function, Code Below :

String to Int :

int strtonum(string s,int length) {
    int num=0;
    for(int i=0;i<length;i++)
        num=num*10+(int)s[i]-48;
    return num;
}

Integer to String :

string numtostring(int number){
    int digit=floor(log10(number))+1;
    string s="";
    for(int i=0;i<digit;i++){
        s+=(char)(48+number%10);
        number/=10;
    }
    reverse(s.begin(),s.end());
    return s;
}

Both Functions are giving desired output, But I am worried about the complexity of Conversion.

For String to Int it is : O(n)

For Int to String it is : O(n)+O(n/2)

Q. Is there any better Solution to this problem ?

Q. How Complex are the Inbuilt Function

Q.How do they Convert one data type to another?

That code isn't portable. You can make it portable by replacing `48` with `'0'`. — molbdnilo, Mar 20 '17 at 11:08
Remember that for typical implementations, `long long` has a max just less than 10E19. That means *n* is never more than 19 - which means it doesn't matter. — Martin Bonner supports Monica, Mar 20 '17 at 11:09
If you want to avoid the `reverse` operation, then as you know the length of the result, insert your generated chars from the end... — Jean-Baptiste Yunès, Mar 20 '17 at 11:13
Possible duplicate of [Easiest way to convert int to string in C++](http://stackoverflow.com/questions/5590381/easiest-way-to-convert-int-to-string-in-c) — Jean-Baptiste Yunès, Mar 20 '17 at 11:14
`numtostring` isn't going to handle zero well :-) - or negative numbers. `strtonum` doesn't handle negative numbers, non-digits, or overflow. — Martin Bonner supports Monica, Mar 20 '17 at 11:25
Some tips: **1.** Don't use `#include `, https://stackoverflow.com/a/31816096/13647419 **2.** Use `'0'` instead of `48` (more specifically, use the character itself instead of its ASCII code). It's easier to read, and I think not on all systems the characters have the same code — H-005, Oct 20 '20 at 18:17

score 0 · Answer 1 · answered Mar 20 '17 at 11:54

0

You probably wanted to use atoi, and its complement strtod, or some related functions in this category.

answered Mar 20 '17 at 11:54

Dmitry Baksheev

36
3

score 0 · Accepted Answer · answered Mar 20 '17 at 11:59

0

Error : 'stoi' was not declared in this scope

try to add "std::"

//stoi // x

std::stoi(...) // ok

answered Mar 20 '17 at 11:59

sailfish009

2,561
1
24
31

Complexity of Converting String to Int in C++

2 Answers2