Question

How to check if a string is a valid positive integer in C++

Answer 1

#include <iostream>

bool is_valid_positive_integer(const std::string& s) {
    if (s.empty()) return false;

    size_t start = 0;
    if (s[0] == '+') {
        if (s.size() == 1) return false; 
        start = 1;
    }
    for (size_t i = start; i < s.size(); ++i) {
        if (!std::isdigit(s[i])) {
            return false;
        }
    }

    return true;
}

int main() {
    
    std::cout << (is_valid_positive_integer("123") ? "yes" : "no") << "\n";
    std::cout << (is_valid_positive_integer("+8492") ? "yes" : "no") << "\n";
    std::cout << (is_valid_positive_integer("-300") ? "yes" : "no") << "\n";
    std::cout << (is_valid_positive_integer("12W") ? "yes" : "no") << "\n";
}



/*
run:

yes
yes
no
no

*/

 

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