Converting string to integer using stoi() and atoi()

Question

C++ compiler g++ 17 GCC 9.1.0

I was trying to convert the digts that are present in the string and sum them up but i was not able to do it, on geeksforgeeks article I found out that they used atoi and coverted the string to c style string using c_str(). After reading about atoi and stoi I found out that atoi does not report error and hence is not adviced to use. Here it seems like the issue of atoi not reporting error is helping, I am not sure though. Can someone tell me why atoi is working and stoi is not, and is there anyother way in which stoi wont give me the error.

#include <bits/stdc++.h>

using namespace std;

int main() {
    string s;
    cin>>s; // s has 1xyz23
    
    int n = s.size();
    int i = 0;
    int sum = 0;
    while(i < n){
        string temp = "";
        while(i < n && isdigit(s[i])){
            temp.push_back(s[i]);
            i++;
        }
        cout<<temp<<" "; // o/p is 1   23 output spacing is weird though 
        int temp_int = atoi(temp.c_str());
        sum += temp_int;
        i++;
    }
    cout<<sum; // sum is 24
    return 0;
}

1 23 24 (weird spacing is shown as a comment in the code)

Why does the above code produce the output I want but the below one produces an error.

#include <bits/stdc++.h>

using namespace std;

int main() {
    string s;
    cin>>s; // s has 1xyz23000
    
    int n = s.size();
    int i = 0;
    int sum = 0;
    while(i < n){
        string temp = "";
        while(i < n && isdigit(s[i])){
            temp.push_back(s[i]);
            i++;
        }
        cout<<temp<<" ";
        int temp_int = stoi(temp);
        sum += temp_int;
        i++;
    }
    cout<<sum; 
    return 0;
}

terminate called after throwing an instance of 'std::invalid_argument' what(): stoi Command terminated by signal 6

It is not necessary to check for errors if you make sure that the string always contains at least one digit, ***and only digits***. Which is what the sample program in that article does, and your code does not. Try to figure out how to change the logic in your code accordingly. — Sam Varshavchik, Aug 08 '21 at 16:25
Don't use geeksforgeeks to study C++. That site is just a garbage collection and has nothing to do with professional programming. Consider reading some [good books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) instead. — Evg, Aug 08 '21 at 17:12

score 1 · Accepted Answer · answered Aug 08 '21 at 16:15

If you have multiple non-digits in a row in the input, you'll try to convert an empty string to an integer. You need to check if temp is empty before trying to convert it.

#include <bits/stdc++.h>

using namespace std;

int main() {
    string s;
    cin>>s; // s has 1xyz23000
    
    int n = s.size();
    int i = 0;
    int sum = 0;
    while(i < n){
        string temp = "";
        while(i < n && isdigit(s[i])){
            temp.push_back(s[i]);
            i++;
        }
        if (temp != "") {
            cout<<temp<<" ";
            int temp_int = stoi(temp);
            sum += temp_int;
        }
        i++;
    }
    cout<<sum; 
    return 0;
}

Converting string to integer using stoi() and atoi()

1 Answers1