atoi() Not Working with std::string::substr()

Question

This is a snippet of my code:

#include <iostream>
#include <fstream>
#include <string>
#include <stdlib.h> // atoi()


int main() {

    std::string line;

    std::ifstream numbers("numbertest.txt");

    if (numbers.is_open()) {
        while (std::getline(numbers, line)) {
            for (int i = 0; i < line.length() - 4; i++) {
                for (int n = 0; n < 5; n++) {
                    std::cout << atoi((line.substr(i, 5)[n]).c_str());
                }

I want to operate with numbers in groups of 5, from a file. Why is atoi() not working here? It says "expression must have class type" under the second parentheses on the atoi line.

Answer 1

Try atoi( line.substr(i,5).c_str() )

Or if you want for each character

std::cout << ((line.substr(i, 5)[n]) - '0');

Or even better

std::cout << (line[i+n]) - '0');

Note that: atoi is not ascii to integer. It converts a ctype string to number. For a single character, this conversion should be done using arithmetic or lookup table.

Also there is no point converting characters to integer and then print it (back to chacters). You should better print digit character itself.

Moreover in C++, I would prefer to use stringstream instead or atoi. On C++11 there are even more advanced solutions like sto*.